Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2016	May. 04, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q1
Trading Symbol	CTMX
Entity Registrant Name	CytomX Therapeutics, Inc.
Entity Central Index Key	0001501989
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		36,087,768

CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 67,897	$ 59,822
Short-term investments	112,739	126,889
Accounts receivable	316	372
Related party accounts receivable	158	372
Prepaid expenses and other current assets	3,438	2,299
Total current assets	184,548	189,754
Property and equipment, net	3,479	3,481
Intangible assets	1,750	1,750
Goodwill	949	949
Restricted cash	917	917
Other assets	309	364
Total assets	191,952	197,215
Current liabilities:
Accounts payable	4,958	4,697
Accrued liabilities	4,738	4,912
Deferred revenues, current portion	7,307	6,130
Total current liabilities	17,003	15,739
Deferred revenue, net of current portion	61,725	54,703
Deferred tax liability	510	507
Other long-term liabilities		198
Total liabilities	$ 79,238	$ 71,147
Commitments and contingencies (Note 11)
Preferred stock, $0.00001 par value; 10,000,000 shares authorized at March 31, 2016; no shares issued and outstanding at March 31, 2016 and December 31, 2015, respectively
Common stock, $0.00001 par value; 75,000,000 authorized at March 31, 2016; 36,085,809 and 36,033,209 shares issued and outstanding at March 31, 2016 and December 31, 2015, respectively	$ 1	$ 1
Stockholders notes receivable	(78)	(78)
Additional paid-in capital	246,256	243,687
Accumulated other comprehensive income / (loss)	30	(76)
Accumulated deficit	(133,495)	(117,466)
Total stockholders' equity	112,714	126,068
Total liabilities and stockholders' equity	$ 191,952	$ 197,215

CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.00001	$ 0.00001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	36,085,809	36,033,209
Common stock, shares outstanding	36,085,809	36,033,209

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Revenues	$ 1,783	$ 1,395
Revenues from related parties	440	347
Total revenues	2,223	1,742
Operating expenses:
Research and development	13,365	4,664
General and administrative	5,040	1,946
Total operating expenses	18,405	6,610
Loss from operations	(16,182)	(4,868)
Interest income	490	138
Interest expense	(353)	(230)
Other income (expense), net	19	(1,251)
Loss before provision for income taxes	(16,026)	(6,211)
Provision for income taxes	3
Net loss	(16,029)	(6,211)
Accretion to redemption value and cumulative dividends on preferred stock		(1,432)
Net loss attributable to common stockholders	$ (16,029)	$ (7,643)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.44)	$ (7.67)
Shares used to compute net loss per share attributable to common stockholders, basic and diluted	36,063,425	996,551
Other comprehensive loss:
Changes in unrealized gains / (losses) on short-term investments	$ 106	$ (10)
Total other comprehensive gain / (loss)	106	(10)
Comprehensive loss	$ (15,923)	$ (6,221)

CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	Mar. 31, 2016	Mar. 31, 2015
Cash flows from operating activities:
Net loss	$ (16,029)	$ (6,211)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	355	268
Amortization of debt discount		13
Accretion of discount on short-term investments	350	124
Stock-based compensation expense	2,362	175
Issuance of stock in connection with services	159
Deferred income taxes	3
Changes in operating assets and liabilities
Accounts receivable	56	2
Related party accounts receivable	214	1,663
Prepaid expenses and other current assets	(1,139)	(410)
Other assets	55	22
Accounts payable	246	(567)
Accrued liabilities	(371)	(169)
Deferred revenue	8,199	(1,532)
Net cash used in operating activities	(5,540)	(5,369)
Cash flows from investing activities:
Purchases of property and equipment	(327)	(255)
Purchases of short-term investments	(33,844)	(51,788)
Maturities of short-term investments	47,750
Net cash provided by / (used in) investing activities	13,579	(52,043)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		1,191
Proceeds from exercise of stock options	48
Repayment of notes payable		(356)
Payment of deferred offering costs	(12)
Net cash provided by financing activities	36	835
Net increase (decrease) in cash and cash equivalents	8,075	(56,577)
Cash and cash equivalents, beginning of period	59,822	64,396
Cash and cash equivalents, end of period	67,897	7,819
Supplemental disclosures of cash flow information:
Cash paid for interest		84
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	$ 26	463
Accretion to redemption value and cumulative dividends on preferred stock		1,432
Convertible Preferred Stock Liability
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of liability		1,031
Convertible Preferred Stock Warrant Liability
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of liability		$ 222

Description of the Business	3 Months Ended
Description of the Business	Mar. 31, 2016
Disclosure Text Block [Abstract]
Description of the Business	1. Description of the Business CytomX Therapeutics, Inc. (the “Company”) is an oncology-focused biopharmaceutical company focused on developing Probody therapeutics for the treatment of cancer. Probody therapeutics are masked antibodies that remain inert in healthy tissue but are activated specifically in the disease microenvironment. The Company is located in South San Francisco, California and was incorporated in the state of Delaware in September 2010. Initial Public Offering On October 7, 2015, the Company’s registration statement on Form S-1 (File No. 333-206658) relating to its initial public offering (“IPO”) of its common stock was declared effective by the Securities and Exchange Commission (“SEC”) and the shares of its common stock began trading on The NASDAQ Global Select Market on October 8, 2015. The public offering price of the shares sold in the IPO was $12.00 per share. The IPO closed on October 14, 2015, pursuant to which the Company sold 7,666,667 shares of common stock, including the sale of 1,000,000 shares of common stock to the underwriters upon their exercise of their option to purchase additional shares. The Company received net proceeds of approximately $81.8 million, after underwriting discounts, commissions and offering expenses. Immediately prior to the consummation of the IPO, all outstanding shares of convertible preferred stock and redeemable convertible preferred stock converted into common stock. Reverse Stock Split On October 2, 2015, the Company effected a one-for-62.997 reverse stock split of the Company’s issued and outstanding shares of common stock, redeemable convertible preferred stock and convertible preferred stock. The par values of the common stock, redeemable convertible preferred stock and convertible preferred stock were not adjusted as a result of the reverse split. All authorized and issued and outstanding shares of common stock, redeemable convertible preferred stock and convertible preferred stock and per share amounts contained in the accompanying condensed financial statements have been retroactively adjusted to reflect this reverse stock split for all periods presented.

Liquidity	3 Months Ended
Liquidity	Mar. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Liquidity	2. Liquidity Since inception, the Company has incurred recurring net operating losses. As of March 31, 2016 and December 31, 2015, the Company had an accumulated deficit of $133.5 million and $117.5 million, respectively, and expects to incur losses for the foreseeable future. Since its inception, the Company has funded its operations primarily through sales of its common stock in conjunction with the IPO, sales of convertible preferred securities and payments received under its collaboration agreements. As of March 31, 2016 and December 31, 2015, the Company had cash, cash equivalents and short-term investments of $180.6 million and $186.7 million, respectively. In May and June 2015, the Company received aggregate net proceeds of $73.2 million from the issuance of its Series C and Series D redeemable convertible preferred stock. In October 2015, the Company consummated its IPO and raised net proceeds of approximately $81.8 million, after deducting underwriting discounts and commissions and offering expenses.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2016

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

3. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying condensed financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). The Company’s functional and reporting currency is the U.S. dollar.

Unaudited Interim Financial Information

The accompanying interim condensed financial statements and related disclosures are unaudited, have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair statement of the results of operations for the periods presented.

The year-end condensed balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. The condensed results of operations for the three months ended March 31, 2016 are not necessarily indicative of the results to be expected for the full year or for any other future year or interim period. The accompanying condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2015 filed with the SEC.

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 that management believes is of high credit quality. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents in highly rated money market funds and its short-term investments in U.S. Government Bonds.

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

	Revenue						Accounts Receivable, net
	Three Months Ended March 31,						March 31,			December 31,
	2016			2015			2016			2015
Customer A		80	%		80	%		67	%		50	%
Customer B		20	%		20	%		33	%		50	%

All of the Company’s customers are located in the United States of America.

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, who is the Company’s chief operating decision maker. 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.

