0001564590-21-008081 | 10-K | XBRL Viewer

Basis of Presentation and Summary of Significant Accounting Policies

2. Basis of Presentation and Summary of Significant Accounting Policies

Basis of Presentation

The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”).

Use of Estimates

The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates.

Concentration of Credit Risk and Other Risks and Uncertainties

The Company is subject to a number of risks similar to other biopharmaceutical companies in the early stage, including, but not limited to, the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, the need to obtain marketing approval for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s products, and protection of proprietary technology. If the Company does not successfully obtain regulatory approval, commercialize or partner any of its product candidates, it will be unable to generate revenue from product sales or achieve profitability.

Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents, short-term investments and accounts receivable. Substantially all the Company’s cash is held by one financial institution. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents and short-term investments in highly rated money market funds and its short-term investments in U.S. Government Bonds.

Customers and collaboration partners who represent 10% or more of the Company’s total revenue during each period presented or accounts receivable balance at each respective balance sheet date are as follows:

	Revenue						Accounts Receivable, net
	Year Ended December 31,						December 31,
	2020		2019		2018		2020		2019
	(in thousands)						(in thousands)
AbbVie Ireland Unlimited Company	$	38,192	$	5,878	$	18,997	$	—	$	—
Astellas Pharma Inc.		13,931		—		—		798		—
Bristol-Myers Squibb Company		39,630		47,740		32,780		—		13
Total revenue from customers who represent 10% or more of the Company's total revenue	$	91,753	$	53,618	$	51,777	$	798	$	13

The Company’s customers are located in the United States of America, Ireland and Japan.

Segments

Management has determined that it has one business activity and operates as one operating segment as it only reports financial information on an aggregate basis to its chief executive officer and chief financial officer, who are the Company’s chief operating decision makers. All long-lived assets are maintained in the United States of America.

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents.

Restricted Cash

Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets that sum to the total of the amounts shown in the statement of cash flows:

	December 31
	2020		2019
	(in thousands)
Cash and cash equivalents	$	191,859	$	188,425
Restricted cash		917		917
	$	192,776	$	189,342

Short-term Investments

All investments have been classified as available-for-sale (“AFS”) and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Generally, those investments with contractual maturities less than 12 months are considered short-term investments. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Dividend and interest income are recognized when earned. Realized gains and losses are included in earnings and are derived using the specific identification method for determining the cost of securities sold.

The Company assesses impairment of its AFS debt securities investments at each reporting period. Unrealized gains resulting from the excess of the fair value over the amortized cost basis of an investment are reported as a component of accumulated other comprehensive income (loss), net of tax. Unrealized losses or impairments resulting from the fair value of the AFS debt security being below the amortized cost basis are evaluated, using the discounted cash flow model, for identification of credit losses and non-credit related losses. Any credit losses are charged to earnings against the allowance for credit losses of the security, limited to the difference between the fair value and the amortized cost basis of the security. Any difference between the fair value of the security and the amortized cost basis, less the allowance for credit losses, are reported in other comprehensive income (loss). Expected cash inflows due to improvements in credit are recognized through a reversal of the allowance for credit losses subject to the total allowance previously recognized.

In the event of impairment of any security, if management (i) has the intent to sell such security or (ii) will more-likely-than-not be required to sell such security before recovery of its amortized cost basis, such AFS debt security’s amortized cost basis will be written down to its fair value through earnings along with any existing allowance for credit losses.

Property and Equipment, net

Property and equipment are recorded at cost net of accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the respective assets. The useful lives of property and equipment are as follows:

Machinery and equipment		5 years
Computer equipment and software		3 years
Furniture and fixtures		3 years
Leasehold improvements		Shorter of remaining lease term or estimated life of the assets

Maintenance and repairs that do not extend the life or improve the asset are expensed when incurred.

Goodwill and Intangible Assets

Goodwill represents the excess of the purchase price paid over the fair value of tangible and identifiable intangible assets acquired in business combinations. Goodwill and other intangible assets with indefinite lives are not amortized, but are assigned to reporting units and tested for impairment annually, or whenever there is an impairment indicator. Intangible assets are comprised of in-process research and development. The Company assesses impairment indicators annually as of December 31 or more frequently, if a change in circumstances or the occurrence of events suggests the remaining value may not be recoverable. Intangible assets that are not deemed to have an indefinite life are amortized over their estimated useful lives. There was no impairment of goodwill or intangible assets identified during the years ended December 31, 2020, 2019 and 2018.

Impairment of Long-Lived Assets

Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset (or asset group) may not be recoverable and prior to any goodwill impairment test. An impairment loss is recognized when the total of estimated undiscounted future cash flows expected to result from the use of the asset (or asset group) and its eventual disposition is less than its carrying amount. Impairment, if any, would be assessed using discounted cash flows or other appropriate measures of fair value. There was no impairment of long-lived assets during the years ended December 31, 2020, 2019 and 2018.

Revenue Recognition

On January 1, 2018, the Company adopted Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in Accounting Standards Codification (ASC) Topic 605, Revenue Recognition (ASC 605), using the modified retrospective transition method applied to those contracts which were not completed as of January 1, 2018. The impact of the adoption of ASC 606 was an increase in the balance of deferred revenue and an increase in the accumulated deficit balance of $10.9 million on January 1, 2018 in the Company’s Balance Sheet.

The Company’s revenues are primarily derived through its license, research, development and commercialization agreements. The terms of these types of agreements may include (i) licenses for the Company’s technology or programs, (ii) research and development services, and (iii) services or obligations in connection with participation in research or steering committees. Payments to the Company under these arrangements typically include one or more of the following: nonrefundable upfront and license fees, research funding, milestone and other contingent payments to the Company for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products.

The Company assesses whether the promises in its arrangements with customers are considered as distinct performance obligations that should be accounted for separately. Judgment is required to determine whether the license to the Company’s intellectual property is distinct from the research and development services or participation on steering committees.

The Company’s collaboration and license agreements may include contingent payments related to specified research, development and regulatory milestones. Such milestone payments are typically payable under the collaborations when the collaboration partner claims or selects a target, or initiates or advances a covered product candidate in preclinical or clinical development, upon submission for marketing approval of a covered product with regulatory authorities; or upon receipt of actual marketing approvals of a covered product or for additional indications. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At each reporting date, the Company re-evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price by using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price in such period of determination.

The Company’s collaboration and license agreements may also include contingent payments related to sales-based milestones. Sales-based milestones are typically payable when annual sales of a covered product reach specified levels. Sales-based milestones are recognized at the later of when the associated performance obligation has been satisfied or when the sales occur. Unlike other contingency payments, such as regulatory milestones, sales-based milestones are not included in the transaction price based on estimates at the inception of the contract, but rather, are included when the sales or usage occur.

The transaction price in each arrangement is allocated to the identified performance obligations based on the relative standalone selling price (“SSP”) of each distinct performance obligation, which requires judgment. In instances where SSP is not directly observable, such as when a license or service is not sold separately, SSP is determined using information that may include market conditions and other observable inputs. Due to the early stage of the Company’s licensed technology, the license of such technology is typically combined with research and development services and steering committee participation as one performance obligation. In the event that the Company receives non-cash consideration such as consideration in the form of a research license and research support services from the counterparty, the transaction price of a non-monetary exchange that has commercial substance is estimated based on the fair value of the non-cash consideration received, which may be determined through a valuation analysis.

In certain cases, the Company’s performance creates an asset that does not have an alternative use to the customer and the Company has an enforceable right to payment at all times for performance completed to date. In these cases, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

AbbVie Ireland Unlimited Company (“AbbVie”), one of the Company’s collaboration partners, entered into a license agreement with Seattle Genetics, Inc. (“SGEN”) to license certain intellectual property rights. As part of the Company’s collaboration agreement with AbbVie, the Company is required to pay SGEN sublicense fees for certain milestone achievements and annual maintenance fee. These sublicense fees are treated as reductions to the transaction price and combined with the performance obligation to which they relate.

Comprehensive Loss

Comprehensive loss represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s non-credit related unrealized gains and losses on short-term investments and impact of adoption of new accounting pronouncements during the period represent the components of other comprehensive income (loss) that is excluded from the reported net loss.

Contract Balances

Customer payments are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the Company satisfies its performance obligations under these arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional.

Stock-Based Compensation

The Company measures its stock-based awards made to employees based on the fair values of the awards as of the grant date using the Black-Scholes option-pricing model. Stock-based compensation expense is recognized over the requisite service period using the ratable method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. Forfeitures are recognized as they occur.

On January 2019, the Company adopted ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. Under this new standard, stock options granted to non-employees as consideration for services received are measured on the date of grant using the Black-Scholes option-pricing model. The related grant date fair values of stock options are expensed on a straight-line basis over the period, generally the vesting period, during which the non-employee is required to provide service in exchange for the award.

Prior to the adoption of ASU 2018-07, such stock-based compensation expense was measured on the date of performance at the fair value of the consideration received or the fair value of the equity instruments issued, using the Black-Scholes option-pricing model, whichever can be more reliably measured. Stock-based compensation expense for options granted to non-employees was periodically remeasured as the underlying options vest.

Income Taxes

The Company accounts for income taxes using an asset and liability approach. Deferred tax assets and liabilities reflect the net tax effects of temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The Company records a valuation allowance to reduce its deferred tax assets to reflect the net amount that it believes as more likely than not to be realized. Realization of the deferred tax assets is dependent on the generation of future taxable income, the amount and timing of which are uncertain. The valuation allowance requires an assessment of both positive and negative evidence when determining whether it is more likely than not that deferred tax assets are recoverable. Based upon the weight of available evidence at December 31, 2020, the Company continues to maintain a full valuation allowance against all of its deferred tax assets after management considered all available evidence, both positive and negative, including but not limited to its historical operating results, income or loss in recent periods, cumulative income in recent years, forecasted earnings, future taxable income, and significant risk and uncertainty related to forecasts.

The Company recognizes the tax effects of an uncertain tax position only if it is more likely than not that it will be sustained based solely on its technical merits as of the reporting date and only in an amount more likely than not that it will be sustained upon review by the tax authorities. The Company evaluates uncertain tax positions on a quarterly basis and adjust the liability for changes in facts and circumstances, such as new regulations or interpretations by the taxing authorities, new information obtained during a tax examination, significant amendment to an existing tax law, or resolution of an examination. To the extent that the final tax outcome of these matters is different than the amounts recorded, such differences will impact the income tax provision in the period in which such determination is made. The resolution of its uncertain income tax positions is dependent on uncontrollable factors such as law changes, new case law, and the willingness of the income tax authorities to settle, including the timing thereof and other factors. Although the Company does not anticipate significant changes to its uncertain income tax positions in the next twelve months, items outside of its control could cause its uncertain income tax positions to change in the future, which would be recorded in its statements of operations. Interest and/or penalties related to income tax matters are recognized as a component of income tax expense.

Research and Development Expenses

The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and includes these costs in accrued liabilities in the balance sheets and within research and development expense in the statements of operations. These costs are a significant component of the Company’s research and development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations.

Research and development expenses include costs directly attributable to the conduct of research and development programs, including the cost of salaries, payroll taxes, employee benefits, materials, supplies, depreciation on and maintenance of research equipment, the cost of services provided by outside contractors, and the allocated portions of facility costs, such as rent, utilities, insurance, repairs and maintenance, depreciation, and general support services. All costs associated with research and development are expensed as incurred.

In January 2019, the Company acquired certain technology know-how that is complementary to the Company’s proprietary Probody technology from a third party for $5.0 million. The Company plans to use this technology in certain of the Company’s discovery stage projects, and has concluded that the technology acquired does not have an alternative future use. Accordingly, the $5.0 million has been recorded as research and development expense for 2019.

Leases

The Company adopted the Accounting Standard Update (“ASU”) No. 2016-02, Leases (Topic 842) on January 1, 2019, using the modified retrospective approach. As a result of the adoption of ASC 842, the Company recorded a right-of-use asset of $28.0 million and a lease liability $30.1 million on January 1, 2019. There was no impact on the Company’s accumulated deficit as of the adoption date. The Company determines if an arrangement is or contains a lease at inception. Operating leases are recorded as operating lease right-of-use (“ROU”) assets and operating lease liabilities in the Company’s balance sheet. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses an implicit rate when readily available, or its incremental borrowing rate based on the information available at lease commencement date in determining the present value of lease payments. The operating lease ROU assets also include any lease prepayments made and reduced by lease incentives. The Company’s lease terms may include options to extend the lease when it is reasonably certain that such option will be exercised. Lease expenses are recognized on a straight-line basis over the lease term. The Company elected the short-term lease recognition exemption. The Company’s operating lease arrangement includes lease and non-lease components which are generally accounted for separately.

Adopted and Recent Accounting Pronouncements

12 Months Ended

Dec. 31, 2020

Adopted and Recent Accounting Pronouncements

3. Adopted and Recent Accounting Pronouncements

Adopted Accounting Pronouncements

Financial Instruments - Credit Losses

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The new standard changes the impairment model for most financial assets and certain other instruments. Under the new standard, entities holding financial assets and net investment in leases that are not accounted for at fair value through net income are to be presented at the net amount expected to be collected. An allowance for credit losses will be a valuation account that will be deducted from the amortized cost basis of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. This new standard also requires that credit losses related to available-for-sale debt securities be recorded as an allowance through net income rather than reducing the carrying amount under the previous other-than-temporary-impairment model. The Company adopted this standard on January 1, 2020 using a modified retrospective approach. There was no impact on the Company’s accumulated deficit as of January 1, 2020 as there was no incremental impairment loss to be recognized upon the adoption of this ASU.

Simplification of the Test for Goodwill Impairment

In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. The new standard simplifies the measurement of goodwill by eliminating the Step 2 impairment test. Step 2 measures a goodwill impairment loss by comparing the implied fair value of a reporting unit's goodwill with the carrying amount of that goodwill. The new guidance requires an entity to compare the fair value of a reporting unit with its carrying amount and recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value. Additionally, an entity should consider income tax effects from any tax-deductible goodwill on the carrying amount of the reporting unit when measuring the goodwill impairment loss, if applicable. The Company adopted this standard on January 1, 2020 and there was no material impact on its financial statement.

Fair Value Measurement Disclosure Requirements Modification

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820). The amendments in this ASU modify the disclosure requirements on fair value measurements in Topic 820, Fair Value Measurement. Various disclosure requirements have been removed, including the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, the policy for timing of transfers between levels, the valuation processes for Level 3 fair value measurements held at the end of the reporting period. The ASU also modified various disclosure requirements and added some disclosure requirements for Level 3 fair value measurements. The amendments in this ASU were effective for the Company on January 1, 2020 and there was no material impact on its financial statements upon adoption of this ASU.

Internal Use Software Guidance for Cloud Computing Arrangement

In August 2018, the FASB issued ASU No. 2018-15, Intangibles-Goodwill and Other- Internal-Use Software (Subtopic 350-40). The amendments in this ASU on the accounting for implementation, setup and other upfront costs (collectively “implementation costs”) apply to entities that are a customer in a hosting arrangement. The amendments under this ASU align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. Accordingly, the amendments in this ASU require an entity in a hosting arrangement that is a service contract to follow the guidance in Subtopic 350-40 to determine which implementation costs to expense. They also require an entity to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. The Company adopted this standard on January 1, 2020 and there was no material impact on its financial statement upon adoption of this ASU.

Collaborative Arrangements and Revenue Recognition

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the interaction between Topic 808 and Topic 606. The amendments in this ASU targeted improvements to generally accepted accounting principles for collaborative arrangements by clarifying that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In addition, unit-of-account guidance in Topic 808 was aligned with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606. The ASU was effective for the Company on January 1, 2020, and there was no material impact on its financial statements upon adoption of this ASU.