Short-term Investments

All investments have been classified as “available-for-sale” and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Those investments with contractual maturities greater than 12 months at the date of purchase are considered long-term investments. Unrealized gains and losses, deemed temporary in nature, are reported as a component of accumulated other comprehensive income (loss), net of tax.

A decline in the fair value of any security below cost that is deemed other than temporary results in a charge to earnings and the corresponding establishment of a new cost basis for the security. Premiums (discounts) are amortized (accreted) over the life of the related security as an adjustment to yield using the straight-line interest method. 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.

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 (“IPR&D”). The Company assesses impairment indicators annually 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 three months ended March 31, 2016 and the year ended December 31, 2015.

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 periods presented in these condensed financial statements.

Convertible Preferred Stock Warrant Liability

Freestanding warrants for shares that are contingently redeemable are classified as liabilities on the balance sheet at their estimated fair value because the shares underlying the warrants may obligate the Company to transfer assets to the holders at a future date under certain circumstances such as a deemed liquidation event. The warrants are subject to re-measurement at each balance sheet date and the change in fair value, if any, is included in other income (expense), net. The Company adjusted the liability for changes in fair value until the consummation of its IPO in October 2015, at which time all convertible preferred stock warrants were net exercised into shares of common stock and the related convertible preferred stock warrant liability was reclassified to additional paid-in capital.

Convertible Preferred Stock Liability

The obligation to issue additional shares of Series B-1 and Series C redeemable convertible preferred stock at a future date was determined to be a freestanding instrument that should be accounted for as a liability. At initial recognition, the Company recorded the convertible preferred stock liability on the balance sheets at its estimated fair value. The liability is subject to remeasurement at each balance sheet date, with changes in fair value recognized as a component of other income (expense), net. At the time of each funding, the Company remeasured the liability, with the change in fair value recognized as a component of other income (expense), net and then reclassifies the fair value associated with the convertible preferred stock liability to the applicable series of redeemable convertible preferred stock. Immediately prior to the consummation of the Company’s IPO in October 2015, the convertible preferred stock converted to 27,135,453 shares of common stock.

Comprehensive Gain and Loss

Comprehensive gain and loss represents all changes in stockholders’ deficit except those resulting from distributions to stockholders. The Company’s unrealized gains and losses on short-term investments represent the only component of other comprehensive loss that is excluded from the reported net loss.

Revenue Recognition

The Company recognizes revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; transfer of technology has been completed or services have been rendered; the price to the customer is fixed or determinable and collectability is reasonably assured.

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 to the Company’s technology, (ii) research and development services, and (ii) 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.

In arrangements involving the delivery of more than one element, each required deliverable is evaluated to determine whether it qualifies as a separate unit of accounting. The determination is based on whether the deliverable has “standalone value” to the customer. If a deliverable does not qualify as a separate unit of accounting, it is combined with the other applicable undelivered item(s) within the arrangement and these combined deliverables are treated as a single unit of accounting.

The arrangement’s consideration that is fixed or determinable is allocated to each separate unit of accounting based on the relative selling price methodology in accordance with the selling price hierarchy, which includes vendor-specific objective evidence (“VSOE”) of selling price, if available, or third-party evidence of selling price if VSOE is not available, or the best estimate of selling price, if neither VSOE nor third-party evidence is available.

Payments or reimbursements for the Company’s research and development efforts for the arrangements where such efforts are considered as deliverables are recognized as the services are performed and are presented on a gross basis. When upfront payments are received and if there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, the Company recognizes revenue ratably over the associated period of performance.

The Company’s collaboration and license agreements may include contingent payments related to specified research, development and regulatory milestones and sales-based milestones. Such 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, upon receipt of actual marketing approvals of a covered product or for additional indications, or upon the first commercial sale of a covered product. Sales-based milestones are typically payable when annual sales of a covered product reach specified levels. Each contingent and milestone payment is evaluated to determine whether it is substantive and at risk to both parties. The Company recognizes any payment that is contingent upon the achievement of a substantive milestone entirely in the period in which the milestone is achieved. Any payments that are contingent upon achievement of a non-substantive milestone are recognized as revenue prospectively, when such payments become due and collectible, over the remaining expected performance period under the arrangement, which is generally the remaining period over which the research and development services are expected to be provided.

Research and Development Expenses

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.

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 straight-line method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. As such, the Company’s stock-based compensation is reduced for the estimated forfeitures at the date of grant and revised in subsequent periods if actual forfeitures differ from those estimates.

Stock-based compensation expense for options granted to non-employees as consideration for services received is 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. Compensation expense for options granted to non- employees is periodically remeasured as the underlying options vest.

Income Taxes

The Company accounts for income taxes under the liability method which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses and research and development credit carryforwards. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized.

The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.

Net Loss per Share Attributable to Common Stockholders

Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, without consideration of potentially dilutive securities. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since the effect of potentially dilutive securities is anti-dilutive.

Recent Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers, which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The ASU will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. The new standard will be effective for the Company on January 1, 2018, which is the effective date for public companies. Early application is permitted as of January 1, 2017. The standard permits the use of either the retrospective or cumulative effect transition method. Additionally, in March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), which clarifies the implementation guidance on principal versus agent considerations in ASU 2014-09. The Company is evaluating the effect that ASU 2014-09 will have on its financial statements and related disclosures. The Company has not yet selected a transition method nor has it determined the effect of the standard on its ongoing financial reporting.

In August 2014, the FASB issued Accounting Standards Update No. 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern. This standard update provides guidance around management’s responsibility to evaluate whether there is substantial doubt about an entity’s ability to continue as a going concern and to provide related footnote disclosures. The new guidance is effective for all annual and interim periods ending after December 15, 2016. The Company does not believe that adopting ASU 2014-15 will have a material impact on its financial statements.

In 2015, the FASB issued new guidance related to balance sheet classification of deferred taxes. The new guidance requires that deferred tax assets and liabilities be classified as noncurrent in a classified statement of financial position. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. As the Company’s deferred tax balance is already classified as noncurrent, the adoption of this new guidance had no financial statement impact.

In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)” (“ASU 2016-02”). Under ASU 2016-2, an entity will be required to recognize right-of-use assets and lease liabilities on its balance sheet and disclose key information about leasing arrangements. ASU 2016-02 offers specific accounting guidance for a lessee, a lessor and sale and leaseback transactions. Lessees and lessors are required to disclose qualitative and quantitative information about leasing arrangements to enable a user of the financial statements to assess the amount, timing and uncertainty of cash flows arising from leases. For public companies, ASU 2016-02 is effective for annual reporting periods beginning after December 15, 2018, including interim periods within that reporting period, and requires a modified retrospective adoption, with early adoption permitted. The Company plans to adopt this guidance beginning with its first quarter ending March 31, 2019. The Company is in the process of evaluating the future impact of ASU 2016-02 on its financial statements.

In March 2016, the FASB issued ASU No. 2016-09, “Compensation – Stock Compensation (Topic 718)” (“ASU 2016-09”). ASU 2016-09 simplifies several aspects of accounting for share-based payment award transactions, including income tax consequences, classification of awards as either equity or liabilities and classification on the statement of cash flows. ASU 2016-09 is effective for fiscal years, and interim periods within those years, beginning after December 15, 2016, with early adoption permitted. The Company is in the process of assessing the impact of adoption of ASU 2016-09 of its financial statements.

Fair Value Measurements and Short-term Investments

3 Months Ended

Mar. 31, 2016

Fair Value Disclosures [Abstract]

Fair Value Disclosures

4. Fair Value Measurements and Short-Term Investments

The Company records its financial assets and liabilities at fair value. The accounting guidance for fair value provides a framework for measuring fair value, clarifies the definition of fair value, and expands disclosures regarding fair value measurements. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The accounting guidance establishes a three-tiered hierarchy, which prioritizes the inputs used in the valuation methodologies in measuring 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 and II assets and Level III liabilities. Level I assets consist primarily of highly liquid money market funds, some of which are included in restricted cash. The Company’s Level II assets consist of U.S. government bonds that are included in short-term investments. The Company’s Level III liabilities include the convertible preferred stock warrant liability and the convertible preferred stock liability. The determination of the fair value of the convertible preferred stock warrant liability is discussed in Note 10. The determination of the fair value of the convertible preferred stock liability is discussed in Note 12.

The following tables set forth the fair value of the Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements (in thousands):

	March 31, 2016
	Level I		Level II		Level III		Total
Assets
Money market funds	$	58,547	$	—	$	—	$	58,547
Restricted cash (money market funds)		917		—		—		917
U.S. Government bonds		—		112,739		—		112,739
	$	59,464	$	112,739	$	—	$	172,203

	December 31, 2015
	Level I		Level II		Level III		Total

Assets
Money market funds	$	44,714	$	—	$	—	$	44,714
Restricted cash (money market funds)		917		—		—		917
U.S. Government bonds		—		140,392				140,392
	$	45,631	$	140,392	$	—	$	186,023

The following is a summary of the gross unrealized gains and losses on the Company’s short-term investments (in thousands):

	March 31, 2016
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value

Investment Securities
U.S. Government bonds		112,709		31		(1	)		112,739
Total securities	$	112,709	$	31	$	(1	)	$	112,739

The contractual maturities of securities classified as available-for-sale as of March 31, 2016 were as follows (in thousands):

	March 31, 2016
Due within one year		112,739
Total	$	112,739

- Definition

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

3 Months Ended

Mar. 31, 2016

Property Plant And Equipment [Abstract]

Property and Equipment

5. Property and Equipment

Property and equipment, net consisted of the following (in thousands):

	March 31,			December 31,
	2016			2015

Machinery and equipment	$	5,153		$	4,910
Computer equipment and software		506			452
Furniture and fixtures		51			51
Leasehold improvements		720			720
Construction in progress		225			169
		6,655			6,302
Less: accumulated depreciation and amortization		(3,176	)		(2,821	)
	$	3,479		$	3,481

Depreciation and amortization expense was $355,000 and $268,000 for the three months ended March 31, 2016 and 2015, respectively.

Goodwill and Intangible Assets

3 Months Ended

Mar. 31, 2016

Goodwill And Intangible Assets Disclosure [Abstract]

Goodwill and Intangible Assets

6. Goodwill and Intangible Assets

Goodwill and in-process research and development assets result from a series of integrated financing transactions in 2010 that was accounted for as a business combination. The in-process research and development relates to the Company’s proprietary Probody technology platform and is accounted for as an indefinite-lived intangible asset until the underlying project is completed or abandoned.

Goodwill and intangible assets consisted of the following (in thousands):

	March 31,		December 31,
	2016		2015
Goodwill	$	949	$	949
In-process research and development		1,750		1,750

- Definition

The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final. May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.

+ References + Details

Accrued Liabilities

3 Months Ended

Mar. 31, 2016

Accrued Liabilities Current [Abstract]

Accrued Liabilities

7. Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	March 31,		December 31,
	2016		2015
Research and clinical expenses	$	2,371	$	1,562
Payroll and related expenses		1,227		2,839
Legal and professional expenses		854		296
Other accrued expenses		286		215
Total	$	4,738	$	4,912

Research and Collaboration Agreements	3 Months Ended
Research and Collaboration Agreements	Mar. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Research and Collaboration Agreements	8. Research and Collaboration Agreements 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 Probody drug conjugates (“PDCs”) for research project targets nominated by Pfizer. Pfizer nominated two research targets in 2013 and had the option of nominating two additional research targets. In December 2014, Pfizer selected an additional research target. The Pfizer Agreement provides Pfizer with an option to acquire an exclusive development and commercialization license for each research project target. Upon exercise of the option, Pfizer (1) will receive an exclusive development and commercialization license for use of the Probody therapeutic during the development, manufacturing and commercialization of the potential product, and (2) will be responsible for the development, manufacturing and commercialization of such potential products. Pursuant to the Pfizer Agreement, the Company received an upfront payment of $6 million and is entitled to contingent payments of up to an aggregate of $626.5 million as follows: (i) $1.5 million for each of the two additional targets; (ii) up to $12.0 million upon exercise of the license options, (iii) up to $25.0 million from the achievement of development milestones for each research target program, or up to $82.0 million if the maximum of four research targets are selected by Pfizer; and (iv) up to $98.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program or up to $249.5 million if the maximum of four research targets are selected and (v) up to $100.0 million in sales milestones payments per research target program, or up to $280.0 million if the maximum of four research targets are selected by Pfizer. The Company is entitled to receive royalties in the mid-single digits to low teens on initial targets and mid-single digit royalties on additional targets from potential future sales of product candidates. The Company will also receive research and development service fees based on a prescribed full-time employee (“FTE”) rate per year that is capped. In accordance with ASC 605-25, the Company identified the following deliverables at the inception of the Pfizer Agreement: (1) the research license, (2) the research services and (3) the obligation to participate in the joint research committee. The Company determined that the research license does not have stand-alone value to Pfizer due to specialized nature of the research services to be provided by the Company, and accordingly, this deliverable was combined with the research services and participation in the joint research committee as a single unit of accounting. The Company concluded that, at the inception of the agreement, Pfizer’s options to obtain an exclusive development and commercialization license for each research project target do not represent deliverables of the agreement because they are substantive options and do not contain a significant or incremental discount. The upfront payment of $6.0 million was recorded as deferred revenue and is being recognized on a ratable basis over the estimated performance period of seven years. In December 2014, Pfizer selected an additional target and paid $1.5 million, which was recorded as deferred revenue and will be recognized over the remaining performance period. During the three months ended March 31, 2016 and 2015, the Company recognized revenue of $0.4 million and $0.3 million respectively. As of March 31, 2016 and December 31, 2015, deferred revenue relating to the Pfizer Agreement was $4.7 million and $4.9 million, respectively. The amount due from Pfizer under the Agreement was $0.2 million and $0.4 million as of March 31, 2016 and December 31, 2015, respectively. ImmunoGen, Inc. In January 2014, the Company and ImmunoGen, Inc. (“ImmunoGen”) entered into the Research Collaboration Agreement (the “ImmunoGen Agreement”). The ImmunoGen Agreement provides the Company with the right to use ImmunoGen’s Antibody Drug Conjugate (“ADC”) technology in combination with the Company’s Probody technology to create Probody Drug Conjugates (“PDC”) 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 PDCs. 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 technology to create PDCs directed at two targets under the research license and to subsequently obtain exclusive, worldwide development and commercialization licenses to develop and commercialize such PDCs. Under the research licenses, the parties have one replacement right for each target, which needs to be made before the third anniversary of the agreement execution. Under the terms of the agreement, both the Company and ImmunoGen are required to perform research activities on behalf of the other party for no monetary consideration. The research activities for a particular target will last until January 2018 unless they are terminated by one of the parties or when a development and commercialization license is obtained with respect to that target. Each party is solely responsible for the development, manufacturing and commercialization of any products resulting from the exclusive development and commercialization license obtained by such party under the agreement. Each party may be liable to pay annual maintenance fees to the other party if the licensed product candidate covered under each development and commercialization license has not progressed to the clinical stage of development within six years of the exercise of the development and commercialization license. In consideration for the exclusive development and commercialization license that may be obtained by ImmunoGen, the Company is entitled to receive up to $30.0 million in development and regulatory milestone payments per the research program target, up to $50.0 million in sales milestone payments per target and royalties in the mid-single digits on the commercial sales of any resulting product. For the development and commercialization license that may be obtained by the Company, ImmunoGen is entitled to receive up to $60.0 million in development and regulatory milestone payments, up to $100.0 million in sales milestone payments and royalties in the mid to high single digits on the commercial sales of any resulting product. The Company accounted for the ImmunoGen Agreement based on the fair value of the assets and services exchanged. The Company identified the following significant deliverables at the inception of the ImmunoGen Agreement: (1) the research license, (2) the research services, (3) the obligation to participate in the joint research committee, (4) the exclusive research, development and commercialization license and (5) the obligation to provide future technology improvements, when available. The Company determined that the research license, participation in the joint steering committee and the research services do not have stand-alone value from the development and commercialization license and therefore those deliverables were combined into one unit of accounting. The Company considered factors such the limited economic benefits to ImmunoGen if development and commercialization license is not obtained and the lack of sublicensing rights in the research license. The estimated total fair value of the consideration of $13.2 million was recorded as deferred revenue, of which $13.0 million was allocated to the unit of accounting comprised of the research license, research services, participation in the joint research committee and the development and commercialization license, and $0.2 million was allocated to the future technological improvements. The Company will recognize $13.0 million upon delivery of development and commercialization licenses and will recognize amount allocated to the future technology improvements over the term of the license. The estimated fair value of assets and services received was also $13.2 million, of which $12.7 million was allocated to the licenses received and was charged to research and development expense, with the remaining amount of $0.5 million was allocated to the research services, joint research committee participation and technology improvements, which is being expensed over the period of services to be provided. Bristol-Myers Squibb Company On May 23, 2014, the Company and Bristol-Myers Squibb Company (“BMS”) 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 technology. The effective date of the BMS Agreement was July 7, 2014. Under the terms of the BMS Agreement, the Company granted BMS exclusive worldwide rights to develop and commercialize Probody therapeutics for up to four oncology targets. BMS will have additional rights to substitute up to two collaboration targets. Each collaboration target has a two year research term and the two additional targets must be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target can be extended in one year increments up to three times. Pursuant to the BMS Agreement, the financial consideration from BMS was comprised of an upfront payment of $50.0 million and contingent payments of up to an aggregate of $1,217.0 million as follows: (i) up to $25.0 million for additional targets; (ii) up to $114.0 million in development milestone payments per research target program or up to $456.0 million if the maximum of four research targets are selected; (iii) up to $124.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program or up to $496.0 million if the maximum of four research targets are selected, and (iv) up to $60.0 million in sales milestones payments per research target program or up to $240.0 million if maximum of four research targets are selected. The Company is entitled to royalty payments in the mid to high single digits to low teens from potential future sales. The Company will also receive research and development service fees based on a prescribed FTE rate that is capped. The BMS Agreement also provides the Company to sell to BMS the Company’s common stock upon an IPO. In connection with the IPO in October 2015, BMS purchased 833,333 shares of the Company’s common stock at the initial public offering price and on the same terms as other purchasers in the offering. The Company identified the following deliverables at the inception of the BMS Agreement: (1) the exclusive research, development and commercialization license (“license”), (2) the research and development services and (3) the obligation to participate in the joint research committee. The Company determined that the license does not have stand-alone value to BMS without the Company’s research services and expertise related to the development of the product candidates, and accordingly, it was combined with the research services and participation in the joint research committee as a single unit of accounting. The Company received an upfront payment of $50.0 million from BMS in July 2014. The upfront payment was recorded as deferred revenue and being recognized on a ratable basis over the estimated performance period of ten years. The Company determined that the remaining contingent payments under the Agreement do not constitute substantive milestones and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments do not meet the definition of a substantive milestone because the achievement of these events solely depends on BMS’s performance. Accordingly, any revenue from these contingent payments would be subject to an allocation of arrangement consideration and would be recognized over any remaining period of performance obligations, if any, relating to this arrangement. If there are no remaining performance obligations under the arrangement at the time the contingent payment is triggered, the contingent payment will be recognized as revenue in full upon the triggering event. In January 2016, BMS selected an additional target pursuant to the BMS Agreement. Under the terms of the BMS Agreement, BMS paid the Company a $10 million milestone payment. This amount has been recorded as deferred revenue and will be recognized over the remaining performance period. During the three months ended March 31, 2016 and 2015, the Company recognized revenue of $1.8 million and $1.4 million, respectively. As of March 31, 2016 and December 31, 2015, deferred revenue relating to the BMS Agreement was $51.1 million and $42.6 million, respectively. The amount due from BMS under the BMS Agreement was $0.3 million and $0.4 million as of March 31, 2016 and December 31, 2015, respectively. MD Anderson In November 2015, the Company entered into a research collaboration agreement with MD Anderson to research Probody-enabled chimeric antigen receptor killer (CAR-NK) cell therapies, known as ProCAR-NK cell therapies. Under this collaboration, MD Anderson will use the Company’s Probody technology to conduct research of ProCAR-NK cell therapies against certain targets selected by the Company in cancer immunotherapy. Under the research collaboration agreement, the Company has the right to exercise an option, during the option period expiring on November 2, 2019 and upon payment of an option exercise fee, to negotiate and acquire a worldwide, exclusive, sublicensable license from MD Anderson for development and commercialization of products directed against any of the selected targets. The research collaboration agreement will continue in effect until the earlier of (i) the date that the Company exercises the option to acquire the license from MD Anderson and (ii) the expiration of the option period. The impact of this agreement was not material for the financial statements for the three months ended March 31, 2016.