Share-Based Payment to Customer

In November 2019, the FASB issued ASU 2019-08, Compensation - Stock Compensation (Topic 718) and Revenue from Contracts with Customers (Topic 606): Codification Improvements – Share-Based Consideration Payable to a Customer. The amendments in this ASU require that an entity measure and classify share-based payment awards granted to a customer by applying the guidance in Topic 718. Under ASC 606, these awards are considered a reduction of the transaction price, unless the awards are payment for a distinct good or service received from the customer and should be recorded as a reduction of the transaction price. However, the ASU requires these awards to be measured on the basis of the grant-date fair value of the share-based payment award in accordance with Topic 718 and should be recognized at the later of when the award is promised and when the entity recognizes revenue for the transfer of the related goods or services in accordance with ASC 606. The ASU is effective for the Company on January 1, 2020, and there was no material impact on its financial statements upon adoption of this ASU.

Recent Accounting Pronouncements

In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. The amendments in this ASU simplify the accounting for income taxes by removing certain exceptions to the general principles of ASC 740 in order to reduce cost and complexity of its application. The ASU removes the exception related to the incremental approach for intraperiod tax allocation as well as two exceptions related to accounting for outside basis differences of equity method investments and foreign subsidiaries. The ASU also amends the scope of ASC 740 related to a franchise tax (or similar tax) that is partially based on income; clarifies when a step-up in the tax basis of goodwill should be considered part of the business combination in which the book goodwill was originally recognized and when it should be considered a separate transaction; specifies that an entity is not required to allocate income tax expense to a legal entity that is both not subject to tax and disregarded by the taxing authority; and clarifies that all tax effects, both deferred and current, should be accounted for in the interim period that includes the enactment date. The ASU is effective for the Company on January 1, 2021, and interim periods within those fiscal years. The Company does not expect the adoption of this ASU will have a material impact on its financial statements.

Net Loss Per Share

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Net Loss Per Share

4. Net Loss Per Share

Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding for the period. Diluted net loss per share is calculated using the weighted-average number of common shares outstanding, plus potential dilutive common stock during the period. Diluted net loss per share is the same as basic net loss per share since the effect of the potentially dilutive securities is anti-dilutive.

The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share for the periods presented, because including them would have been anti-dilutive:

	Year Ended December 31,
	2020		2019		2018
Options and ESPP to purchase common stock		11,388,691		9,687,844		7,478,755

Fair Value Measurements and Short-Term Investments

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Fair Value Measurements and Short-Term Investments

5. Fair Value Measurements and Short-Term Investments

In accordance with Accounting Standards Codification (“ASC”) 820-10, Fair Value Measurements and Disclosures, the Company determines the fair value of financial and non-financial assets and liabilities using the fair value hierarchy, which establishes three levels of inputs that may be used to measure fair value, as follows:

•

Level I: Inputs which include quoted prices in active markets for identical assets and liabilities.

•

Level II: Inputs other than Level I that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

•

Level III: Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

The carrying amounts of the Company’s financial instruments, including restricted cash, accounts receivable, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. The Company’s financial instruments consist of Level I assets which consist primarily of highly liquid money market funds, some of which are included in restricted cash and U.S. government bonds that are included in short-term investments.

The following tables set forth the fair value of the Company’s short-term investments subject to fair value measurements on a recurring basis and the level of inputs used in such measurements:

		December 31, 2020
	Valuation Hierarchy	Amortized Cost		Allowance for Credit Losses		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Aggregate Fair Value
		(in thousands)
Assets
Money market funds	Level I	$	131,121	$	—	$	—	$	—	$	131,121
Restricted cash (money market funds)	Level I		917		—		—		—		917
U.S. Government bonds	Level I		124,254		—		6		—		124,260
Total Securities		$	256,292	$	—	$	6	$	—	$	256,298

		December 31, 2019
	Valuation Hierarchy	Amortized Cost		Allowance for Credit Losses		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Aggregate Fair Value
		(in thousands)
Assets
Money market funds	Level I	$	170,757	$	—	$	—	$	—	$	170,757
Restricted cash (money market funds)	Level I		917		—		—		—		917
U.S. Government bonds	Level I		107,610		—		110		—		107,720
Total securities		$	279,284	$	—	$	110	$	—	$	279,394

No securities have contractual maturities of greater than twelve months.

Property Plant And Equipment [Abstract]

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment

12 Months Ended

Dec. 31, 2020

Property and Equipment

6. Property and Equipment

Property and equipment, net consisted of the following:

	December 31
	2020			2019
	(in thousands)
Machinery and equipment	$	13,772		$	12,124
Computer equipment and software		1,600			1,555
Furniture and fixtures		1,024			1,024
Leasehold improvements		1,728			1,483
Construction in progress		252			236
		18,376			16,422
Less: accumulated depreciation and amortization		(11,426	)		(9,050	)
	$	6,950		$	7,372

Depreciation and amortization expense was $2.4 million, $2.5 million and $1.7 million for the years ended December 31, 2020, 2019 and 2018, respectively.

Goodwill and Intangible Assets

12 Months Ended

Dec. 31, 2020

Goodwill And Intangible Assets Disclosure [Abstract]

Goodwill and Intangible Assets

7. Goodwill and Intangible Assets

Goodwill and in-process research and development assets resulted from a series of integrated financing transactions in 2010 that was accounted for as a business combination. The in-process research and development was related to the Company’s proprietary Probody platform and was accounted for as an indefinite-lived intangible asset until the underlying project was completed or abandoned. In connection with the collaboration agreements, the Company began amortizing the intangible asset in 2017. The intangible asset is being amortized over the estimated lives of the patents which average 12 years. The amortization expense for the years ended December 31, 2020, 2019 and 2018 was $0.1 million, $0.1 million and $0.1 million, respectively.

Goodwill and intangible assets consisted of the following:

Goodwill
	December 31,
	2020			2019
	(in thousands)
Goodwill	$	949		$	949

Intangible assets
	December 31,
	2020			2019
	(in thousands)
In-process research and development	$	1,750		$	1,750
Less accumulated amortization		(583	)		(438	)
	$	1,167		$	1,312

Accrued Liabilities

12 Months Ended

Dec. 31, 2020

Accrued Liabilities Current [Abstract]

Accrued Liabilities

8. Accrued Liabilities

Accrued liabilities consisted of the following:

	December 31,
	2020		2019
	(in thousands)
Research and clinical expenses	$	10,092	$	19,006
Payroll and related expenses		8,362		6,721
Legal and professional expenses		815		1,062
Operating lease liabilities - short term		3,195		2,810
Other accrued expenses		595		452
Total	$	23,059	$	30,051

Research and Collaboration Agreements

12 Months Ended

Dec. 31, 2020

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Research and Collaboration Agreements

9. Research and Collaboration Agreements

The following table summarizes the revenue by collaboration partners:

	Year Ended December 31,
	2020		2019		2018
	(in thousands)
AbbVie	$	38,192	$	5,878	$	18,997
Amgen		8,609		3,871		4,899
Astellas		13,931		—		—
Bristol Myers Squibb		39,630		47,740		32,780
ImmunoGen		—		—		1,471
Pfizer		—		—		1,355
Total revenue	$	100,362	$	57,489	$	59,502

AbbVie Ireland Unlimited Company

In April 2016, the Company and AbbVie entered into two agreements, a CD71 Co-Development and Licensing Agreement (the “CD71 Agreement”) and a Discovery Collaboration and Licensing Agreement (as amended and restated in June 2019, the “Discovery Agreement” and together with the CD71 Agreement the “AbbVie Agreements”). Under the terms of the CD71 Agreement, the Company and AbbVie will co-develop a conditionally activated antibody-drug conjugate (“ADC”) against CD71, with the Company responsible for preclinical and early clinical development. AbbVie will be responsible for later development and commercialization, with global late-stage development costs shared between the two companies. The Company will assume 35% of the net profits or net losses related to later development and commercialization unless it opts-out. If the Company opts-out from participation of co-development of the CD71 conditionally activated ADC, which includes CX-2029, AbbVie will have sole right and responsibility for the further development, manufacturing and commercialization of such CD71 conditionally activated ADC.

Under the CD71 Agreement, the Company received an upfront payment of $20.0 million in April 2016, and was eligible to initially receive up to $470.0 million in development, regulatory and commercial milestone payments, a 35% profit split on U.S. sales, and royalties on ex-U.S. sales at percentages in the high teens to low twenties if the Company participates in the co-development of the CD71 conditionally activated ADC subject to a reversion to a royalty on U.S. sales, and reduction in royalties on ex-U.S. sales, if the Company opts-out from the co-development of the CD71 conditionally activated ADC. The Company’s share of later stage co-development costs for each CD71 conditionally activated ADC is capped, provided that AbbVie may offset the Company’s co-development cost above the capped amounts from future payments such as milestone payments and royalties. In July 2017, the Company received a milestone payment of $14.0 million (net of payment of an associated sublicense fee of $1.0 million to SGEN under the Seattle Genetics Agreement) from AbbVie for achieving certain milestones required to be met to begin GLP toxicology studies under the CD71 Agreement. In May 2018, the United States Food and Drug Administration (“FDA”) cleared the IND application for CX-2029. As a result, the Company achieved the IND success criteria under the CD71 Agreement and received a $21.0 million milestone payment (net of the payment of an associated sublicense fee of $4.0 million to SGEN). In March 2020, the Company earned a $40.0 million milestone payment for satisfying the CD71 dose escalation success criteria under the CD71 Agreement

Under the terms of the Discovery Agreement, AbbVie receives exclusive worldwide rights to develop and commercialize conditionally activated ADCs against up to two targets, one of which was selected in March 2017. The Company shall perform research services to discover the Probody therapeutics and create conditionally activated ADCs for the nominated collaboration targets. From that point, AbbVie shall have sole right and responsibility for development and commercialization of products comprising or containing such conditionally activated ADCs (“Discovery Licensed Products”).

Under the Discovery Agreement, the Company received an upfront payment of $10.0 million in April 2016 and subsequently earned an additional $10.0 million milestone payment triggered by selection of the second target by AbbVie in June 2019. The Company is also eligible to receive up to $265.0 million for each target, in development, regulatory and commercial milestone payments and royalties at percentages in the high single to low teens from commercial sales of any resulting conditionally activated ADCs. The second target was selected under the Discovery Agreement that allows AbbVie to select a target for developing a conditionally activated ADC or a Probody.

The Company has determined that the AbbVie Agreements should be combined and evaluated as a single arrangement in determining revenue recognition, because both agreements were concurrently negotiated and executed.

The Company identified the following performance obligations at the inception of the AbbVie Agreements:

(1)

the research, development and commercialization license for CD71 Probody therapeutic,

(2)

the research services related to CD71 Probody therapeutic,

(3)

the obligation to participate in the CD71 Agreement joint research committee,

(4)

the research services related to the first discovery target

(5)

the research, development and commercialization license for the first discovery target, and

(6)

the obligation to participate in the Discovery Agreement joint research committee.

The Company concluded that AbbVie’s option for the second discovery target was not a material right and was therefore not a performance obligation at the inception of the AbbVie Agreements. However, it was subsequently included in the total transaction price in June 2019 as a performance obligation upon AbbVie’s selection of such second target as further discussed below.

The Company determined that the research, development and commercialization licenses for CD71 and discovery targets are not distinct from the Company’s respective research services and expertise. The Company considered factors such as novelty of the Probody therapeutic and conditionally activated ADC technology and lack of other parties’ expertise in this space, the Company’s rights to technology relating to a proprietary platform to enable the Probody therapeutic development and AbbVie’s contractual obligation to use the Company’s research services. The Company determined that the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee were a combined performance obligation and were distinct from the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee. Therefore, the Company concluded that there are two distinct performance obligations:

(1)

the CD71 Agreement performance obligation consisting of the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee, and

(2)

the Discovery Agreement performance obligation consisting of the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee.

The total transaction price for the Discovery Agreement and CD71 Agreement, collectively, upon adoption of ASC 606 on January 1, 2018 of $39.8 million consists of $30.0 million in upfront payments, and a $14.0 million milestone payment received under the CD71 Agreement (net of the payment of an associated sublicense fee of $1.0 million to SGEN), less $4.2 million of estimated sublicense fees. The upfront payments under the AbbVie Agreements are allocated between the two performance obligations based on the estimated relative standalone selling prices. The $30.0 million of upfront payments is allocated $20.0 million to the CD71 Agreement, with the remaining $10.0 million allocated to the Discovery Agreement. The $14.0 million milestone payment received (net of the payment of an associated sublicense fee of $1.0 million to SGEN) and the estimated sublicense fees of $4.2 million are allocated to the CD71 Agreement performance obligation as they are directly related to the development of the CX-2029.

Therefore, of the $39.8 million total initial transaction price discussed above, the Company allocated $29.8 million to the CD71 Agreement performance obligation and recognized revenue using a cost-based input measure, the common measure of progress for the performance obligation. In applying the cost-based input method, revenue is recognized based on actual full-time employee (“FTE”) hours incurred as a percentage of total estimated FTE hours for completing its combined performance obligation over the estimated service period. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. During 2019, as a result of ongoing dose escalation in the continued development program, there was a change in estimate of the research service period as well as an increase in the projected FTE hours-to-completion. The research service period for the CD71 Agreement performance obligation was extended from April 2021 to March 2022 in 2019. During the second quarter of 2020, the Company further increased the projected FTE hours-to-completion and extended the research service period for the CD71 Agreement performance obligation from March 2022 to September 2022 in response to the reduced rate of operation as impacted by the COVID-19 pandemic.

The remaining $10.0 million of the total initial transaction price of $39.8 million allocated to the Discovery Agreement performance obligation represents an obligation to continuously make the Company’s Probody therapeutic technology platform available to AbbVie. The $10.0 million was initially recognized on a straight-line basis over five years, which represented the estimated research service period for the first target, through April 2021 using the time-elapsed input method as the common measure of progress over the entire performance obligation. During the fourth quarter of 2020, the Company extended the estimated research service period for this first target for an additional twelve months from April 2021 to April 2022.

In May 2018, the Company earned a $21.0 million milestone payment (net of the payment of an associated sublicense fee of $4.0 million to SGEN) under the CD71 Agreement. The $21.0 million milestone payment was included as part of the transaction price in May 2018 and a revenue adjustment of $9.9 million was recognized in the second quarter of 2018 reflecting the percentage completed to-date on the project related to this milestone.

In June 2019, the Company earned a $10.0 million milestone payment for the second target selected by AbbVie under the Discovery Agreement. It is recognized also using the time-elapse measure of progress of the related obligation and straight line over the estimated research service period of five years through June 2024.

The $40.0 million milestone payment earned in March 2020 for satisfying the CD71 dose escalation success criteria under the CD71 Agreement was included as part of the transaction price as it was unconstrained during the first quarter of 2020 and $26.6 million was recognized as revenue related to this milestone reflecting the percentage completed to-date on the project.

The Company determined that the remaining potential milestone payments of both agreements are probable of significant revenue reversal as their achievement is highly dependent on factors outside the Company’s control. Therefore, these payments continued to be fully constrained and were not included in the transaction price as of December 31, 2020.

The Company recognized revenue of $38.2 million, $5.9 million and $19.0 million for the years ended December 31, 2020, 2019 and 2018, respectively, related to the AbbVie Agreements. As of December 31, 2020 and 2019, deferred revenue related to the CD71 Agreement performance obligation was $25.2 million and $20.0 million, respectively, and deferred revenue related to the Discovery Agreement performance obligation was $8.0 million and $11.6 million, respectively. No amounts were due from AbbVie as of December 31, 2020 and 2019.