License Agreement

3 Months Ended

Mar. 31, 2016

Research And Development [Abstract]

License Agreement

9. License Agreement

The Company has an exclusive, worldwide license agreement (the “UC Agreement”) with the Regents of the University of California (the “UC Regents”) relating to the use of certain patents and technology relating to its core technology, including its therapeutic antibodies. Pursuant to the UC Agreement, the Company is obligated to (i) make royalty payments to the UC Regents on net sales of its products covered under the agreement, subject to annual minimum amounts,(ii) make milestone payments to the UC Regents upon the occurrence of certain events, (iii) make a milestone payment to the UC Regents upon occurrence of an IPO or change of control, and (iv) reimburse the UC Regents for prosecution and maintenance of the licensed patents. If the Company sublicenses its rights under the UC Agreement, it is obligated to pay the UC Regents a percentage of the total gross proceeds received in consideration of the grant of the sublicense, which total amount would be first reduced by the aggregate amount of certain research and development related expenses incurred by the Company.

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

In the three months ended March 31, 2016 and 2015, the Company incurred expenses of $537,000 and $173,000, respectively, to the UC Regents under the provisions of the UC Agreement.

Royalty obligations

The Company has future minimum royalty obligations due under the terms of certain exclusive licensed patent rights. These minimum future obligations are as follows (in thousands):

Year ended December 31,
2016 (nine months remaining)	$	—
2017		150
Total minimum royalty obligations	$	150

Long-term Debt	3 Months Ended
Long-term Debt	Mar. 31, 2016
Debt Disclosure [Abstract]
Long-term Debt	10. Long-term Debt In May 2012, the Company entered into a Master Loan and Security Agreement (the “Debt Facility”). Under the terms of the agreement, an aggregate of $2.0 million could be drawn down during the initial basic loan term of 42 months. In January and December 2013, the Company amended the Debt Facility to borrow an additional $0.3 million and $3.0 million, respectively, with similar terms. Borrowings under the debt facility bear interest at 11.74% per annum. The Company’s obligations under the Debt Facility are collateralized by a security interest in substantially all of its assets, excluding its intellectual property and certain other assets. The Debt Facility also contains customary conditions related to borrowing, events of default, and covenants, including covenants limiting the Company’s ability to dispose of assets, undergo a change in control, merge with or acquire other entities, incur debt, incur liens, pay dividends or other distributions to holders of its capital stock, repurchase stock and make investments, in each case subject to certain exceptions. The agreement also allows the lender to call the debt in the event there is a material adverse change in the Company’s business or financial condition. In connection with the execution and the amendment of the Debt Facility, the Company issued warrants to the lender to purchase an aggregate of 81,620 shares of the Company’s Series B-1 redeemable convertible preferred stock. The warrants were exercisable in cash at an exercise price of $3.084396 per share or through a cashless exercise provision. In connection with the consummation of the IPO in October 2015, all of the warrants were net exercised, resulting in issuance of an aggregate of 60,640 shares of our common stock. Upon issuance of the warrants, the Company recorded a preferred stock warrant liability based on its initial fair value estimated using the Black-Scholes model with an offset to debt discount. The debt discount is amortized to interest expense using the effective interest method over the term of the Debt Facility. The warrant liability is subject to remeasurement to fair value at each balance sheet date until the earliest of the exercise or expiration of the convertible preferred stock warrant, and any change in fair value is recognized in other income (expense), net. The Company repaid and terminated the Debt Facility in September 2015.