Amgen, Inc.

On September 29, 2017, the Company and Amgen, Inc. (“Amgen”) entered into a Collaboration and License Agreement (the “Amgen Agreement”). Pursuant to the Amgen Agreement, the Company received an upfront payment of $40.0 million in October 2017. Concurrent with the entry into the Amgen Agreement, the Company and Amgen entered into a Share Purchase Agreement (the “Purchase Agreement”) pursuant to which Amgen purchased 1,156,069 shares of the Company’s common stock at a price of $17.30 per share (calculated based on a 20-day volume-weighted average price), for total proceeds of $20.0 million, which the Company received on October 6, 2017, the closing date of the transaction. The Company estimated a premium on the stock sold to Amgen of $0.5 million, which takes into account a discount due to the lack of marketability resulting from the six-month lockup period.

Under the terms of the Amgen Agreement, the Company and Amgen will co-develop a conditionally activated T-cell engaging bi-specific therapeutic targeting epidermal growth factor receptor (the “EGFR Products”). The Company is responsible for early-stage development of EGFR Products and all related costs up to certain pre-set costs and certain limits based on clinical trial size. Amgen will be responsible for late-stage development, commercialization, and all related costs of EGFR Products. Following early-stage development, the Company will have the right to elect to participate financially in the global co-development of EGFR Products with Amgen, during which the Company would bear certain of the worldwide development costs for EGFR Products and Amgen would bear the rest of such costs (the “EGFR Co-Development Option”). If the Company exercises its EGFR Co-Development Option, the Company will share in somewhat less than 50% of the profit and losses from sales of such EGFR Products in the U.S., subject to certain caps, offsets, and deferrals. If the Company chooses not to exercise its EGFR Co-Development Option, the Company will not bear any costs of later stage development. The Company is eligible to receive up to $455.0 million in development, regulatory, and commercial milestone payments for EGFR Products, and royalties in the low-double-digit to mid-teen percentage of worldwide commercial sales, provided that if the Company exercises its EGFR Co-Development option, it shall receive a profit and loss split of sales in the United States and royalties in the low-double-digit to mid-teen percentage of commercial sales outside of the United States.

Amgen also has the right to select a total of up to three targets, including the two additional targets discussed below. The Company and Amgen collaborate in the research and development of conditionally activated T-cell engaging bi-specifics products directed against such targets. Amgen has selected one such target (the “Amgen Other Product”). If Amgen exercises its option within the specified period of time, it can select two such additional targets (the “Amgen Option Products” and, together with the Amgen Other Product, the “Amgen Products”). Except with respect to preclinical activities to be conducted by CytomX, Amgen will be responsible, at its expense, for the development, manufacture, and commercialization of all Amgen Products. If Amgen exercises all of its options and advances all three of the Amgen Products, CytomX is eligible to receive up to $950.0 million in upfront, development, regulatory, and commercial milestones and tiered high single-digit to low-teen percentage royalties. The Company concluded that, at the inception of the agreement, Amgen’s option to select the two additional targets is not a material right and does not represent a performance obligation of the agreement.

At the initiation of the collaboration, CytomX had the option to select, from programs specified in the Amgen Agreement, an existing pre-clinical stage T-cell engaging bispecific product from the Amgen pre-clinical pipeline. In March 2018, CytomX selected the program. CytomX is responsible, at its expense, for converting this program to a conditionally activated T-cell engaging bispecific product, and thereafter, will be responsible for development, manufacturing, and commercialization of the product (“CytomX Product”). Amgen is eligible to receive up to $203.0 million in development, regulatory, and commercial milestone payments for the CytomX Product, and tiered mid-single digit to low double-digit percentage royalties.

The Company considered the criteria for combining contracts in ASC 606 and determined that the Amgen Agreement and the Purchase Agreement should be combined into one contract. The Company accounted for the Amgen Agreement based on the fair values of the assets and services exchanged. The Company identified the following performance obligations at the inception of the Amgen Agreement:

(1)

the research, development and commercialization license,

(2)

the research and development services for the EGFR Products and the Amgen Other Product, and

(3)

the obligation to participate in the joint steering committee (“JSC”) and the joint research committee (“JRC”).

The Company determined that research, development and commercialization license and the participation in the JSC and JRC are not distinct from the research and development services and therefore those performance obligations were combined into one combined performance obligation. The Amgen Other Products are accounted for as a separate performance obligation from the EGFR Products as the nature of the services being performed is not the same and the value that Amgen can derive from one program is not dependent on the success of the other. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

Concurrent with the execution of the Amgen Agreement, the Company entered into a sublicense agreement whereby the Company granted Amgen a sublicense of its rights to one patent family that it co-owns with UCSB, that is exclusively licensed to the Company under the UCSB Agreement covering Probody antibodies and other pro-proteins in the fields of therapeutics, in vivo diagnostics and prophylactics. This sublicense was incremental to the patents, patent applications and know-how covering conditionally activated T-cell engaging bispecific molecules that were developed and owned by the Company and licensed to Amgen. Under the UCSB Agreement, as amended, the Company is obligated to make a sublicense payment to UCSB equal to up to 7.5% of certain upfront and milestone payments owed to or received by the Company. There were no sublicense fees payable to UCSB as of December 31, 2020 and 2019.

The total transaction price of $51.2 million, consisting of the $40.0 million upfront payment, an estimated fair value of $10.7 million for the CytomX Product and $0.5 million of premium on the sale of the Company’s common stock, was allocated between the two performance obligations based on the relative standalone selling price of each performance obligation. To determine the standalone selling price, the Company used the discounted cash flow method by calculating risk-adjusted net present values of estimated cash flows. The Company determined that the remaining potential milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company’s control. As a result, these payments were fully constrained and were not included in the transaction price as of January 1, 2018, the adoption date of ASC 606.

Of the $51.2 million total transaction price, the Company allocated $46.4 million to the EGFR Products performance obligation and $4.8 million to the Amgen Other Product performance obligations. The transaction price of the EGFR Product performance obligation was recognized using a cost-based input measure. In applying the cost-based input method of revenue recognition, the Company uses actual FTE hours incurred relative to estimated total FTE hours expected to be incurred for the combined performance obligation over the research service period. At the end of the second quarter of 2019, the Company determined that it will undertake additional testing and assessment of the molecules being evaluated under the EGFR project. As a result, the estimated FTE hours-to-completion and research service period were increased to eight years. In the second quarter of 2020, the Company completed the clinical candidate

characterization phase and has moved into the IND-enabling phase earlier than planned. As a result, the estimated FTE hours-to-completion and research service period were decreased from eight to seven years.

The $4.8 million transaction price allocated to the Amgen Other Product performance obligation represents an obligation to continuously make the Probody therapeutic technology platform available to Amgen, which is recognized over the common measure of progress for the entire performance obligation over the estimated research service period of six years.

The Company recognized revenue of $8.6 million, $3.9 million and $4.9 million for the years ended December 31, 2020, 2019 and 2018, respectively, related to the Amgen Agreement. As of December 31, 2020 and 2019, deferred revenue related to the EGFR Products performance obligation was $29.8 million and $37.6 million, respectively. As of December 31, 2020 and 2019, deferred revenue related to the Amgen Other Products performance obligation was $2.2 million and $3.0 million, respectively. No amounts were due from Amgen as of December 31, 2020 and 2019.

Astellas Pharma Inc.

The Company and Astellas Pharma, Inc. (“Astellas”) entered into a Collaboration and License Agreement (the “Astellas Agreement”) on March 23, 2020, the effective date, to collaborate on preclinical research activities to discover and develop certain antibody compounds for the treatment of cancer using the Company’s Probody therapeutic technology.

Under the terms of the Astellas Agreement, the Company granted Astellas an exclusive, worldwide, rights to develop and commercialize Probody therapeutics for up to four collaboration targets including one initial target and three additional targets (“Additional Targets”). In addition, Astellas has the right to expand the number of Additional Targets from three up to five (the “Expansion Option”) before the third anniversary of the effective date. Furthermore, for a specified number of targets, at a pre-specified time prior to the initiation of the first pivotal study of a product against such target, the Company may elect to participate in certain development costs and share in the profits generated in the United States with respect to such product (“Cost Share Option”). The Cost Share Option, if exercised, will also provide the option for the Company to co-commercialize such product in the United States. The Company does not consider the Cost Share Option as a performance obligation at the inception of the agreement as the participation is at the Company’s discretion.

Pursuant to the Astellas Agreement, the consideration from Astellas is comprised of an upfront fee of $80.0 million and contingent payments for development, regulatory and sales milestones of up to an aggregate of approximately $1.6 billion. If Astellas exercises its Expansion Option for the two Additional Targets, the Company would be eligible to receive additional upfront and milestone payments aggregating to approximately $0.9 billion. The Company is also entitled to tiered royalties from high-single digit to mid-teen percentage royalties from potential future sales. Astellas is responsible for all preclinical research costs incurred by either party as set forth in the preclinical research plan and the Company will receive research and development service fees based on a prescribed FTE rate.

The Company identified the following performance obligations at the inception of the Astellas Agreement:

(1)

the exclusive research, development and commercialization license;

(2)

the research and development services; and

(3)

the obligation to participate in the joint research committee.

The Company determined that the license, the research services and expertise related to the development of the product candidates should be combined with the research services and participation in the joint research committee as one combined performance obligation. The Company concluded, that at the inception of the agreement, Astellas’ Expansion Option for two Additional Targets were not material rights and therefore not considered performance obligations. As such, each option will be accounted for as a separate arrangement upon exercise.

The initial transaction price of $90.0 million consists of the upfront fee of $80.0 million and research and development service fees of $10.0 million. The Company determined that the potential development and regulatory milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company’s control. Therefore, the potential development and regulatory milestone payments were fully constrained and were not included in the initial transaction price and continued to be fully constrained as of December 31, 2020. The Company will re-evaluate the transaction price at each reporting date or as uncertain events are resolved or other changes in circumstances occur.

The upfront fee of $80.0 million for the combined obligation to continuously make the Probody therapeutic technology platform available to Astellas is recognized on a straight-line basis for the entire performance obligation over the estimated research service period of five years, which ends in March 2025. The research and development service fees, estimated to be $10.0 million, will be recognized when services are provided based on the prescribed FTE rate.

The Company recognized revenue of $13.9 million for the year ended December 31, 2020, which included the research and development service fee of $1.5 million. As of December 31, 2020, deferred revenue relating to the Astellas Agreement was $67.6 million. The amount due from Astellas under the Astellas Agreement was $0.8 million as of December 31, 2020.

Bristol Myers Squibb Company

On May 23, 2014, the Company and Bristol Myers Squibb Company (“Bristol Myers Squibb”) entered into a Collaboration and License Agreement (the “BMS Agreement”) to discover and develop compounds for use in human therapeutics aimed at multiple immuno-oncology targets using the Company’s Probody therapeutic technology. The effective date of the BMS Agreement was July 7, 2014.

Under the terms of the BMS Agreement, the Company granted Bristol Myers Squibb exclusive worldwide rights to develop and commercialize Probody therapeutics for up to four oncology targets. Bristol Myers Squibb had additional rights to substitute up to two collaboration targets within three years of the effective date of the BMS Agreement. These rights expired in May 2017. Each collaboration target had a two-year research term and the two additional targets had to be nominated by Bristol Myers Squibb within five years of the effective date of the BMS Agreement. The research term for each collaboration target could be extended in one year increments up to three times.

Pursuant to the BMS Agreement, the financial consideration from Bristol Myers Squibb was comprised of an upfront payment of $50.0 million, and the Company was initially entitled to receive contingent payments of up to $25.0 million for additional targets and up to an aggregate of $1,192.0 million for development, regulatory and sales milestones. In addition, the Company is entitled to royalty payments in the mid-single digits to low double-digit percentages from potential future sales. The Company also receives research and development service fees based on a prescribed FTE rate that is capped.

The Company identified the following performance obligations at the inception of the BMS Agreement:

(1)

the exclusive research, development and commercialization license;

(2)

the research and development services; and

(3)

the obligation to participate in the joint research committee.

The Company determined that the license, the Company’s research services and expertise related to the development of the product candidates should be combined with the research services and participation in the joint research committee as one combined performance obligation. The Company concluded that, at the inception of the agreement, Bristol Myers Squibb’s options for the third and fourth targets were not material rights and not performance obligations. As such, each option was accounted for as a separate arrangement upon exercise. Additionally, the Company considered whether the services performed for each target should be considered as separate performance obligations and concluded that all targets should be accounted for as one combined performance obligation.

The Company received an upfront payment of $50.0 million from Bristol Myers Squibb in July 2014. In January and December 2016, Bristol Myers Squibb selected the third and fourth targets, respectively, and paid the Company $10.0 million and $15.0 million, respectively, pursuant to the terms of the BMS Agreement. In December 2016, Bristol Myers Squibb selected a clinical candidate pursuant to the BMS Agreement, which triggered a $2.0 million pre-clinical milestone payment to the Company. In November 2017, the Company recognized a $10.0 million milestone payment from Bristol Myers Squibb upon approval of the investigational new drug application for the CTLA-4-directed Probody therapeutic.

On March 17, 2017, the Company and Bristol Myers Squibb entered into Amendment Number 1 to Extend Collaboration and License Agreement (“Amendment 1”). Amendment 1 granted Bristol Myers Squibb exclusive worldwide rights to develop and commercialize Probody therapeutics for up to eight additional targets. The effective date of Amendment 1 was April 25, 2017 (“Amendment Effective Date”). Under the terms of Amendment 1, the Company continued to have obligations to Bristol Myers Squibb to discover and conduct preclinical development of Probody therapeutics against any targets they chose to select during the research period under the terms of Amendment 1.

Pursuant to Amendment 1, the financial consideration from Bristol Myers Squibb was comprised of an upfront payment of $200.0 million and the Company was initially eligible to receive contingent payments for development, regulatory and sales milestones of up to an aggregate of $3,586.0 million for the eight targets. The Company was also entitled to tiered mid-single to low double-digit percentage royalties from potential future sales. Amendment 1 did not change the term of the Bristol Myers Squibb’s royalty obligation under the BMS Agreement. Bristol Myers Squibb’s royalty obligation continues on a licensed-product by licensed-product basis until the later of (i) the expiration of the last claim of the licensed patents covering the licensed products in the country, (ii) the twelfth anniversary of the first commercial sale of a licensed product in a country, or (iii) the expiration of any applicable regulatory, pediatric, orphan drug or data exclusivity with respect to such product.

The initial transaction price for the BMS Agreement and Amendment 1, collectively, was $272.8 million consisting of the upfront fees of $250.0 million, research and development service fees of $10.8 million and milestone payments received to date of $12.0 million. The Company determined that the remaining potential milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company’s control. Therefore, these payments were fully constrained and were not included in the transaction price upon the adoption of ASC 606 on January 1, 2018. The BMS Agreement represents an obligation to continuously make the Probody therapeutic technology platform available to Bristol Myers Squibb. Therefore, the initial transaction price is recognized over the estimated research service period, which ends on April 25, 2025.

During the first quarter of 2019, Bristol Myers Squibb terminated pre-clinical activities on three of the first four collaboration targets selected under the original 2014 BMS Agreement. The first and second targets under the BMS Agreement were combined into a single performance obligation. The Company determined that termination of pre-clinical activities on the second target does not impact the Company’s continuing obligation to Bristol Myers Squibb for the first target, CTLA-4, as it is still being actively developed by Bristol Myers Squibb. Therefore, the Company concluded that it will continue to amortize the related deferred revenue over the original performance period. The Company has determined that upon the termination of pre-clinical activities on the third and the fourth collaboration targets selected by Bristol Myers Squibb in January and December of 2016, respectively, under the BMS Agreement, it has no further obligations. As a result of termination of three of the four collaboration targets, the Company is no longer eligible to receive up to an aggregate of $894.0 million contingent payments for development, regulatory and sales milestones as well as the related royalty payments for such targets. As a result, the Company recognized $18.1 million of revenue for the third and fourth targets in the first quarter of 2019, of which $17.4 million represented the accelerated recognition of all of the related deferred revenue upon the effective date of termination. The Company continues to be obligated to perform research work under Amendment 1 executed in March 2017.

In February 2020, Bristol Myers Squibb dosed the first patient in the Part 2 cohort expansion portion of its ongoing BMS-986249 clinical study for the CTLA-4 program, which triggered a $10.0 million milestone payment to the Company pursuant to the terms of the BMS Agreement. The $10.0 million milestone payment was recognized as revenue in the first quarter of 2020. The Company reevaluated the remaining potential milestone payments and determined that significant revenue reversal was still probable as the achievement of such milestones was highly dependent on factors outside the Company’s control. As a result, these payments continued to be fully constrained and were not included in the transaction price on December 31, 2020.

In February 2021, the Company and Bristol Myers Squibb entered into Amendment Number 2 to amend the Collaboration and License Agreement (“Amendment 2”), as amended by Amendment 1. Subsequent to Amendment 2, Bristol Myers Squibb has the exclusive worldwide rights to develop and commercialize Probody therapeutics for up to five oncology targets. Under the terms of Amendment 2, the period for target selection has been extended and the Company will continue to collaborate with Bristol Myers Squibb to discover and conduct preclinical development of Probody therapeutics against targets selected by Bristol Myers Squibb. Pursuant to Amendment 2, the Company is eligible to receive contingent payments for development, regulatory and sales milestones of up to an aggregate of $1,779.0 million. It is also entitled to tiered mid-single to low double-digit percentage of royalties from potential future sales.

The Company recognized revenue of $39.6 million, $47.7 million and $32.8 million for the years ended December 31, 2020, 2019, and 2018, respectively. As of December 31, 2020 and 2019, deferred revenue relating to the BMS Agreement was $128.3 million and $158.0 million, respectively. The amounts due from Bristol Myers Squibb under the BMS Agreement were $0 and $13,000 as of December 31, 2020 and 2019, respectively.

ImmunoGen, Inc.

In January 2014, the Company and ImmunoGen, Inc. (“ImmunoGen”) entered into the Research Collaboration Agreement (the “ImmunoGen Research Agreement”). The ImmunoGen Research Agreement provided the Company with the right to use ImmunoGen’s Antibody Drug Conjugate (“ADC”) technology in combination with the Company’s Probody therapeutic technology to create a conditionally activated ADC directed at one specified target under a research license, and to subsequently obtain an exclusive,

worldwide development and commercialization license to use ImmunoGen’s ADC technology to develop and commercialize such conditionally activated ADCs. The Company made no upfront cash payment in connection with the execution of the agreement. Instead, the Company provided ImmunoGen with the rights to CytomX’s Probody therapeutic technology to create conditionally activated ADCs directed at two targets under the ImmunoGen Research Agreement and to subsequently obtain exclusive, worldwide development and commercialization licenses to develop and commercialize such conditionally activated ADCs. In February 2016, the Company exercised its option to obtain a development and commercialization license for praluzatamab ravtansine (CX-2009) pursuant to the terms of the ImmunoGen Research Agreement (the “CX-2009 License”).

In February 2017, ImmunoGen exercised its first option to obtain a development and commercialization license for one of the two targets. Substitution rights for this first target selection program terminated in February 2017 and ImmunoGen discontinued the program in July 2017. The Company recognized the remaining deferred revenue related to the discontinued program upon the termination of the program. ImmunoGen exercised its second option to obtain a development and commercialization license pursuant to the ImmunoGen Research Agreement (the “ImmunoGen 2017 License”) for a target in December 2017. In December 2019, the parties entered into a license agreement (the “ImmunoGen 2019 License”) pursuant to which the ImmunoGen 2017 License was terminated and ImmunoGen granted a license for all of ImmunoGen’s rights under the ImmunoGen 2017 License to the Company. See Note 10. License Agreement, for more information.

Under the terms of the ImmunoGen Research Agreement, both the Company and ImmunoGen performed research activities on behalf of the other party for no monetary consideration through January 2018. In December 2017, the Company entered into the ImmunoGen 2017 License arrangement and extended the Company’s obligation to provide research services under the ImmunoGen Research Agreement to June 30, 2018. The estimated fair value of the consideration of $13.2 million for the performance obligation to ImmunoGen was recognized as revenue over the research period that ended on June 30, 2018. No further research services were provided by the Company after June 20, 2018 under the ImmunoGen 2017 License arrangement. The Company recognized revenue of $1.5 million for the year ended December 31, 2018.

In February 2020, the Company dosed the first patient in the praluzatamab ravtansine Phase 2 clinical trial and triggered a $3.0 million milestone payment to ImmunoGen pursuant to the CX-2009 License which continued to remain in effect following the termination of the ImmunoGen 2017 License in December 2019. The Company recorded a $3.0 million charge to research and development expense in the first quarter of 2020, in connection with this milestone payment to ImmunoGen.

Pfizer Inc.

In May 2013, the Company and Pfizer Inc. (“Pfizer”) entered into a Research Collaboration, Option and License Agreement (the “Pfizer Agreement”) to collaborate on the discovery and preclinical research activities related to Probody therapeutics, and conditionally activated ADCs for research project targets nominated by Pfizer. Pfizer nominated two research targets in 2013 and, pursuant to the Pfizer Agreement, had the option of nominating two additional research targets. In December 2014, Pfizer selected an additional research target and paid the Company $1.5 million. The option to select a fourth target lapsed in May 2016. Pfizer discontinued the epidermal growth factor receptor (“EGFR”) program and decided to terminate the remaining two targets in February and March 2018. In March 2018, Pfizer terminated the Pfizer Agreement. As such, the Company had no further performance obligations under this agreement after the first quarter of 2018. The Company recognized revenue of $1.4 million for the year ended December 31, 2018.

Contract Liabilities

The following table presents changes in the Company’s total contract liabilities for the year ended December 31, 2020 (in thousands):

	Balance at 12/31/2019		Additions		Deductions		Balance at 12/31/2020
	(in thousands)
Contract liabilities:
Deferred revenue	$	230,239	$	120,000	$	89,109	$	261,130

There were $120.0 million of additions to deferred revenue during the year ended December 31,2020. Of such amount, $40.0 million was related to the milestone payment triggered by AbbVie’s CD71 dose escalation success criteria which was achieved in March 2020, and an $80.0 million addition was related to the upfront fee payable under the Astellas Agreement entered into in March 2020. Deductions of $89.1 million related to revenue recognized included in the contract liability balance at the beginning of the period plus the revenue recognized related to the $120.0 million additions during the year ended December 31, 2020.

The Company expects that the $261.1 million of deferred revenue related to the following contracts as of December 31, 2020 will be recognized as revenue as set forth below. However, the timing of revenue recognition could differ from the estimates depending on facts and circumstances impacting the various contracts, including progress of research and development, resources assigned to the contracts by the Company or its collaboration partners or other factors outside of the Company’s control.

•

The $25.2 million of deferred revenue related to the CD71 Agreement as of December 31, 2020 is expected to be recognized based on actual FTE effort and program progress until approximately September 2022.

•

The $1.0 million of deferred revenue related to the first target under the Discovery Agreement as of December 31, 2020 is expected to be recognized ratably until approximately April 2022.

•

The $7.0 million of deferred revenue related to the second target under the Discovery Agreement as of December 31, 2020 is expected to be recognized ratably until approximately June 2024.

•

The $29.8 million of deferred revenue related to the Amgen EGFR Products as of December 31, 2020 is expected to be recognized based on actual FTE effort and program progress until approximately September 2024.

•

The $2.2 million of deferred revenue related to the Amgen Other Products as of December 31, 2020 is expected to be recognized ratably until approximately September 2023.

•

The $67.6 million of deferred revenue related to the Astellas Agreement as of December 31, 2020 is expected to be recognized ratably until approximately March 2025.

•

The $128.3 million of deferred revenue related to the BMS Agreement as of December 31, 2020 is expected to be recognized ratably until approximately April 2025.

License Agreement

12 Months Ended

Dec. 31, 2020

Research And Development [Abstract]

License Agreement

10. License Agreement

UCSB

The Company has an exclusive, worldwide license agreement with UCSB (the “UCSB Agreement”), relating to the use of certain patents and technology relating to its core technology, including its therapeutic antibodies, and to certain patent rights the Company co-owns with UCSB covering Probody antibodies and other pro-proteins.

Pursuant to the UCSB Agreement, the Company is obligated to (i) make royalty payments to UCSB on net sales of its products covered under the agreement, subject to annual minimum amounts, (ii) make milestone payments to UCSB upon the occurrence of certain events, (iii) make a milestone payment to UCSB upon occurrence of an IPO or change of control, and (iv) reimburse UCSB for prosecution and maintenance of the licensed patents. If the Company sublicenses its rights under the UCSB Agreement, it is obligated to pay UCSB a percentage of the total sublicense revenue received, which total amount would be first reduced by the aggregate amount of certain research and development related expenses incurred by the Company and other permitted deductions. As part of the UCSB Agreement, the Company has annual minimum royalty obligations of $0.2 million under the terms of certain exclusive licensed patent rights. The royalty obligations are cancellable any time by giving notice to the licensor, with the termination being effective 60 days after giving notice.

In 2013, the Company amended the UCSB Agreement to reduce certain amounts due to UCSB upon receipt by the Company of upfront payments, milestone payments and royalties from sublicensees. In exchange for this amendment, the Company issued to UCSB 157,332 shares of common stock. The UCSB Agreement, as amended, will remain in effect until the expiration or abandonment of the last to expire of the licensed patents.

In April 2019, the Company entered into Amendment No.3 to the UCSB Agreement to adjust and clarify certain sublicense terms (“Amendment No.3”). In connection with the amendment, the Company issued to UCSB 150,000 shares of CytomX common stock with a fair value of $10.68 per share. Under the terms of Amendment No.3, the Company and UCSB agreed to modify the determination of sublicense revenues payable by the Company to UCSB on certain existing collaboration agreements and on collaboration agreements executed subsequent to Amendment No.3. In exchange, the Company agreed to make an upfront payment of $1.0 million as well as additional annual license maintenance fees of $0.8 million through 2031. In the event that the Company terminates the agreement due to material concern of the safety or efficacy of the related technology, 50% of all remaining maintenance fees will become due immediately. Otherwise, all remaining maintenance fees will become due immediately upon early termination of the agreement unless there is a material breach by UCSB. Pursuant to Amendment No.3, the Company recorded in research and development expense a charge of $3.4 million relating to sublicense and maintenance fees representing the 150,000 shares issued with a fair value of $1.6 million, the upfront payment of $1.0 million and the additional annual maintenance fee of $0.8 million during the second quarter of 2019.

In June 2019, the Company incurred an additional $0.8 million of sublicense fees related to the $10.0 million milestone payment for the second target selected by AbbVie under the Discovery Agreement.

In February 2020, the Company recorded $0.8 million of sublicense fees triggered by the $10.0 million milestone payment from Bristol Myers Squibb’s dosing of the first patient in the Part 2 cohort expansion portion of its ongoing BMS-986249 clinical study for the CTLA-4 program. In March 2020, the Company incurred additional sublicense fees of $6.0 million and $1.4 million related to the $80.0 million upfront fee received pursuant to the Astellas Agreement entered into in March 2020, and the $40.0 million milestone payment from AbbVie for satisfying the CD71 dose escalation success criteria under the CD71 Agreement, respectively.

During the years ended December 31, 2020, 2019 and 2018, the Company incurred sublicense expenses of $9.1 million, $4.3 million and $0.6 million, respectively, under the provisions of the UCSB Agreement. As of December 31, 2020 and 2019, there was $0 and $0.2 million sublicense fee payable to UCSB, respectively.

ImmunoGen

In December 2019, the Company entered into a License Agreement (the “ImmunoGen 2019 License”) with ImmunoGen, Inc. to obtain an exclusive license with respect to epithelial cell adhesion molecule (“EPCAM”). Under the ImmunoGen 2019 License, ImmunoGen agreed to transfer its know-how, patents, intellectual property rights, and technology transfer materials and information related to its EpCAM program. The license gives the Company the sole ability to develop, manufacture, use and commercialize any licensed product that incorporates, is comprised of, or otherwise derived from a Probody that targets EpCAM in any human therapeutic field on a worldwide basis. In exchange, the Company agreed to make non-refundable and non-creditable payments including an upfront license payment of $7.5 million and certain clinical development, approval and commercialization milestone payments, if achieved and royalties on product sales.

The upfront license fee of $7.5 million was recorded as research and development expense in December 2019 and included in accrued liabilities as of December 31, 2019. The upfront license fee was paid in January 2020.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

Legal Proceedings

On March 4, 2020, Vytacera Bio, LLC filed a patent infringement lawsuit against the Company in the U.S. District Court for the District of Delaware. The lawsuit alleges that the Company’s use, offers to sell, and/or sales of the Probody® technology platform for basic research applications constitutes infringement. The complaint seeks unspecified monetary damages. The Company filed an Answer, Affirmative Defenses, and Counterclaims on May 26, 2020. Vytacera Bio, LLC filed its Answer to CytomX Therapeutics Inc.’s Counterclaims on June 5, 2020. The parties have agreed to a case schedule, which is pending Court approval. Discovery is in the initial phases. The Company believes that the lawsuit is without merit and intends to vigorously defend itself and has not recorded any amount for claims associated with this lawsuit as of December 31, 2020.

On May 21, 2020, a putative securities class action lawsuit was commenced in the U.S. District Court for the Northern District of California naming as defendants the Company and three current and former officers. The complaint alleges that the defendants violated Sections 10(b) and 20(a) of the Securities Exchange Act of 1934, as amended, and Rule 10b-5 thereunder, by making false and misleading statements and omissions of material fact related to the product candidates praluzatamab ravtansine (CX-2009) and pacmilimab (CX-072). The plaintiff sought to represent all persons who purchased or otherwise acquired CytomX securities between May 17, 2018, and May 13, 2020. On January 14, 2021, the class action was dismissed without prejudice.

Indemnifications

In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has also entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company currently has directors’ and officers’ insurance.

Leases

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Leases

12. Leases

Operating Lease

On December 10, 2015, the Company entered into a lease (the “2016 Lease”) with HCP Oyster Point III LLC (the “Landlord”) to lease approximately 76,000 rentable square feet of office and laboratory space located in South San Francisco, California for the Company’s new corporate headquarters.

The term of the Lease commenced on October 1, 2016. The 2016 Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the 2016 Lease.

The Lease provided for annual base rent of approximately $3.1 million in the first year of the lease term. The annual base rent for the second twelve months was approximately $4.3 million, which will increase on an annual basis beginning from the 25^th month to approximately $5.5 million for the tenth year of the lease. The Company utilized the full amount of the one-time improvement allowance of $12.6 million, of which $2.3 million is recoverable by the landlord through increased rent which continues through the expiration of the initial lease term.

In addition, the Company obtained a standby letter of credit (the “Letter of Credit”) in an amount of approximately $0.9 million, which may be drawn by the Landlord to be applied for certain purposes upon the Company’s breach of any provisions under the 2016 Lease. The Company has recorded the $0.9 million Letter of Credit as non-current restricted cash on its balance sheet as of December 31, 2020 and 2019.