Commitments and Contingencies

3 Months Ended

Mar. 31, 2016

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

Operating Lease

New Lease Agreement

On December 10, 2015, the Company entered into a lease (the “New Lease”) with HCP Oyster Point III LLC (the “Landlord”) to lease approximately 76,173 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 New Lease commences on the later of (i) the date that the Landlord’s construction and tenant improvements have been completed pursuant to the New Lease and (ii) October 1, 2016. The New 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 New Lease.

The New Lease provides 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 will be 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 will be entitled to a one-time improvement allowance of up to $12.6 million.

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 New Lease. The Company has recorded the $0.9 million Letter of Credit in restricted cash as a non-current asset on its balance sheet.

Amendment to Current Lease

In March 2016, the Company entered into an agreement to terminate the current lease, which was due to expire on January 31, 2019 (“Lease Termination”) with its current landlord. The Lease Termination provides for an early termination of the current lease effective on November 30, 2016. The Company will not be required to pay the landlord a termination payment in connection with the early termination of the lease.

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 as a deferred rent liability.

The minimum lease payments for all the Company’s facility leases are as follows (in thousands):

Year Ending December 31:
2016 (nine months remaining)	$	1,053
2017		3,387
2018		4,374
2019		4,506
2020		34,144
Total	$	47,464

Rent expense during the three months ended March 31, 2016 was a credit of $2,000 due to a one-time adjustment to deferred rent pursuant to the termination of the current lease. Rent expense during the three months ended March 31, 2015 was $235,000.

Legal Proceedings

The Company is subject to claims and assessments from time to time in the ordinary course of business but do not believe that any such matters, individually or in the aggregate, will have a material adverse effect on the Company’s financial position, results of operations or cash flows.

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.

Convertible Preferred Stock	3 Months Ended
Convertible Preferred Stock	Mar. 31, 2016
Equity [Abstract]
Convertible Preferred Stock	12. Convertible Preferred Stock In December 2014, the Company granted a second tranche option (“Second Tranche Option”) to one of its investors to purchase 659,209 shares of its Series C redeemable convertible preferred stock upon the achievement of certain milestones. At initial recognition, the Company recorded the Second Tranche Option as a derivative liability on the balance sheet at its estimated fair value of $395,000. In May 2015, the Company achieved the relevant milestones and the investor exercised their right to purchase 659,209 shares of Series C convertible redeemable preferred stock for net proceeds of $3.5 million. Immediately prior to the closing of this tranche, the Company remeasured the preferred stock liability to its then fair value and recorded a loss from remeasurement of $1.1 million in other income (expense), net. The fair value of the preferred stock liability in the amount of $1.5 million was reclassified to redeemable convertible preferred stock. In connection with the consummation of the IPO in October 2015, all outstanding shares of Series A-1, Series A-2, Series B-1, Series B-2, Series C and Series D were converted into 27,135,453 shares of common stock on a one-for-one basis. As such, no convertible preferred stock shares were outstanding as of March 31, 2016 and December 31, 2015.

Common Stock

3 Months Ended

Mar. 31, 2016

Equity [Abstract]

Common Stock

13. Common Stock

In October 2015, the Company’s board of directors and stockholders approved the Company’s amended and restated certificate of incorporation. The amended and restated certificate of incorporation was effective as of October 14, 2015, and provides for 75,000,000 authorized shares of common stock with a par value of $0.00001 per share and 10,000,000 shares of preferred stock with a par value of $0.00001 per share.

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 March 31, 2016 and December 31, 2015, no dividends on common stock had been declared by the Board of Directors.

The Company had reserved shares of common stock for issuance, on an as-converted basis, as follows:

	March 31, 2016		December 31, 2015
Options issued and outstanding		6,125,654		5,270,751
Shares available for future stock option grants		2,927,926		2,401,406
		9,053,580		7,672,157

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(d),(e))
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Section S99
-Paragraph 2
-Subparagraph (SAB TOPIC 4.E)
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

Reference 5: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29-31)
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Reference 6: http://www.xbrl.org/2003/role/presentationRef
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
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Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Subparagraph (SX 210.3-04)
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Reference 9: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
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Reference 10: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section E

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section C

+ Details

Stock Option Plans

3 Months Ended

Mar. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock Option Plans

14. Stock Option Plans

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”). In conjunction with adopting the 2015 Plan, the Company discontinued the 2011 Plan with respect to new equity awards.

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 shares 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 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.

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


	Options Outstanding
	Number of Options			Weighted-Average Exercise Price Per Share
Balances at December 31, 2015		5,270,751		$	3.694
Options granted		905,246			14.459
Options exercised		(41,600	)		1.116
Option forfeited		(8,743	)		3.904
Balances at March 31, 2016		6,125,654		$	5.302
Options exercisable at March 31, 2016		2,156,260		$	2.191

Stock Based Compensation

3 Months Ended

Mar. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock Based Compensation

15. Stock Based Compensation

Total stock-based compensation recorded related to option granted to employees and nonemployees was as follows (in thousands):


	March 31,
	2016		2015
Stock-based compensation expense:
Research and development	$	1,349	$	79
General and administrative		1,172		96
Total stock-based compensation expense	$	2,521	$	175

Related Party Transactions	3 Months Ended
Related Party Transactions	Mar. 31, 2016
Related Party Transactions [Abstract]
Related Party Transactions Disclosure	16. Related Party Transactions Certain employees of Third Rock Ventures, a stockholder of the Company, provide consulting services to the Company. General and administrative expense for these services of $12,000 and $8,400 were recorded for the three months ended March 31, 2016 and 2015, respectively. The amounts outstanding and included in accounts payable were $12,000 and $0 as of March 31, 2016 and December 31, 2015, respectively. The Company entered into full recourse loans (“stockholder notes” or “loans”) with current and former executive officers. Principal and interest under these loans are due at the earliest of (i) the fifth anniversary of the related note, (ii) the sale of the shares securing the notes, or (iii) thirty days after the termination of services. The principal loan amount and the accrued interest are reported as a deduction from stockholders’ deficit on the Company’s balance sheets. Loans made to two of the Company’s current and former executive officers were repaid and terminated in August 2015. The remaining balance of these loans was approximately $78,000 and $78,000 at March 31, 2016 and December 31, 2015, respectively. Interest income earned on the loans was insignificant during the three months ended March 31, 2016 and 2015. Revenues from related parties refer to our collaboration agreement with Pfizer, one of our stockholders. During the three months ended March 31, 2016 and 2015, the Company recognized revenue of $0.4 million and $0.3 million, respectively (Note 8). As of March 31, 2016 and December 31, 2015, deferred revenue relating to the Pfizer Agreement was $4.7 million and $4.9 million, respectively. The amount due from Pfizer under the agreement was $0.2 million and $0.4 million as of March 31, 2016 and December 31, 2015, respectively.

- Definition

The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.

+ References + Details

Employee Benefit Plans	3 Months Ended
Employee Benefit Plans	Mar. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Pension and Other Postretirement Benefits Disclosure [Text Block]	17. Employee Benefit Plans 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 three months ended March 31, 2016 and 2015, the Company made contributions to the plan of $141,000 and $18,000, respectively. Employee Stock Purchase Plan Concurrent with the completion of the IPO in October 2015, the Company’s 2015 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. No shares were issued under the ESPP as of March 31, 2016 and December 31, 2015.

Net Loss Per Share Attributable to Common Stockholders

3 Months Ended

Mar. 31, 2016

Earnings Per Share [Abstract]

Net Loss Per Share Attributable to Common Stockholders

18. Net Loss Per Share Attributable to Common Stockholders

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

	Three months Ended March 31,
	2016		2015
Redeemable convertible preferred stock (on an as-converted basis)		—		18,612,177
Convertible preferred stock (on an as-converted basis)		—		244,782
Options to purchase common stock		5,951,381		2,563,466
Convertible preferred stock warrants		—		81,620
Total		5,951,381		21,502,045

A reconciliation of the numerator and denominator used in the calculation of the basic and diluted net loss per share attributable to common stockholders is as follows (in thousands except share and per share amounts):

	Three months ended March 31,
	2016			2015
Numerator:
Net loss	$	(16,029	)	$	(6,211	)
Add: accretion to redemption value and cumulative dividends on preferred stock		—			(1,432	)
Net loss attributable to common stockholders	$	(16,029	)	$	(7,643	)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted		36,063,425			996,551
Net loss per share attributable to common stockholders, basic and diluted	$	(0.44	)	$	(7.56	)

Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2016
Subsequent Events [Abstract]
Subsequent Events	19. Subsequent Events On April 21, 2016, the Company entered into the following two collaboration agreements with AbbVie Ireland Unlimited Company (“AbbVie”). Co-Development Agreement Pursuant to a Co-Development and License Agreement (the “Co-Development Agreement”), the Company and AbbVie will collaborate in the research, development and commercialization of PDCs and products. The Company will be responsible for the research and development of a PDC through the completion of Phase I and all related costs. AbbVie will be responsible for the subsequent clinical studies, and the Company will financially participate in 35% of the global development costs following a Phase II study unless it elects to opt out of the co-development (the “Co-Development Opt-Out”). AbbVie has the sole right to commercialize the PDCs and co-development products worldwide at its own cost and expense, subject to the Company’s right to elect to assume a portion of the co-promotion effort in the U.S. for each product (the “Co-Promotion Option”). The Company grants to AbbVie a worldwide, exclusive and sublicensable license to certain patents and know-how for the development and commercialization of PDCs and co-development products. The parties will establish a joint research committee and a joint development committee to oversee the PDC research and development activities, respectively, and, if the Company exercises the Co-Promotion Option, a joint commercialization committee to oversee the commercialization of the co-development products. Pursuant to the Co-Development Agreement, the financial consideration from AbbVie was comprised of an upfront payment of $20 million and, subject to a reduction by 25% if the Company exercises the Co-Development Opt-Out, a total of up to $470 million in development, regulatory and commercial milestone payments. Unless the Company exercises the Co-Development Opt-Out, AbbVie and the Company will share 65% and 35%, respectively, of the net profits and net losses from sales of the co-development products in the U.S. (the “U.S. Profit Sharing”), and the Company will be eligible to receive tiered royalties at double-digit percentages, subject to a reduction in royalties to royalties in the high-single digits to low teens if the Company exercises the Co-Development Opt-Out, on net sales of the co-development products from the ex-U.S. territory. If the Company elects to opt out of the U.S. Profit Sharing, it will receive the tiered royalties, subject to reduction, on global net sales of the co-development products. AbbVie’s royalty obligation continues with respect to each country and each licensed product until the later of (i) the expiration, invalidation or abandonment date of the last claim of the licensed patents covering the manufacture, use or sale of such licensed product in such country, (ii) the expiration of any applicable regulatory exclusivity with respect to such product in such country or (iii) the tenth anniversary of the first commercial sale of a licensed product in such country. The Co-Development Agreement will continue in effect until the date of expiration of the last royalty term for the last licensed product and, if later, the date on which no co-development product is being developed or commercialized in or for the U.S. AbbVie may terminate the agreement in its entirety or on a country-by-country basis after April 21, 2018 for no reason or at any time for certain development, regulatory or commercialization reasons. Either party may terminate the agreement upon the other party’s uncured material breach or insolvency. Discovery Collaboration Agreement Pursuant to a Discovery Collaboration and License Agreement (the “Discovery Collaboration Agreement”), AbbVie has the right to select a total of up to two targets and the Company and AbbVie will collaborate in the research and development of Probodies against the selected targets. AbbVie has the sole right to develop, manufacture and commercialize the PDCs and products directed toward the targets worldwide at its own cost and expense. The Company grants to AbbVie a worldwide, exclusive and sublicensable license, on a target-by-target basis, to certain patents and know-how for the development, manufacture and commercialization of the PDCs and licensed products. The parties will establish a joint research committee to oversee the research and discovery of Probodies against the selected targets and the conjugation of Probodies into PDCs. Under the Discovery Collaboration Agreement, the Company will receive from AbbVie an upfront payment of $10 million, an additional milestone payment payable upon the selection by AbbVie of the second target and additional milestone and royalty payments per target, should AbbVie ultimately pursue these targets. AbbVie’s royalty obligation continues with respect to each country on a licensed-product-by-licensed product basis until the later of (i) the expiration, invalidation or abandonment date of the last claim of the licensed patents covering the manufacture, use or sale of such licensed product in such country, (ii) the expiration of any applicable regulatory exclusivity with respect to such product in such country or (iii) the tenth anniversary of the first commercial sale of a licensed product in such country. The Discovery Collaboration Agreement will continue in effect until the date of expiration of the last royalty term for the last licensed product. AbbVie may terminate the agreement in its entirety or on a country-by-country or target-by-target basis for no reason after April 21, 2017 or at any time for certain development, regulatory or commercialization reasons. Either party may terminate the agreement upon the other party’s uncured material breach or insolvency.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2016

Accounting Policies [Abstract]

Basis of Presentation

Unaudited Interim Financial Information

Use of Estimates

Concentration of Credit Risk and Other Risks and Uncertainties

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

	Revenue						Accounts Receivable, net
	Three Months Ended March 31,						March 31,			December 31,
	2016			2015			2016			2015
Customer A		80	%		80	%		67	%		50	%
Customer B		20	%		20	%		33	%		50	%

All of the Company’s customers are located in the United States of America.

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.

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

Convertible Preferred Stock Warrant Liability

Convertible Preferred Stock Liability

Comprehensive Gain and Loss

Revenue Recognition

Research and Development Expenses

Stock-Based Compensation

Income Taxes

Net Loss per Share Attributable to Common Stockholders

Recent Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2016

Accounting Policies [Abstract]

Schedule of Customers Represent 10% of More of Total Revenue or Net Accounts Receivable Balance

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

	Revenue						Accounts Receivable, net
	Three Months Ended March 31,						March 31,			December 31,
	2016			2015			2016			2015
Customer A		80	%		80	%		67	%		50	%
Customer B		20	%		20	%		33	%		50	%

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

Fair Value Measurements and Short-term Investments (Tables)

3 Months Ended

Mar. 31, 2016

Fair Value Disclosures [Abstract]

Schedule of Financial Assets and Liabilities Subject to Fair Value Measurements on a Recurring Basis

	March 31, 2016
	Level I		Level II		Level III		Total
Assets
Money market funds	$	58,547	$	—	$	—	$	58,547
Restricted cash (money market funds)		917		—		—		917
U.S. Government bonds		—		112,739		—		112,739
	$	59,464	$	112,739	$	—	$	172,203

	December 31, 2015
	Level I		Level II		Level III		Total

Assets
Money market funds	$	44,714	$	—	$	—	$	44,714
Restricted cash (money market funds)		917		—		—		917
U.S. Government bonds		—		140,392				140,392
	$	45,631	$	140,392	$	—	$	186,023

Summary of Gross Unrealized Gains on the Company's Short-term Investments

The following is a summary of the gross unrealized gains and losses on the Company’s short-term investments (in thousands):

	March 31, 2016
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value

Investment Securities
U.S. Government bonds		112,709		31		(1	)		112,739
Total securities	$	112,709	$	31	$	(1	)	$	112,739

Schedule of Contractual Maturities of Securities Classified as Available-for-sale

The contractual maturities of securities classified as available-for-sale as of March 31, 2016 were as follows (in thousands):

	March 31, 2016
Due within one year		112,739
Total	$	112,739

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Property and Equipment (Tables)

3 Months Ended

Mar. 31, 2016

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):

	March 31,			December 31,
	2016			2015

Machinery and equipment	$	5,153		$	4,910
Computer equipment and software		506			452
Furniture and fixtures		51			51
Leasehold improvements		720			720
Construction in progress		225			169
		6,655			6,302
Less: accumulated depreciation and amortization		(3,176	)		(2,821	)
	$	3,479		$	3,481

Goodwill and Intangible Assets (Tables)

3 Months Ended

Mar. 31, 2016

Goodwill And Intangible Assets Disclosure [Abstract]

Schedule of Goodwill and Intangible Assets

Goodwill and intangible assets consisted of the following (in thousands):

	March 31,		December 31,
	2016		2015
Goodwill	$	949	$	949
In-process research and development		1,750		1,750