Rent expense is recognized on a straight-line basis over the term of the lease and accordingly the Company records the difference between cash rent payments and the recognition of rent expense against the operating lease ROU asset. Rent expense during the years ended December 31, 2020, 2019 and 2018 was $5.1 million, $4.8 million and $4.2 million, respectively.

Supplemental information related to leases are as follows:

	Year Ended
	December 31, 2020			December 31, 2019
	(in thousands)
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$	4,990		$	4,855

	December 31, 2020			December 31, 2019
	(in thousands)
Supplemental balance sheet information related to leases:
Operating lease right-of-use assets	$	22,495		$	25,382
Current operating lease liabilities		3,195			2,810
Non-current operating lease liabilities		21,675			24,871
Total operating lease liabilities	$	24,870		$	27,681

Weighted-average remaining lease term (in years)
Operating lease	5.75				6.85
Weighted-average discount rate
Operating lease		8.25	%		8.25	%

	December 31, 2020
	(in thousands)
Maturity of operating lease liabilities
2021		5,129
2022		5,273
2023		5,420
2024		5,572
2025 and beyond		10,116
Total lease payments		31,510
Less imputed interest		(6,640	)
Present value of lease liabilities	$	24,870

Common Stock

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Common Stock

13. Common Stock

In October 2015, the Company’s board of directors and stockholders approved the amendment and restatement of the Company’s certificate of incorporation. The Amended and Restated Certificate of Incorporation was effective as of October 14, 2015, which provided for 75,000,000 authorized shares of common stock with par value of $0.00001 per share and 10,000,000 shares of preferred stock with a par value of $0.00001 per share. In June 2020, the board of directors and stockholders of the Company approved an increase in the authorized shares of common stock from 75,000,000 to 150,000,000.

Common stockholders are entitled to dividends if and when declared by the board of directors subject to the prior rights of the preferred stockholders. As of December 31, 2020 and 2019, no dividends on common stock had been declared by the Board of Directors.

In July 2018, the Company completed an underwritten public offering of 5,867,347 shares of common stock at a price of $24.50 per share, which included 765,306 shares issued pursuant to the underwriters’ exercise of their option to purchase additional shares of common stock. The aggregate net proceeds received by the Company from the offering were approximately $134.6 million after deducting underwriting discounts and commissions and offering expenses of $9.2 million.

In February 2020, the Company entered into an Open Market Sale Agreement (the “Sales Agreement”) with Jefferies LLC (“Jefferies”), to sell shares of the Company’s common stock, par value $0.00001 per share, with aggregate gross sales proceeds of up to $75,000,000, from time to time upon the Company’s request, through an at the market offering under which Jefferies will act as sales agent. Pursuant to the Sales Agreement, Jefferies as the sales agent will receive a commission of 3.0% of the gross sales price for shares of common stock sold under the Sales Agreement. During December 2020, the Company sold 1,535,217 shares at an average price of $7.62 per shares and received net proceeds of approximately $11.3 million after deducting the 3.0% sales commission and related issuance cost.

The Company had reserved shares of common stock for issuance, as follows:

	December 31,
	2020		2019
Options issued and outstanding		10,929,530		9,936,168
Shares available for future stock option grants		2,903,398		3,145,266
Shares available for future employee stock purchase plan		1,935,683		1,609,137
Total		15,768,611		14,690,571

Stock-based Compensation

12 Months Ended

Dec. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

14. Stock-based Compensation

The 2010 Plan and 2011 Plan

In 2010, the Company adopted its 2010 Stock Incentive Plan (the “2010 Plan”) which provided for the granting of stock options to employees, directors and consultants of the Company. Options granted under the 2010 Plan were either incentive stock options (“ISOs”) or nonqualified stock options (“NSOs”).

In February 2012, the Company adopted its 2011 Stock Incentive Plan (the “2011 Plan”). The 2011 Plan is divided into two separate equity programs, an option and stock appreciation rights grant program and a stock award program. In conjunction with adopting the 2011 Plan, the Company discontinued the 2010 Plan and released the shares reserved and still available under that plan.

In connection with the consummation of the IPO in October 2015, the board of directors adopted the Company’s 2015 Equity Incentive Plan (the “2015 Plan” and collectively with the 2010 Plan and 2011 Plan, the “Plans”). In conjunction with adopting the 2015 Plan, the Company discontinued the 2011 Plan with respect to new equity awards.

The 2015 Plan

Options under the 2015 Plan may be granted for periods of up to ten years. All options issued to date have had a 10-year life. Under the terms of the 2015 Plan, options may be granted at an exercise price not less than the estimated fair value of the Company’s common stock on the date of grant, as determined by the Company’s board of directors. For employees holding more than 10% of the voting rights of all classes of stock, the exercise price of ISOs and NSOs may not be less than 110% of the estimated fair value of the shares on the date of grant, as determined by the board of directors. To date, options granted under the 2015 Plan generally vest over four years and vest at a rate of 25% upon the first anniversary of the issuance date and 1/48th per month thereafter.

The initial number of shares of common stock available for future issuance under the 2015 Plan was 2,444,735. Beginning on January 1, 2016 and continuing until the expiration of the 2015 Plan, the total number of shares of common stock available for issuance under the 2015 Plan will automatically increase annually on January 1 by 4% of the total number of issued and outstanding shares of common stock as of January 1 of the same year. As of December 31, 2020 and 2019, 2,698,798 shares and 1,808,066 shares of common stock were available for future issuance under the 2015 Plan.

The 2019 Plan

In September 2019, the Board of Directors adopted the 2019 Employment Inducement Incentive Plan (the “2019 Plan”) which provides for the grant of stock options and other equity awards to any employee who has not previously been an employee or director of the Company or who is commencing employment with the Company following a bona fide period of nonemployment by the Company. Awards granted under the 2019 Plan are intended to constitute “employment inducement awards” under Nasdaq Listing Rule 5635(c)(4). Options granted under the 2019 Plan are nonqualified stock options (“NSOs”) which may be exercisable for periods of up to ten years and the options shall be granted at an exercise price of not less than 100% of the fair market value of the Company’s common stock on the date of grant.

The initial number of shares of common stock available for future issuance under the 2019 Plan was 1,815,000. As of December 31, 2020 and 2019, 204,600 shares and 1,337,200 shares, respectively, of common stock were available for future issuance under the 2019 Plan.

Activity under the Company’s stock option plans is set forth below:

							Options Outstanding
	Options Available for Grant			Number of Options			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Life (years)		Aggregate Intrinsic Value
											(in thousands)
Balances at December 31, 2019		3,145,266			9,936,168			12.26
Options authorized		1,820,923			—			—
Options granted		(4,219,044	)		4,219,044			7.23
Options exercised		—			(1,064,830	)		4.58
Options cancelled		2,156,253			(2,160,852	)		13.75
Balances at December 31, 2020		2,903,398			10,929,530		$	10.77		6.4	$	6,844
Options Exercisable—December 31, 2020					6,145,652		$	11.69		4.7	$	6,031
Options vested and expected to vest—December 31, 2020					10,929,530		$	10.77		6.4	$	6,844

The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the estimated fair value of the underlying common stock as of December 31, 2020.

The aggregate intrinsic value of stock options exercised in the years ended December 31, 2020, 2019 and 2018 was $4.4 million, $0.9 million and $14.6 million, respectively.

The options granted in the years ended December 31, 2020, 2019 and 2018 had weighted-average per share grant-date fair values of $3.94, $7.03 and $14.21, respectively. As of December 31, 2020, the unrecognized compensation expense with respect to options granted was $23.2 million and is expected to be recognized over 2.8 years.

Early Exercise of Employee Options

Certain stock options granted under the Plans provide option holders the right to elect to exercise unvested options in exchange for restricted common stock. Such unvested restricted shares are subject to a repurchase right held by the Company at the original issuance price in the event the optionee’s service to the Company is terminated either voluntarily or involuntarily. The right usually lapses 25% on the first anniversary of the vesting start date and in 36 equal monthly amounts thereafter. These repurchase terms are considered to be a forfeiture provision. The cash or full recourse notes received from employees for exercise of unvested options is treated as a refundable deposit and is classified as a liability on the balance sheets.

Employee Stock Purchase Plan

Concurrent with the completion of the IPO in October 2015, the Company’s Employee Stock Purchase Plan (“ESPP”) became effective. The ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their eligible compensation, subject to any plan limitations. The ESPP generally provides for six-month offering periods, and at the end of each offering period, employees are able to purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the last trading day of the offering period. The Company issued 128,684 shares and 142,949 shares of common stock under the ESPP in 2020 and 2019, respectively.

Shares available for future purchase under the ESPP were 1,935,683 shares and 1,609,137 shares at December 31, 2020 and 2019, respectively. The compensation expense related to the ESPP was $0.4 million, $0.6 million and $0.5 million for the years ended December 31, 2020, 2019 and 2018, respectively. As of December 31, 2020, there was $0.2 million of unrecognized compensation cost related to the ESPP, which the Company expects to recognize over 5 months.

Stock Based Compensation

Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Research and development	$	6,825	$	9,226	$	8,313
General and administrative		7,961		9,874		8,565
Total stock-based compensation expense	$	14,786	$	19,100	$	16,878

Stock-based compensation expense for employees was $14.7 million, $18.9 million and $16.7 million for the years ended December 31, 2020, 2019 and 2018, respectively.

Stock-based compensation expense for non-employees was $0.1 million, $0.2 million and $0.2 million for the years ended December 31, 2020, 2019 and 2018, respectively.

The Company estimated the fair value of employee stock options and ESPP using the Black-Scholes valuation model based on the date of grant with the following assumptions:

	Options			ESPP
	Year Ended December 31,			Year Ended December 31,
	2020	2019	2018	2020	2019	2018
Expected volatility	64.4% - 73.3%	64.4% - 68.6%	65.6% - 69.3%	47.3% - 122.1%	60.8% - 71.9%	46.4% - 70.5%
Risk-free interest rate	0.2% - 1.3%	1.4% - 2.5%	2.5% - 3.0%	0.1% - 0.2%	1.6% - 2.4%	2.1% - 2.6%
Dividend yield	— %	— %	— %	— %	— %	— %
Expected term (in years)	4.7 - 4.9	4.9 - 5.0	4.7 - 4.9	0.5	0.5	0.5

Expected term. The expected term of stock options represents the period that the stock options are expected to remain outstanding and is based on vesting terms, exercise term and contractual lives of the options. The expected term of the ESPP shares is equal to the six-month look-back period.

Expected volatility. The expected stock price volatility for the Company’s stock options is based on the historical stock price volatility which is commensurate with the estimated expected term of the stock awards. Volatility for ESPP shares is equal to the Company’s historical volatility over a six-month offering period.

Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury yield with a maturity equal to the expected term of the stock options in effect at the time of grant.

Dividend yield. The expected dividend is assumed to be zero as the Company has never paid dividends and has no current plan to pay any dividends on its common stock.

Income Taxes

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Income Taxes

15. Income Taxes

The Company derives its income only from the United States. The components of the provision for (benefit from) income taxes are as follows (in thousands):

	Years Ended December 31,
	2020			2019			2018
Current:
Federal	$	(13,912	)	$	(390	)	$	14,302
State		1			—			1
Total current		(13,911	)		(390	)		14,303
Deferred:
Federal		—			(37	)		—
State		—			—			—
Total deferred		—			(37	)		—
Provision for (benefit from) income taxes	$	(13,911	)	$	(427	)	$	14,303

A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows:

	Years Ended December 31,
	2020			2019			2018
U.S. federal taxes at statutory rate		21.0	%		21.0	%		21.0	%
State tax, net of federal benefit		3.3	%		1.3	%		0.7	%
Stock compensation		(2.2	)%		(1.2	)%		1.7	%
Tax credits		11.0	%		2.5	%		6.2	%
Change in valuation allowance		(31.2	)%		(22.8	)%		(49.5	)%
Net operating loss carryback		27.9	%		0.0	%		0.0	%
Return to provision adjustment		0.0	%		(0.3	)%		0.0	%
Other		(0.1	)%		(0.1	)%		(0.5	)%
Total		29.7	%		0.4	%		(20.4	)%

The types of temporary differences that give rise to significant portions of the Company’s deferred income tax assets and liabilities are set out below (in thousands):

	Year Ended December 31,
	2020			2019			2018
Net operating loss carryforwards	$	37,099		$	38,967		$	15,468
Research and development credits		16,803			10,940			7,514
Lease liability		5,223			5,813			—
Intangible assets		3,923			3,320			—
Deferred revenue		38,832			47,180			56,881
Accruals and deferred rent		1,512			1,296			1,955
Stock-based compensation		8,578			7,837			5,746
Other		20			—			39
Total gross deferred income tax assets		111,990			115,353			87,603
Less: valuation allowance		(106,448	)		(109,174	)		(86,466	)
Deferred tax assets, net of valuation allowance		5,542			6,179			1,137
Fixed assets		(433	)		(591	)		(854	)
Right-of-use assets		(4,724	)		(5,330	)		—
Intangible assets		—			—			(108	)
Prepaid expenses		(385	)		(243	)		(175	)
Other		—			(15	)		—
Deferred tax liabilities		(5,542	)		(6,179	)		(1,137	)
Net deferred income tax liabilities	$	—		$	—		$	—

On March 27, 2020, the Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”) was enacted in response to the COVID-19 pandemic. The tax relief measures under the CARES Act for businesses include a five-year net operating loss carryback, suspension of annual deduction limitation of 80% of taxable income from net operating losses generated in a tax year beginning after December 31, 2017, changes in the deductibility of interest, acceleration of alternative minimum tax credit refunds, payroll tax relief, and a technical correction to allow accelerated deductions for qualified improvement property.

The Company recognized income tax benefit of $13.9 million for the year ended December 31, 2020, through the net operating loss carryback under the CARES Act which generated a refund of income taxes paid for 2018.

The Company has established a valuation allowance against all of its net deferred tax assets. Management considered all available evidence, both positive and negative, including but not limited to our historical operating results, income or loss in recent periods, cumulative losses in recent years, forecasted earnings, future taxable income, and significant risk and uncertainty related to forecasts, and concluded the deferred tax assets are not more likely than not to be realized. The net change in the total valuation allowance for the years ended December 31, 2020, 2019 and 2018 was an increase(decrease) of $(2.7) million, $22.7 million and $35.7 million, respectively.

The Company had net operating loss carryforwards for federal and state income tax purposes of approximately $168.6 million and $24.2 million, respectively, as of December 31, 2020, available to reduce future taxable income. Of the federal net operating loss carryforwards, $65.6 million will begin to expire in 2034, if not utilized and $103 million will carryforward indefinitely. The state net operating loss carryforwards will begin to expire in 2032, if not utilized.

The Company also has federal and state research and development tax credits carryforwards of $15.6 million and $9.0 million, respectively, as of December 31, 2020 available to reduce future income taxes. The federal research and development tax credits will begin to expire in 2031 if not utilized. The state research and development tax credits have no expiration date.

Internal Revenue Code section 382 (“IRC Section 382”) places a limitation (the “Section 382 Limitation”) on the amount of taxable income that can be offset by net operating loss (“NOL”) carryforwards after a change in control (generally greater than 50% change in ownership) of a loss corporation. California has similar rules. The Company has performed an IRC Section 382 analysis and determined there was an ownership change in 2017 that resulted in 382 limitations. When an ownership change occurs, IRC Section 382 limits the use of NOLs and credits in subsequent periods based on the annual 382 limitations. The annual 382 limitations may limit the full use of available tax attributes in one year but the identified ownership changes may not result in expiration of tax attributes for use prior to expiration of their respective carryforward periods. Accordingly, none of the tax attributes have been reduced but limited the full use in 2018. The Company has determined that, while an ownership change has occurred, the applicable limits would not impair the value or anticipated use of the Company’s federal and state net operating losses. Although realization is not assured, management believes it is more likely than not that any limitation under IRC Section 382 will not impair the realizability of the deferred income tax assets related to federal and state net operating loss carryforwards. The Company reviewed its stock ownership for the year ended December 31, 2020 and concluded no ownership changes occurred in current year which would result in a reduction of its net operating loss or in its research and development credits expiring unused. If additional ownership change occurs, the utilization of net operating loss and credit carryforwards could be significantly reduced.

The Company had approximately $6.5 million, $5.2 million and $3.8 million of unrecognized tax benefits as of December 31, 2020, 2019 and 2018, respectively, and approximately $0, $0.9 million and $1.0 million, respectively, would affect the Company’s effective tax rate if recognized.

A reconciliation of the beginning and ending unrecognized tax benefit amount is as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Balance at the beginning of the year	$	5,249	$	3,756	$	4,320
Additions based on tax positions related to current year		1,205		1,403		1,166
Adjustment based on tax positions related to prior years		—		90		(1,730	)
Balance at end of the year	$	6,454	$	5,249	$	3,756

The Company recognizes interest and penalties related to uncertain tax positions in income tax expense. To the extent accrued interest and penalties do not ultimately become payable, amounts accrued will be reduced and reflected as a reduction of the provision for income taxes in the period that such determination is made. Interest and penalties have not been accrued for 2020, 2019 and 2018.

The Company files income tax returns in the United States, including California state jurisdiction. The tax years 2010 to 2019 remains open to U.S. federal and state examination to the extent of the utilization of net operating loss and credit carryovers. As of December 31, 2020, the Company is under examination by the State of California.

Compensation And Retirement Disclosure [Abstract]

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Defined Contribution Plan

12 Months Ended

Dec. 31, 2020

Defined Contribution Plan

16. Defined Contribution Plan

The Company sponsors a defined contribution plan under Section 401(k) of the Internal Revenue Code covering substantially all full-time U.S. employees. Employee contributions are voluntary and are determined on an individual basis subject to the maximum allowable under federal tax regulations. During the years ended December 31, 2020, 2019 and 2018, the Company made contributions to the plan of $0.8 million, $0.8 million and $0.6 million, respectively.

The entire disclosure for retirement benefits.

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Subsequent Events

12 Months Ended

Dec. 31, 2020

Subsequent Events [Abstract]

Subsequent Events

17. Subsequent Events

In January 2021, the Company completed an underwritten public offering of 14,285,714 shares of common stock at a price of $7.00 per share. The aggregate net proceeds received by the Company from the offering were approximately $93.6 million, after deducting underwriting discounts and commissions and estimated offering expenses of $6.4 million. The Company also granted the underwriters the option, for 30 days, to purchase up to 2,142,857 additional shares of common stock at the public offering price, less the underwriting discounts and commissions. In February 2021, the underwriters exercised the option in full which resulted in additional net proceeds of $14.1 million to the Company, after deducting the underwriting discounts and commissions of $0.9 million.

Basis of Presentation and Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2020

Concentration of Credit Risk and Other Risks and Uncertainties

Basis of Presentation

The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”).

Use of Estimates

	Revenue						Accounts Receivable, net
	Year Ended December 31,						December 31,
	2020		2019		2018		2020		2019
	(in thousands)						(in thousands)
AbbVie Ireland Unlimited Company	$	38,192	$	5,878	$	18,997	$	—	$	—
Astellas Pharma Inc.		13,931		—		—		798		—
Bristol-Myers Squibb Company		39,630		47,740		32,780		—		13
Total revenue from customers who represent 10% or more of the Company's total revenue	$	91,753	$	53,618	$	51,777	$	798	$	13

The Company’s customers are located in the United States of America, Ireland and Japan.

Segments

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents.

Restricted Cash

Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015.

	December 31
	2020		2019
	(in thousands)
Cash and cash equivalents	$	191,859	$	188,425
Restricted cash		917		917
	$	192,776	$	189,342

Short-term Investments

Property and Equipment, net

Machinery and equipment		5 years
Computer equipment and software		3 years
Furniture and fixtures		3 years
Leasehold improvements		Shorter of remaining lease term or estimated life of the assets

Maintenance and repairs that do not extend the life or improve the asset are expensed when incurred.

Goodwill and Intangible Assets

Impairment of Long-Lived Assets

Revenue Recognition

Comprehensive Loss

Contract Balances

Stock-Based Compensation

Income Taxes

Research and Development Expenses

Leases

Adopted Accounting Pronouncements and Recent Accounting Pronouncements

Adopted Accounting Pronouncements

Financial Instruments - Credit Losses

Simplification of the Test for Goodwill Impairment

Fair Value Measurement Disclosure Requirements Modification

Internal Use Software Guidance for Cloud Computing Arrangement

Collaborative Arrangements and Revenue Recognition

Share-Based Payment to Customer

Recent Accounting Pronouncements

Basis of Presentation and Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2020

Schedule of Customers and Collaboration Partners who Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance

	Revenue						Accounts Receivable, net
	Year Ended December 31,						December 31,
	2020		2019		2018		2020		2019
	(in thousands)						(in thousands)
AbbVie Ireland Unlimited Company	$	38,192	$	5,878	$	18,997	$	—	$	—
Astellas Pharma Inc.		13,931		—		—		798		—
Bristol-Myers Squibb Company		39,630		47,740		32,780		—		13
Total revenue from customers who represent 10% or more of the Company's total revenue	$	91,753	$	53,618	$	51,777	$	798	$	13

Schedule of Cash, Cash Equivalents, and Restricted Cash

	December 31
	2020		2019
	(in thousands)
Cash and cash equivalents	$	191,859	$	188,425
Restricted cash		917		917
	$	192,776	$	189,342

Schedule of Useful Lives of Property and Equipment

The useful lives of property and equipment are as follows:

Machinery and equipment		5 years
Computer equipment and software		3 years
Furniture and fixtures		3 years
Leasehold improvements		Shorter of remaining lease term or estimated life of the assets

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share

	Year Ended December 31,
	2020		2019		2018
Options and ESPP to purchase common stock		11,388,691		9,687,844		7,478,755

Fair Value Measurements and Short-Term Investments (Tables)

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis

The following tables set forth the fair value of the Company’s short-term investments subject to fair value measurements on a recurring basis and the level of inputs used in such measurements:

		December 31, 2020
	Valuation Hierarchy	Amortized Cost		Allowance for Credit Losses		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Aggregate Fair Value
		(in thousands)
Assets
Money market funds	Level I	$	131,121	$	—	$	—	$	—	$	131,121
Restricted cash (money market funds)	Level I		917		—		—		—		917
U.S. Government bonds	Level I		124,254		—		6		—		124,260
Total Securities		$	256,292	$	—	$	6	$	—	$	256,298

		December 31, 2019
	Valuation Hierarchy	Amortized Cost		Allowance for Credit Losses		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Aggregate Fair Value
		(in thousands)
Assets
Money market funds	Level I	$	170,757	$	—	$	—	$	—	$	170,757
Restricted cash (money market funds)	Level I		917		—		—		—		917
U.S. Government bonds	Level I		107,610		—		110		—		107,720
Total securities		$	279,284	$	—	$	110	$	—	$	279,394

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2020

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following:

	December 31
	2020			2019
	(in thousands)
Machinery and equipment	$	13,772		$	12,124
Computer equipment and software		1,600			1,555
Furniture and fixtures		1,024			1,024
Leasehold improvements		1,728			1,483
Construction in progress		252			236
		18,376			16,422
Less: accumulated depreciation and amortization		(11,426	)		(9,050	)
	$	6,950		$	7,372

Goodwill and Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2020

Goodwill And Intangible Assets Disclosure [Abstract]

Schedule of Goodwill and Intangible Assets

Goodwill and intangible assets consisted of the following:

Goodwill
	December 31,
	2020			2019
	(in thousands)
Goodwill	$	949		$	949

Intangible assets
	December 31,
	2020			2019
	(in thousands)
In-process research and development	$	1,750		$	1,750
Less accumulated amortization		(583	)		(438	)
	$	1,167		$	1,312

Accrued Liabilities (Tables)

12 Months Ended

Dec. 31, 2020

Accrued Liabilities Current [Abstract]

Schedule of Accrued Liabilities

Accrued liabilities consisted of the following:

	December 31,
	2020		2019
	(in thousands)
Research and clinical expenses	$	10,092	$	19,006
Payroll and related expenses		8,362		6,721
Legal and professional expenses		815		1,062
Operating lease liabilities - short term		3,195		2,810
Other accrued expenses		595		452
Total	$	23,059	$	30,051

Research and Collaboration Agreements (Tables)

12 Months Ended

Dec. 31, 2020

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Schedule of Revenue by Collaboration Partners

The following table summarizes the revenue by collaboration partners:

	Year Ended December 31,
	2020		2019		2018
	(in thousands)
AbbVie	$	38,192	$	5,878	$	18,997
Amgen		8,609		3,871		4,899
Astellas		13,931		—		—
Bristol Myers Squibb		39,630		47,740		32,780
ImmunoGen		—		—		1,471
Pfizer		—		—		1,355
Total revenue	$	100,362	$	57,489	$	59,502

Summary of Contract Liabilities

The following table presents changes in the Company’s total contract liabilities for the year ended December 31, 2020 (in thousands):

	Balance at 12/31/2019		Additions		Deductions		Balance at 12/31/2020
	(in thousands)
Contract liabilities:
Deferred revenue	$	230,239	$	120,000	$	89,109	$	261,130

Leases (Tables)

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Summary of Supplemental Information Related to Leases

Supplemental information related to leases are as follows:

	Year Ended
	December 31, 2020			December 31, 2019
	(in thousands)
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$	4,990		$	4,855

	December 31, 2020			December 31, 2019
	(in thousands)
Supplemental balance sheet information related to leases:
Operating lease right-of-use assets	$	22,495		$	25,382
Current operating lease liabilities		3,195			2,810
Non-current operating lease liabilities		21,675			24,871
Total operating lease liabilities	$	24,870		$	27,681

Weighted-average remaining lease term (in years)
Operating lease	5.75				6.85
Weighted-average discount rate
Operating lease		8.25	%		8.25	%

Schedule of Maturity of Operating Lease Liabilities

	December 31, 2020
	(in thousands)
Maturity of operating lease liabilities
2021		5,129
2022		5,273
2023		5,420
2024		5,572
2025 and beyond		10,116
Total lease payments		31,510
Less imputed interest		(6,640	)
Present value of lease liabilities	$	24,870

Common Stock (Tables)

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Schedule Reserved Shares of Common Stock Issuance on Converted Basis

The Company had reserved shares of common stock for issuance, as follows:

	December 31,
	2020		2019
Options issued and outstanding		10,929,530		9,936,168
Shares available for future stock option grants		2,903,398		3,145,266
Shares available for future employee stock purchase plan		1,935,683		1,609,137
Total		15,768,611		14,690,571

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

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Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Schedule of Activity Under Company's Stock Option Plans

Activity under the Company’s stock option plans is set forth below:

							Options Outstanding
	Options Available for Grant			Number of Options			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Life (years)		Aggregate Intrinsic Value
											(in thousands)
Balances at December 31, 2019		3,145,266			9,936,168			12.26
Options authorized		1,820,923			—			—
Options granted		(4,219,044	)		4,219,044			7.23
Options exercised		—			(1,064,830	)		4.58
Options cancelled		2,156,253			(2,160,852	)		13.75
Balances at December 31, 2020		2,903,398			10,929,530		$	10.77		6.4	$	6,844
Options Exercisable—December 31, 2020					6,145,652		$	11.69		4.7	$	6,031
Options vested and expected to vest—December 31, 2020					10,929,530		$	10.77		6.4	$	6,844

Total Stock-based Compensation Recognized

Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Research and development	$	6,825	$	9,226	$	8,313
General and administrative		7,961		9,874		8,565
Total stock-based compensation expense	$	14,786	$	19,100	$	16,878

Schedule of Estimated Fair Value of Employee Stock Options and ESPP Using Black-Scholes Valuation Model

The Company estimated the fair value of employee stock options and ESPP using the Black-Scholes valuation model based on the date of grant with the following assumptions:

	Options			ESPP
	Year Ended December 31,			Year Ended December 31,
	2020	2019	2018	2020	2019	2018
Expected volatility	64.4% - 73.3%	64.4% - 68.6%	65.6% - 69.3%	47.3% - 122.1%	60.8% - 71.9%	46.4% - 70.5%
Risk-free interest rate	0.2% - 1.3%	1.4% - 2.5%	2.5% - 3.0%	0.1% - 0.2%	1.6% - 2.4%	2.1% - 2.6%
Dividend yield	— %	— %	— %	— %	— %	— %
Expected term (in years)	4.7 - 4.9	4.9 - 5.0	4.7 - 4.9	0.5	0.5	0.5

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Schedule of Components of Provision for (Benefit from) Income Taxes

The Company derives its income only from the United States. The components of the provision for (benefit from) income taxes are as follows (in thousands):

	Years Ended December 31,
	2020			2019			2018
Current:
Federal	$	(13,912	)	$	(390	)	$	14,302
State		1			—			1
Total current		(13,911	)		(390	)		14,303
Deferred:
Federal		—			(37	)		—
State		—			—			—
Total deferred		—			(37	)		—
Provision for (benefit from) income taxes	$	(13,911	)	$	(427	)	$	14,303

Schedule of Effective Tax Rate Reconciliation

A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows:

	Years Ended December 31,
	2020			2019			2018
U.S. federal taxes at statutory rate		21.0	%		21.0	%		21.0	%
State tax, net of federal benefit		3.3	%		1.3	%		0.7	%
Stock compensation		(2.2	)%		(1.2	)%		1.7	%
Tax credits		11.0	%		2.5	%		6.2	%
Change in valuation allowance		(31.2	)%		(22.8	)%		(49.5	)%
Net operating loss carryback		27.9	%		0.0	%		0.0	%
Return to provision adjustment		0.0	%		(0.3	)%		0.0	%
Other		(0.1	)%		(0.1	)%		(0.5	)%
Total		29.7	%		0.4	%		(20.4	)%

Schedule of Deferred Income Tax Assets and Liabilities

The types of temporary differences that give rise to significant portions of the Company’s deferred income tax assets and liabilities are set out below (in thousands):

	Year Ended December 31,
	2020			2019			2018
Net operating loss carryforwards	$	37,099		$	38,967		$	15,468
Research and development credits		16,803			10,940			7,514
Lease liability		5,223			5,813			—
Intangible assets		3,923			3,320			—
Deferred revenue		38,832			47,180			56,881
Accruals and deferred rent		1,512			1,296			1,955
Stock-based compensation		8,578			7,837			5,746
Other		20			—			39
Total gross deferred income tax assets		111,990			115,353			87,603
Less: valuation allowance		(106,448	)		(109,174	)		(86,466	)
Deferred tax assets, net of valuation allowance		5,542			6,179			1,137
Fixed assets		(433	)		(591	)		(854	)
Right-of-use assets		(4,724	)		(5,330	)		—
Intangible assets		—			—			(108	)
Prepaid expenses		(385	)		(243	)		(175	)
Other		—			(15	)		—
Deferred tax liabilities		(5,542	)		(6,179	)		(1,137	)
Net deferred income tax liabilities	$	—		$	—		$	—