Accrued Liabilities (Tables)

3 Months Ended

Mar. 31, 2016

Accrued Liabilities Current [Abstract]

Schedule of Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	March 31,		December 31,
	2016		2015
Research and clinical expenses	$	2,371	$	1,562
Payroll and related expenses		1,227		2,839
Legal and professional expenses		854		296
Other accrued expenses		286		215
Total	$	4,738	$	4,912

License Agreement (Tables)

3 Months Ended

Mar. 31, 2016

Research And Development [Abstract]

Schedule of Future Minimum Royalty Obligations Due Under Terms of Certain Exclusive Licensed Patent Rights

The Company has future minimum royalty obligations due under the terms of certain exclusive licensed patent rights. These minimum future obligations are as follows (in thousands):

Year ended December 31,
2016 (nine months remaining)	$	—
2017		150
Total minimum royalty obligations	$	150

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2016

Commitments And Contingencies Disclosure [Abstract]

Schedule of Minimum Lease Payments

The minimum lease payments for all the Company’s facility leases are as follows (in thousands):

Year Ending December 31:
2016 (nine months remaining)	$	1,053
2017		3,387
2018		4,374
2019		4,506
2020		34,144
Total	$	47,464

Common Stock (Tables)

3 Months Ended

Mar. 31, 2016

Equity [Abstract]

Schedule Reserved Shares of Common Stock Issuance on Converted Basis

The Company had reserved shares of common stock for issuance, on an as-converted basis, as follows:

	March 31, 2016		December 31, 2015
Options issued and outstanding		6,125,654		5,270,751
Shares available for future stock option grants		2,927,926		2,401,406
		9,053,580		7,672,157

- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27011957&loc=d3e177068-122764

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21553-112644

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(d),(e))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6403732&loc=d3e21300-112643

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.28,29)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21538-112644

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 30
-Article 5

+ Details

Stock Option Plans (Tables)

3 Months Ended

Mar. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Activity Under Company's Stock Option Plans

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


	Options Outstanding
	Number of Options			Weighted-Average Exercise Price Per Share
Balances at December 31, 2015		5,270,751		$	3.694
Options granted		905,246			14.459
Options exercised		(41,600	)		1.116
Option forfeited		(8,743	)		3.904
Balances at March 31, 2016		6,125,654		$	5.302
Options exercisable at March 31, 2016		2,156,260		$	2.191

Stock Based Compensation (Tables)

3 Months Ended

Mar. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Total Stock-based Compensation Recorded Related to Option Granted to Employees and Nonemployees

Total stock-based compensation recorded related to option granted to employees and nonemployees was as follows (in thousands):


	March 31,
	2016		2015
Stock-based compensation expense:
Research and development	$	1,349	$	79
General and administrative		1,172		96
Total stock-based compensation expense	$	2,521	$	175

Net Loss Per Share Attributable to Common Stockholders (Tables)

3 Months Ended

Mar. 31, 2016

Earnings Per Share [Abstract]

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

	Three months Ended March 31,
	2016		2015
Redeemable convertible preferred stock (on an as-converted basis)		—		18,612,177
Convertible preferred stock (on an as-converted basis)		—		244,782
Options to purchase common stock		5,951,381		2,563,466
Convertible preferred stock warrants		—		81,620
Total		5,951,381		21,502,045

Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share

	Three months ended March 31,
	2016			2015
Numerator:
Net loss	$	(16,029	)	$	(6,211	)
Add: accretion to redemption value and cumulative dividends on preferred stock		—			(1,432	)
Net loss attributable to common stockholders	$	(16,029	)	$	(7,643	)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted		36,063,425			996,551
Net loss per share attributable to common stockholders, basic and diluted	$	(0.44	)	$	(7.56	)

Description of the Business - Additional Information (Detail) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended
	Oct. 14, 2015 USD ($) $ / shares shares	Oct. 02, 2015	Oct. 31, 2015 USD ($)	Mar. 31, 2016
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses \| $			$ 81.8
Reverse stock split				one-for-62.997
Reverse stock split conversion ratio		0.01587
Initial Public Offering \| Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
IPO price per share \| $ / shares	$ 12.00
Common stock shares issued \| shares	7,666,667
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses \| $	$ 81.8
Underwriters \| Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Common stock shares issued \| shares	1,000,000

Liquidity - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	2 Months Ended	3 Months Ended
	Oct. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Mar. 31, 2016	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accumulated deficit				$ (133,495)	$ (117,466)
Cash, cash equivalents, and short-term investments				$ 180,600	$ 186,700
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs			$ 1,191
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses	$ 81,800
Series C and Series D Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		$ 73,200

- Definition

Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

+ References + Details

Summary of Significant Accounting Policies - Schedule of Customers Represent 10% of More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance (Details) - Customer Concentration Risk	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Revenue \| Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	80.00%	80.00%
Revenue \| Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	20.00%	20.00%
Accounts Receivable, net \| Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	67.00%		50.00%
Accounts Receivable, net \| Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	33.00%		50.00%

Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($) Activity Segment	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)	Oct. 31, 2015 shares
Summary Of Significant Accounting Policies [Line Items]
Number of business activities \| Activity	1
Number of operating segments \| Segment	1
Impairment of goodwill	$ 0		$ 0
Impairment of intangible assets	0		$ 0
Impairment of long-lived assets	0	$ 0
Convertible preferred stock, shares issued \| shares				27,135,453
Interest or penalties charged in relation to unrecognized tax benefits	$ 0
Initial Public Offering \| Common Stock
Summary Of Significant Accounting Policies [Line Items]
Convertible preferred stock, shares issued \| shares				27,135,453

Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)	3 Months Ended
	Mar. 31, 2016
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

Fair Value Measurements and Short-term Investments - Schedule of Financial Assets and Liabilities Subject to Fair Value Measurements on a Recurring Basis (Details) - Fair Value Measurements Recurring - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets
Assets, fair value	$ 172,203	$ 186,023
Level I
Assets
Assets, fair value	59,464	45,631
Level II
Assets
Assets, fair value	112,739	140,392
Money market funds
Assets
Assets, fair value	58,547	44,714
Money market funds \| Level I
Assets
Assets, fair value	58,547	44,714
Restricted cash (money market funds)
Assets
Assets, fair value	917	917
Restricted cash (money market funds) \| Level I
Assets
Assets, fair value	917	917
U. S. Government bonds
Assets
Assets, fair value	112,739	140,392
U. S. Government bonds \| Level II
Assets
Assets, fair value	$ 112,739	$ 140,392

Fair Value Measurements and Short-term Investment - Summary of Gross Unrealized Gains and Losses on the Company Short-term Investments (Details) $ in Thousands	Mar. 31, 2016 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 112,709
Gross Unrealized Holding Gains	31
Gross Unrealized Holding Losses	(1)
Aggregate Fair Value	112,739
U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	112,709
Gross Unrealized Holding Gains	31
Gross Unrealized Holding Losses	(1)
Aggregate Fair Value	$ 112,739

Fair Value Measurements and Short-term Investments - Schedule of Contractual Maturities of Securities Classified as Available-for-sale (Details) $ in Thousands	Mar. 31, 2016 USD ($)
Fair Value Disclosures [Abstract]
Due within one year	$ 112,739
Total	$ 112,739

Property and Equipment - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 6,655	$ 6,302
Less: accumulated depreciation and amortization	(3,176)	(2,821)
Property and equipment, net	3,479	3,481
Machinery and equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	5,153	4,910
Computer equipment and software
Property Plant And Equipment [Line Items]
Property and equipment, gross	506	452
Furniture and fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	51	51
Leasehold improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	720	720
Construction in progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 225	$ 169

Property and Equipment - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation and amortization	$ 355	$ 268

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Goodwill And Intangible Assets [Line Items]
Goodwill	$ 949	$ 949
Intangible assets	1,750	1,750
In-process research and development
Goodwill And Intangible Assets [Line Items]
Intangible assets	$ 1,750	$ 1,750

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accrued Liabilities Current [Abstract]
Research and clinical expenses	$ 2,371	$ 1,562
Payroll and related expenses	1,227	2,839
Legal and professional expenses	854	296
Other accrued expenses	286	215
Total	$ 4,738	$ 4,912