Schedule of Reconciliation of Unrecognized Tax Benefits

A reconciliation of the beginning and ending unrecognized tax benefit amount is as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Balance at the beginning of the year	$	5,249	$	3,756	$	4,320
Additions based on tax positions related to current year		1,205		1,403		1,166
Adjustment based on tax positions related to prior years		—		90		(1,730	)
Balance at end of the year	$	6,454	$	5,249	$	3,756

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Customers and Collaboration Partners who Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Revenue		$ 100,362	$ 57,489	$ 59,502
Accounts Receivable, net		798	13
AbbVie Ireland Unlimited Company
Concentration Risk [Line Items]
Revenue	$ 9,900	38,200	5,900	19,000
Major Customer
Concentration Risk [Line Items]
Revenue		91,753	53,618	51,777
Accounts Receivable, net		798	13
Major Customer \| AbbVie Ireland Unlimited Company
Concentration Risk [Line Items]
Revenue		38,192	5,878	18,997
Major Customer \| Bristol Myers Squibb Company
Concentration Risk [Line Items]
Revenue		39,630	47,740	$ 32,780
Accounts Receivable, net			$ 13
Major Customer \| Astellas Pharma Inc.
Concentration Risk [Line Items]
Revenue		13,931
Accounts Receivable, net		$ 798

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) Activity Segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of business activities \| Activity		1
Number of operating segments \| Segment		1
Impairment of goodwill		$ 0	$ 0	$ 0
Impairment of intangible assets		0	0	0
Impairment of long-lived assets		0	0	0
Increase in accumulated deficit balance		(450,115,000)	(417,230,000)
Research and development expense		112,936,000	131,619,000	$ 103,866,000
Right-of-use asset		22,495,000	25,382,000
Lease liability		$ 24,870,000	27,681,000
Probody technology
Summary Of Significant Accounting Policies [Line Items]
Acquired, technology know-how	$ 5,000,000.0
Research and development expense			$ 5,000,000.0
ASC 842
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset					$ 28,000,000.0
Lease liability					$ 30,100,000
Adjustments \| ASC 606
Summary Of Significant Accounting Policies [Line Items]
Increase in accumulated deficit balance						$ (10,900,000)

Basis of Presentation and Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 191,859	$ 188,425
Restricted cash	917	917
Cash, cash equivalents, and restricted cash	$ 192,776	$ 189,342	$ 248,494	$ 178,465

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2020
Machinery and equipment
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	5 years
Computer equipment and software
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Furniture and fixtures
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Leasehold improvements
Property Plant And Equipment [Line Items]
Estimated useful life of leasehold improvements, description	Shorter of remaining lease term or estimated life of the assets

Adopted and Recent Accounting Pronouncements - Additional Information (Details)	Dec. 31, 2020
ASU 2016-13
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
ASU 2017-04
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
ASU 2018-13
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
ASU 2018-15
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
ASU 2018-18
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
ASU 2019-08
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
ASU 2019-12
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2021
Change in accounting principle, accounting standards update, adopted [true false]	false
Change in accounting principle, accounting standards update, immaterial effect [true false]	false

Net Loss Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Options and ESPP to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	11,388,691	9,687,844	7,478,755

Fair Value Measurements and Short-Term Investments - Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 256,292	$ 279,284
Gross Unrealized Holding Gains	6	110
Aggregate Fair Value	256,298	279,394
Cash and cash equivalents	191,859	188,425
Level I \| Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	131,121	170,757
Level I \| Restricted cash (money market funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted cash	917	917
Level I \| U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term Investments, Amortized Cost	124,254	107,610
Short-term Investments, Gross Unrealized Holding Gains	6	110
Short-term Investments, Aggregate Fair Value	$ 124,260	$ 107,720

Fair Value Measurements and Short-Term Investments - Additional Information (Details)	Dec. 31, 2020 USD ($)
Fair Value Disclosures [Abstract]
Contractual maturities of greater than twelve months	$ 0

Property and Equipment - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Machinery and equipment	$ 13,772	$ 12,124
Computer equipment and software	1,600	1,555
Furniture and fixtures	1,024	1,024
Leasehold improvements	1,728	1,483
Construction in progress	252	236
Total property and equipment	18,376	16,422
Less: accumulated depreciation and amortization	(11,426)	(9,050)
Property and equipment, net	$ 6,950	$ 7,372

Property and Equipment - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization	$ 2.4	$ 2.5	$ 1.7

Goodwill and Intangible Assets - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite Lived Intangible Assets [Line Items]
Amortization expense of intangible assets	$ 146	$ 146	$ 146
IPR&D [Member]
Finite Lived Intangible Assets [Line Items]
Average estimated lives of patents	12 years
Amortization expense of intangible assets	$ 100	$ 100	$ 100

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Goodwill
Goodwill	$ 949	$ 949
In-Process Research and Development
Intangible assets
Intangible assets, gross	1,750	1,750
Less accumulated amortization	(583)	(438)
Intangible assets, net	$ 1,167	$ 1,312

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Research and clinical expenses	$ 10,092	$ 19,006
Payroll and related expenses	8,362	6,721
Legal and professional expenses	815	1,062
Operating lease liabilities - short term	3,195	2,810
Other accrued expenses	595	452
Total	$ 23,059	$ 30,051

Research and Collaboration Agreements - Schedule of Revenue by Collaboration Partners (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 100,362	$ 57,489	$ 59,502
AbbVie
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	38,192	5,878	18,997
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	8,609	3,871	4,899
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	39,630	$ 47,740	32,780
Astellas
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 13,931
ImmunoGen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue			1,471
Pfizer
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue			$ 1,355

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details)	1 Months Ended					3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	May 31, 2018 USD ($)	Jul. 31, 2017 USD ($)	Apr. 30, 2016 USD ($) Agreement Target AccountingUnit	Jun. 30, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2017 Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement							$ 100,362,000	$ 57,489,000	$ 59,502,000
Type of Revenue [Extensible List]								us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue							261,130,000	$ 230,239,000
AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment received			$ 21,000,000.0
Revenue recognized from collaborative arrangement						$ 9,900,000	$ 38,200,000	$ 5,900,000	$ 19,000,000.0
Type of Revenue [Extensible List]						us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
AbbVie Ireland Unlimited Company \| ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received					$ 30,000,000.0
Milestone payment received					14,000,000.0
Total transaction price					39,800,000
Estimated sublicense fees					$ 4,200,000
Number of accounting units \| AccountingUnit					2
AbbVie Ireland Unlimited Company \| Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of collaboration agreements \| Agreement					2
Percentage of net profits or net losses related to development and commercialization costs					35.00%
Amount due from agreement							$ 0	$ 0
AbbVie Ireland Unlimited Company \| CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received					$ 20,000,000.0
Milestone payment received	$ 40,000,000.0		21,000,000.0	$ 14,000,000.0
Milestone payment received			21,000,000.0
Deferred revenue							25,200,000	20,000,000.0
AbbVie Ireland Unlimited Company \| CD71 Agreement \| Transferred over Time
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement	$ 26,600,000
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember
AbbVie Ireland Unlimited Company \| CD71 Agreement \| ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received					20,000,000.0
Total transaction price					29,800,000
Estimated sublicense fees					4,200,000
Milestone payment received					$ 14,000,000.0
AbbVie Ireland Unlimited Company \| CD71 Agreement \| U.S
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Profit split on U.S. sales					35.00%
AbbVie Ireland Unlimited Company \| CD71 Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development, regulatory and commercial milestones					$ 470,000,000.0
AbbVie Ireland Unlimited Company \| Seattle Genetics Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid			$ 4,000,000.0	$ 1,000,000.0
AbbVie Ireland Unlimited Company \| Seattle Genetics Agreement \| ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid					1,000,000.0
AbbVie Ireland Unlimited Company \| Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received					10,000,000.0
Number of targets selected \| Target										1
Revenue recognition upon performance obligation					10,000,000.0
Deferred revenue							$ 8,000,000.0	$ 11,600,000
AbbVie Ireland Unlimited Company \| Discovery Agreement \| ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received					10,000,000.0
Total transaction price					$ 10,000,000.0
AbbVie Ireland Unlimited Company \| Discovery Agreement \| Second Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments received		$ 10,000,000.0
AbbVie Ireland Unlimited Company \| Discovery Agreement \| Second Target \| ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price		$ 10,000,000.0
Estimated research service period		5 years
AbbVie Ireland Unlimited Company \| Discovery Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets \| Target					2
AbbVie Ireland Unlimited Company \| Discovery Agreement \| Maximum \| Development, Regulatory and Commercial Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable					$ 265,000,000.0

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details 1)	Apr. 30, 2016
AbbVie Ireland Unlimited Company \| Discovery Agreement \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2016-04-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognition upon performance obligation, service period	5 years

Research and Collaboration Agreements - Amgen, Inc - Additional Information (Details)		1 Months Ended	3 Months Ended		12 Months Ended
	Sep. 29, 2017 USD ($) Target	Oct. 31, 2017 USD ($) $ / shares shares	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, value of shares issued in connection with agreement					$ 11,288,000		$ 134,596,000
Accrued liabilities					23,059,000	$ 30,051,000
Revenue recognized from collaborative arrangement					100,362,000	$ 57,489,000	$ 59,502,000
Type of Revenue [Extensible List]						us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue					261,130,000	$ 230,239,000
Amgen Inc \| EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue					29,800,000
Amgen Inc \| Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue					2,200,000
Collaboration and License Agreement \| Amgen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 40,000,000.0	$ 40,000,000.0
Common stock, shares issuable under agreement \| shares		1,156,069
Common stock, shares issuable under agreement, price per share \| $ / shares		$ 17.30
Common stock, value of shares issued in connection with agreement		$ 20,000,000.0
Period used to calculate weighted average price per share		20 days
Lock-up period for share disposal		6 months
Estimated premium on the stock sold		$ 500,000
Number of targets selected \| Target	1
Number of additional collaboration target \| Target	2
Total transaction price	$ 51,200,000
Portion of transaction price allocated to premium on sale of common stock	500,000
Estimated fair value of products	10,700,000
Revenue recognized from collaborative arrangement					$ 8,600,000	$ 3,900,000	$ 4,900,000
Type of Revenue [Extensible List]					us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Amount due from agreement					$ 0	$ 0
Collaboration and License Agreement \| Amgen Inc \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments payable	203,000,000.0
Collaboration and License Agreement \| Amgen Inc \| EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price	46,400,000
Estimated research service period			8 years	7 years
Deferred revenue					29,800,000	37,600,000
Collaboration and License Agreement \| Amgen Inc \| EGFR Products \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 455,000,000.0
Percentage share of profit and losses	50.00%
Collaboration and License Agreement \| Amgen Inc \| Amgen Products \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 950,000,000.0
Number of targets \| Target	3
Collaboration and License Agreement \| Amgen Inc \| Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price	$ 4,800,000
Estimated research service period	6 years
Revenue recognition upon performance obligation	$ 4,800,000
Deferred revenue					2,200,000	3,000,000.0
Sublicense Agreement \| Amgen Inc \| UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities					$ 0	$ 0
Sublicense Agreement \| Amgen Inc \| Maximum \| UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense payment percentage	7.50%

Research and Collaboration Agreements - Astellas Pharma Inc - Additional Information (Details) $ in Thousands		1 Months Ended	12 Months Ended
	Mar. 23, 2020 USD ($) Target	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement			$ 100,362	$ 57,489	$ 59,502
Deferred revenue			261,130	$ 230,239
Astellas Pharma Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			67,600
Collaboration and License Agreement \| Astellas Pharma Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets selected \| Target	1
Number of additional collaboration target \| Target	3
Upfront payment received	$ 80,000	$ 80,000
Total transaction price	90,000
Research and development service fees	$ 10,000		1,500
Estimated research service period	5 years
Estimated research service termination month & year	2025-03
Revenue recognized from collaborative arrangement			13,900
Deferred revenue			67,600
Amount due from customer			$ 800
Collaboration and License Agreement \| Astellas Pharma Inc. \| Additional Contingent Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 900,000
Collaboration and License Agreement \| Astellas Pharma Inc. \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets \| Target	4
Right to expand the number of additional collaboration target \| Target	5
Contingent milestone payments receivable	$ 1,600,000
Collaboration and License Agreement \| Astellas Pharma Inc. \| Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Right to expand the number of additional collaboration target \| Target	3

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)			3 Months Ended		12 Months Ended
	Apr. 25, 2017 USD ($) Target	Jul. 07, 2014 USD ($) Target Term	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Feb. 28, 2021 USD ($)	Feb. 29, 2020 USD ($)	Nov. 30, 2017 USD ($)	Mar. 17, 2017 Target	Dec. 31, 2016 USD ($)	Jan. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement					$ 100,362,000	$ 57,489,000	$ 59,502,000
Type of Revenue [Extensible List]						us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue					261,130,000	$ 230,239,000
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue					$ 128,300,000
Collaborative Arrangement \| Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration target research term		2 years
Number of additional collaboration target \| Target		2
Research terms					Each collaboration target had a two-year research term and the two additional targets had to be nominated by Bristol Myers Squibb within five years of the effective date of the BMS Agreement. The research term for each collaboration target could be extended in one year increments up to three times.
Extension of research term for each collaboration target		1 year
Upfront payment received	$ 200,000,000.0	$ 50,000,000.0
Contingent milestone payments receivable									$ 10,000,000.0			$ 15,000,000.0	$ 10,000,000.0
Number of research targets selected \| Target	8
Total transaction price		272,800,000
Upfront payment received		250,000,000.0
Research and development service fees		10,800,000
Milestone payment received		$ 12,000,000.0	$ 10,000,000.0
Estimated research service termination date		Apr. 25, 2025
Revenue recognized from collaborative arrangement					$ 39,600,000	$ 47,700,000	$ 32,800,000
Type of Revenue [Extensible List]					us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue					$ 128,300,000	$ 158,000,000.0
Amount due from agreement					$ 0	$ 13,000
Collaborative Arrangement \| Bristol Myers Squibb Company \| Forecast
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable								$ 1,779,000,000.0
Collaborative Arrangement \| Bristol Myers Squibb Company \| Pre Clinical Candidate
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable												$ 2,000,000.0
Collaborative Arrangement \| Bristol Myers Squibb Company \| New Drug Application for CTLA-4
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable										$ 10,000,000.0
Collaborative Arrangement \| Bristol Myers Squibb Company \| Third And Fourth Immuno-Oncology Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement				$ 18,100,000
Contingent milestone payments, no longer eligible to receive				894,000,000.0
Collaborative Arrangement \| Bristol Myers Squibb Company \| Accelerated Recognition of Deferred Revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement				$ 17,400,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of oncology target \| Target		4
Number of collaboration target \| Target		2
Period of nomination of additional target from effective date		5 years
Times of increments for extended collaboration target research time \| Term		3
Contingent milestone payments receivable	$ 3,586,000,000.0	$ 1,192,000,000.0
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Each Of Two Additional Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 25,000,000.0
Collaboration and License Agreement \| Maximum \| Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of additional collaboration target \| Target											8

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details)	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2014 USD ($) Target	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Feb. 29, 2020 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue			$ 261,130,000	$ 230,239,000
Revenue recognized from collaborative arrangement			100,362,000	57,489,000	$ 59,502,000
Charge to research and development expense			$ 112,936,000	$ 131,619,000	103,866,000
Collaborative Arrangement \| Immuno Gen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	$ 0
Number of research targets selected \| Target	2
Estimated total fair value consideration recorded as deferred revenue	$ 13,200,000
Revenue recognized from collaborative arrangement					$ 1,500,000
Milestone payments payable						$ 3,000,000.0
Charge to research and development expense		$ 3,000,000.0

Research and Collaboration Agreements - Pfizer Inc - Additional Information (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Dec. 31, 2014 USD ($)	May 31, 2013 Target	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional target as deferred revenue			$ 120,000
Revenue recognized from collaborative arrangement			$ 100,362	$ 57,489	$ 59,502
Type of Revenue [Extensible List]				us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Collaborative Arrangement \| Pfizer Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets \| Target		2
Nominated number of additional research targets \| Target		2
Additional target as deferred revenue	$ 1,500
Revenue recognized from collaborative arrangement					$ 1,400
Type of Revenue [Extensible List]					us-gaap:LicenseAndServiceMember

Research and Collaboration Agreements - Summary of Contract Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
Revenue From Contract With Customer [Abstract]
Balance at 12/31/2019	$ 230,239
Additions	120,000
Deductions	89,109
Balance at 12/31/2020	$ 261,130

Research and Collaboration Agreements - Contract Liabilities - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additions to deferred revenue	$ 120,000
Deductions to deferred revenue recognized during the period	89,109
Deferred revenue	261,130	$ 230,239
AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additions to deferred revenue	40,000
AbbVie Ireland Unlimited Company \| CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 25,200	$ 20,000
Deferred revenue recognition maturity month and year	2022-09
AbbVie Ireland Unlimited Company \| First Target under Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 1,000
Deferred revenue recognition maturity month and year	2022-04
AbbVie Ireland Unlimited Company \| Second Target under Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 7,000
Deferred revenue recognition maturity month and year	2024-06
Astellas Pharma Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additions to deferred revenue	$ 80,000
Deferred revenue	$ 67,600
Deferred revenue recognition maturity month and year	2025-03
Amgen Inc \| EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 29,800
Deferred revenue recognition maturity month and year	2024-09
Amgen Inc \| Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 2,200
Deferred revenue recognition maturity month and year	2023-09
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 128,300
Deferred revenue recognition maturity month and year	2025-04

License Agreement - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended							3 Months Ended		12 Months Ended
	Mar. 23, 2020	Apr. 25, 2017	Jul. 07, 2014	Mar. 31, 2020	Feb. 29, 2020	Jun. 30, 2019	Apr. 30, 2019	May 31, 2018	Jul. 31, 2017	Apr. 30, 2016	Mar. 31, 2020	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2013
License Agreement [Line Items]
Annual minimum royalty obligations													$ 200
License termination period													60 days
Common stock, shares issued													48,251,819	45,523,088
Research and development													$ 112,936	$ 131,619	$ 103,866
Accrued liabilities													23,059	30,051
AbbVie \| Discovery Agreement \| Second Target
License Agreement [Line Items]
Additional sublicense fees						$ 800
Milestone payment receivable						$ 10,000
Bristol Myers Squibb Company \| Collaboration and License Agreement
License Agreement [Line Items]
Milestone payment receivable					$ 10,000
Sublicense fees					$ 800
Bristol Myers Squibb Company \| Collaborative Arrangement
License Agreement [Line Items]
Upfront payment received		$ 200,000	$ 50,000
Milestone payment received			$ 12,000								$ 10,000
Astellas Pharma Inc. \| Collaboration and License Agreement
License Agreement [Line Items]
Additional sublicense fees				$ 6,000
Upfront payment received	$ 80,000			80,000
AbbVie Ireland Unlimited Company \| Discovery Agreement
License Agreement [Line Items]
Upfront payment received										$ 10,000
AbbVie Ireland Unlimited Company \| CD71 Agreement
License Agreement [Line Items]
Additional sublicense fees				1,400
Upfront payment received										$ 20,000
Milestone payment received				$ 40,000				$ 21,000	$ 14,000
Immuno Gen Inc \| Collaborative Arrangement
License Agreement [Line Items]
Research and development											$ 3,000
Upfront license payment														7,500
UCSB
License Agreement [Line Items]
Common stock, shares issued							150,000									157,332
Fair value of common stock price per share							$ 10.68
Payment of upfront fees							$ 1,000					$ 1,000
Annual license maintenance fees							$ 800					$ 800
License payment term							2031
Percentage of remaining maintenance fees outstanding							50.00%
Fair value of common stock issued							$ 1,600
Sublicense fees													9,100	4,300	$ 600
Accrued liabilities													$ 0	$ 200
UCSB \| Sublicense and Maintenance Fees
License Agreement [Line Items]
Research and development							$ 3,400

Commitments and Contingencies - Additional Information (Details)			12 Months Ended
	May 21, 2020 Defendant	Mar. 04, 2020 USD ($)	Dec. 31, 2020
Patent Infringement Lawsuit
Loss Contingencies [Line Items]
Lawsuit filed date		March 4, 2020
Name of plaintiff			Vytacera Bio, LLC
Loss contingency, answer, affirmative defenses and counterclaims filing date		May 26, 2020
Loss contingency, answer and counterclaims by plaintiff filing date		Jun. 05, 2020
Loss contingency claim amount \| $		$ 0
Securities Class Action Lawsuit
Loss Contingencies [Line Items]
Number of current and former officers named as defendants \| Defendant	3

Leases - Additional Information (Details)		12 Months Ended
Leases - Additional Information (Details)	Dec. 10, 2015 USD ($) ft²	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Operating Leased Assets [Line Items]
Restricted cash		$ 917,000	$ 917,000
Rent expense		5,100,000	4,800,000	$ 4,200,000
Letter of Credit
Operating Leased Assets [Line Items]
Letter of credit outstanding, amount		900,000
Restricted cash		$ 900,000	$ 900,000
New Lease Agreement
Operating Leased Assets [Line Items]
Area of rentable office and laboratory space \| ft²	76,000
Lease term description		The term of the Lease commenced on October 1, 2016. The 2016 Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the 2016 Lease.
Initial lease term	10 years
Extended lease term	5 years
Maximum one-time improvement allowance	$ 12,600,000
Improvement allowance from recoverable rent	2,300,000
New Lease Agreement \| First year of lease term
Operating Leased Assets [Line Items]
Annual lease rent	3,100,000
New Lease Agreement \| Second twelve months of lease term
Operating Leased Assets [Line Items]
Annual lease rent	4,300,000
New Lease Agreement \| Tenth year of lease term
Operating Leased Assets [Line Items]
Annual lease rent	$ 5,500,000

Leases - Summary of Supplemental Information Related to Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$ 4,990	$ 4,855
Supplemental balance sheet information related to leases:
Right-of-use asset	22,495	25,382
Current operating lease liabilities	3,195	2,810
Non-current operating lease liabilities	21,675	$ 24,871
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]		us-gaap:OtherLiabilitiesCurrent
Total operating lease liabilities	$ 24,870	$ 27,681
Weighted-average remaining lease term (in years)
Operating lease	5 years 9 months	6 years 10 months 6 days
Weighted-average discount rate
Operating lease	8.25%	8.25%

Leases - Schedule of Maturity of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Maturity of operating lease liabilities
2021	$ 5,129
2022	5,273
2023	5,420
2024	5,572
2025 and beyond	10,116
Total lease payments	31,510
Less imputed interest	(6,640)
Present value of lease liabilities	$ 24,870	$ 27,681

Common Stock - Additional Information (Details) - USD ($)		1 Months Ended		12 Months Ended
	Jul. 31, 2018	Dec. 31, 2020	Feb. 29, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2020	Oct. 14, 2015
Class Of Stock [Line Items]
Common stock, shares authorized		150,000,000		150,000,000	75,000,000		150,000,000	75,000,000
Common stock, par value		$ 0.00001		$ 0.00001	$ 0.00001			$ 0.00001
Preferred stock, shares authorized								10,000,000
Preferred stock, par value								$ 0.00001
Dividends on common stock				$ 0	$ 0
Proceeds from issuance of common stock, net of issuance costs				$ 11,288,000		$ 134,596,000
Jefferies LLC
Class Of Stock [Line Items]
Common stock, par value			$ 0.00001
Issuance of common stock in follow-on offering, net of issuance costs, shares		1,535,217
Proceeds from issuance of common stock, net of issuance costs		$ 11,300,000
Percentage of sales commission			3.00%
Average price		$ 7.62
Percentage of sales commission and issuance cost		3.00%		3.00%
Jefferies LLC \| Maximum
Class Of Stock [Line Items]
Proceeds from issuance of common stock, net of issuance costs			$ 75,000,000
Common Stock
Class Of Stock [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares				1,535,217	150,000	5,867,347
Proceeds from issuance of common stock, net of issuance costs	$ 134,600,000
Underwriting discounts and commissions and offering expenses	$ 9,200,000
Common Stock \| Underwritten Public Offering
Class Of Stock [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares	5,867,347
Common stock price, per share	$ 24.50
Common Stock \| Underwriters
Class Of Stock [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares	765,306

Common Stock - Reserved Shares of Common Stock Issuance on Converted Basis (Details) - shares	Dec. 31, 2020	Dec. 31, 2019
Equity [Abstract]
Options issued and outstanding	10,929,530	9,936,168
Shares available for future stock option grants	2,903,398	3,145,266
Shares available for future employee stock purchase plan	1,935,683	1,609,137
Total	15,768,611	14,690,571

Stock-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 01, 2016	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option expiration period		10 years
Common stock, capital shares reserved for future issuance		15,768,611	14,690,571
Aggregate intrinsic value of stock options exercised		$ 4,400	$ 900	$ 14,600
Weighted average grant date fair value, options granted		$ 3.94	$ 7.03	$ 14.21
Unrecognized compensation expense, options granted		$ 23,200
Number of shares issued during period		128,684	142,949
Stock-based compensation expense		$ 14,786	$ 19,100	$ 16,878
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation cost, period for recognition		2 years 9 months 18 days
Stock-based compensation expense		$ 14,700	$ 18,900	16,700
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of the shares on the date of grant		85.00%
Common stock, capital shares reserved for future issuance		1,935,683	1,609,137
Unrecognized stock-based compensation cost, period for recognition		5 months
Plan maximum percentage of eligible compensation to purchase shares		15.00%
Stock-based compensation expense		$ 400	$ 600	500
Unrecognized stock-based compensation cost		200
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 100	$ 200	$ 200
Vesting per Month After First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent		2.08%
2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option expiration period		10 years
Employees holding voting rights of all classes of stock, percentage		10.00%
Common stock, capital shares reserved for future issuance		2,698,798	1,808,066			2,444,735
Increase in number of shares available for issuance, as proportion of issued and outstanding shares of common stock, percentage	4.00%
2015 Plan \| Share Based Compensation Award Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest period		4 years
Options granted generally vest percent		25.00%
2019 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance		204,600	1,337,200		1,815,000
Early Exercise of Employee Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest period		36 months
Early Exercise of Employee Options \| Share Based Compensation Award Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent		25.00%
Minimum \| 2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of the shares on the date of grant		110.00%
Minimum \| 2019 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of the shares on the date of grant		100.00%
Maximum \| 2019 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option expiration period		10 years

Stock-based Compensation - Schedule of Activity Under Company's Stock Option Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Options Available for Grant
Balances, beginning of the period	3,145,266
Balances, end of the period	2,903,398	3,145,266
Number of Options
Balances, beginning of the period	9,936,168
Options granted	4,219,044
Options exercised	(1,064,830)
Options cancelled	(2,160,852)
Balances, end of the period	10,929,530	9,936,168
Options Exercisable, end of the period	6,145,652
Options vested and expected to vest, end of the period	10,929,530
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period	$ 12.26
Options granted	7.23
Options exercised	4.58
Options cancelled	13.75
Balances, end of the period	10.77	$ 12.26
Options Exercisable, end of the period	11.69
Options vested and expected to vest, end of the period	$ 10.77
Options Outstanding, Weighted- Average Remaining Contractual Life (years)
Balances, end of the period	6 years 4 months 24 days
Options Exercisable, end of the period	4 years 8 months 12 days
Options vested and expected to vest, end of the period	6 years 4 months 24 days
Aggregate Intrinsic Value
Options exercised	$ 4,400	$ 900	$ 14,600
Balances at December 31, 2020	6,844
Options Exercisable—December 31, 2020	6,031
Options vested and expected to vest—December 31, 2020	$ 6,844
Employee Stock Option
Options Available for Grant
Balances, beginning of the period	3,145,266
Options authorized	1,820,923
Options granted	(4,219,044)
Options cancelled	2,156,253
Balances, end of the period	2,903,398	3,145,266

Stock-based Compensation - Total Stock-based Compensation Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-based compensation expense:
Stock-based compensation expense	$ 14,786	$ 19,100	$ 16,878
Research and development
Stock-based compensation expense:
Stock-based compensation expense	6,825	9,226	8,313
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 7,961	$ 9,874	$ 8,565

Stock-based Compensation - Schedule of Estimated Fair Value of Employee Stock Options and ESPP Using Black-Scholes Valuation Model (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	64.40%	64.40%	65.60%
Expected volatility, maximum	73.30%	68.60%	69.30%
Risk-free interest rate, minimum	0.20%	1.40%	2.50%
Risk-free interest rate, maximum	1.30%	2.50%	3.00%
Employee Stock Option \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	4 years 8 months 12 days	4 years 10 months 24 days	4 years 8 months 12 days
Employee Stock Option \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	4 years 10 months 24 days	5 years	4 years 10 months 24 days
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	47.30%	60.80%	46.40%
Expected volatility, maximum	122.10%	71.90%	70.50%
Risk-free interest rate, minimum	0.10%	1.60%	2.10%
Risk-free interest rate, maximum	0.20%	2.40%	2.60%
Expected term (in years)	6 months	6 months	6 months

Income Taxes - Additional Information (Details) - USD ($)		12 Months Ended
	Mar. 27, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Net operating loss carryback	5 years
Percentage of taxable income from net operating losses for suspension of annual deduction limitation	80.00%
Deferred tax asset, net change in total valuation allowance increase during period		$ (2,700,000)	$ 22,700,000	$ 35,700,000
Unrecognized tax benefits		6,454,000	5,249,000	3,756,000	$ 4,320,000
Unrecognized tax benefits would affect company's effective tax rate		0	900,000	1,000,000.0
Income tax expense (benefit)		(13,911,000)	(427,000)	14,303,000
Interest and penalties accrued		0	$ 0	$ 0
Minimum
Income Tax Contingency [Line Items]
Percentage of change in ownership					50.00%
Federal
Income Tax Contingency [Line Items]
Net operating loss carryforwards		$ 168,600,000
Operating loss carryforwards expiration year		2034
Operating loss carryforwards to expire if not utilized		$ 65,600,000
Operating loss carryforwards not subject to expiration		$ 103,000,000
Open tax year		2010 2011 2012 2013 2014 2015 2016 2017 2018
Federal \| Research and Development Tax Credits
Income Tax Contingency [Line Items]
Tax credit carry forwards		$ 15,600,000
Tax credit carryforward, expiration		2031
State
Income Tax Contingency [Line Items]
Net operating loss carryforwards		$ 24,200,000
Operating loss carryforwards expiration year		2032
Open tax year		2010 2011 2012 2013 2014 2015 2016 2017 2018
State \| Research and Development Tax Credits
Income Tax Contingency [Line Items]
Tax credit carry forwards		$ 9,000,000.0

Income Taxes - Schedule of Components of Provision for (Benefit from) Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ (13,912)	$ (390)	$ 14,302
State	1		1
Total current	(13,911)	(390)	14,303
Deferred:
Federal		(37)
Total deferred		(37)
Provision for (benefit from) income taxes	$ (13,911)	$ (427)	$ 14,303