Research and Collaboration Agreements - Pfizer Inc - Additional Information (Details) - Collaborative Arrangement - Pfizer Inc	1 Months Ended		3 Months Ended
	Dec. 31, 2014 USD ($)	May. 31, 2013 USD ($) Target ResearchTarget SaleIndicator	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets \| Target		2
Nominated number of additional research targets \| Target		2
Deferred revenue			$ 4,700,000		$ 4,900,000
Estimated performance period of agreement		7 years
Additional target as deferred revenue	$ 1,500,000
Revenue recognized from collaborative arrangement			400,000	$ 300,000
Amount due from agreement			$ 200,000		$ 400,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 626,500,000
Achievement Of Additional Nominated Research Target One
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		1,500,000
Achievement Of Additional Nominated Research Target Two
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		1,500,000
Exercise of License Options [Member] \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		12,000,000
Achievement Of Development Milestones For Each Research Target Program \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		25,000,000
Achievement Of Development Milestones If Four Research Targets Selected By Counterparty \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 82,000,000
Number of research targets selected \| ResearchTarget		4
Achieving First Commercial Sale In Various Territories \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 98,000,000
Number of sales indicators \| SaleIndicator		3
Achieving First Commercial Sale If Four Research Targets Selected By Counterparty \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 249,500,000
Number of research targets selected \| ResearchTarget		4
Achieving Sales Milestones For Each Research Target Program \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 100,000,000
Achieving Sales Milestones If Four Research Targets Selected By Counterparty \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 280,000,000
Number of research targets selected \| ResearchTarget		4
Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received		$ 6,000,000
Deferred revenue		$ 6,000,000

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details) - Collaborative Arrangement - Immuno Gen Inc	1 Months Ended
	Jan. 31, 2014 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue	$ 13,200,000
Estimated fair value of assets and services	13,200,000
Licenses Received
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services	12,700,000
Research Services, Joint Research Committee Participation and Technology Improvements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services	500,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development and regulatory milestones	30,000,000
Contingent payments receivable upon achieving sales milestones	50,000,000
Contingent payments payable upon achieving development and regulatory milestones	60,000,000
Contingent payments payable upon achieving sales milestones	100,000,000
Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	0
Research License, Research Services, Joint Research Committee, Commercialization License
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue	13,000,000
Technology Improvements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue	$ 200,000

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)		1 Months Ended	3 Months Ended
	Jul. 07, 2014 USD ($) Target ResearchTarget SaleIndicator Term	Oct. 31, 2015 shares	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Jan. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
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			BMS will have additional rights to substitute up to two collaboration targets. Each collaboration target has a two year research term and the two additional targets must be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target can be extended in one year increments up to three times.
Extension of research term for each collaboration target	1 year
Contingent milestone payments receivable					$ 10,000,000
Estimated performance period of agreement			10 years
Revenue recognized from collaborative arrangement			$ 1,800,000	$ 1,400,000
Deferred revenue			51,100,000			$ 42,600,000
Amount due from agreement			$ 300,000			$ 400,000
Collaborative Arrangement \| Bristol Myers Squibb Company \| Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 50,000,000
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	$ 1,217,000,000
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
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achievement Of Development Milestones For Each Research Target Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	114,000,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achievement Of Development Milestones If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 456,000,000
Number of research targets selected \| ResearchTarget	4
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving First Commercial Sale In Various Territories
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 124,000,000
Number of sales indicators \| SaleIndicator	3
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving First Commercial Sale If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 496,000,000
Number of research targets selected \| ResearchTarget	4
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Sales Milestones For Each Research Target Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 60,000,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Sales Milestones If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 240,000,000
Number of research targets selected \| ResearchTarget	4
Bristol Myers Squibb Agreement \| Initial Public Offering
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, shares issued in connection with agreement \| shares		833,333

License Agreement - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2013
License Agreement [Line Items]
Common stock, shares issued	36,085,809		36,033,209
UC Regents
License Agreement [Line Items]
Common stock, shares issued				157,332
Milestone and minimum annual royalty provision paid	$ 537,000	$ 173,000

Long-term Debt - Additional Information (Details) - USD ($)	1 Months Ended
	Oct. 31, 2015	May. 31, 2012	Mar. 31, 2016	Dec. 31, 2013	Jan. 31, 2013
Debt Instrument [Line Items]
Warrants exercise price			$ 3.084396
Common Stock \| Initial Public Offering
Debt Instrument [Line Items]
Convertible preferred stock, shares issued	60,640
Series B-1 Redeemable Convertible Preferred Stock
Debt Instrument [Line Items]
Warrants issued to purchase stock			81,620
Master Loan and Security Agreement
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 2,000,000
Line of credit facility, initial basic loan term		42 months
Line of credit facility, additional borrowing capacity				$ 3,000,000	$ 300,000
Debt facility interest rate		11.74%

Commitments and Contingencies - Additional Information (Details)		1 Months Ended	3 Months Ended
	Dec. 10, 2015 USD ($) ft²	Mar. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Operating Leased Assets [Line Items]
Restricted cash		$ 917,000	$ 917,000		$ 917,000
One-time adjustment to deferred rent		2,000	2,000
Rent expense				$ 235,000
Letter of Credit
Operating Leased Assets [Line Items]
Letter of credit outstanding, amount		900,000	900,000
Restricted cash		$ 900,000	$ 900,000
New Lease Agreement
Operating Leased Assets [Line Items]
Area of rentable office and laboratory space \| ft²	76,173
Lease term description			The term of the New Lease commences on the later of (i) the date that the Landlord’s construction and tenant improvements have been completed pursuant to the New Lease and (ii) October 1, 2016. The New 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 New Lease.
Initial lease term			10 years
Extended lease term			5 years
Maximum one-time improvement allowance	$ 12,600,000
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
Amendment to Current Lease \| Lease Termination
Operating Leased Assets [Line Items]
Lease expiration date		Jan. 31, 2019
Amendment to Current Lease \| Early Lease Termination
Operating Leased Assets [Line Items]
Lease expiration date		Nov. 30, 2016

Commitments and Contingencies - Schedule of Minimum Lease Payments (Details) $ in Thousands	Mar. 31, 2016 USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2016 (nine months remaining)	$ 1,053
2017	3,387
2018	4,374
2019	4,506
2020	34,144
Total	$ 47,464

Convertible Preferred Stock - Additional information (Details) - USD ($)	1 Months Ended	3 Months Ended
	May. 31, 2015	Mar. 31, 2016	Dec. 31, 2015	Oct. 31, 2015	Dec. 31, 2014
Class Of Stock [Line Items]
Convertible preferred stock, shares issued				27,135,453
Convertible preferred stock, conversion basis		one-for-one
Shares Outstanding		0	0
Series C Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Issued	659,209
Convertible preferred stock liability					$ 395,000
Proceeds from issuance of redeemable preferred stock	$ 3,500,000
Fair value of preferred stock liability reclassified to redeemable convertible preferred stock	1,500,000
Series C Redeemable Convertible Preferred Stock \| Other Income (Expense), Net
Class Of Stock [Line Items]
Gain (loss) on re-measurement of preferred stock liability	$ 1,100,000
Series C Redeemable Convertible Preferred Stock \| Second Tranche Option
Class Of Stock [Line Items]
Preferred stock, shares granted					659,209

Common Stock - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Oct. 14, 2015
Equity [Abstract]
Common stock, shares authorized	75,000,000	75,000,000	75,000,000
Common stock, par value	$ 0.00001	$ 0.00001	$ 0.00001
Preferred stock, shares authorized	10,000,000	10,000,000	10,000,000
Preferred stock, par value	$ 0.00001	$ 0.00001	$ 0.00001
Dividends on common stock	$ 0	$ 0

Common Stock - Reserved Shares of Common Stock Issuance on Converted Basis (Details) - shares	Mar. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Options issued and outstanding	6,125,654	5,270,751
Shares available for future stock option grants	2,927,926	2,401,406
Total	9,053,580	7,672,157

Stock Option Plans - Additional Information (Details)	3 Months Ended
Stock Option Plans - Additional Information (Details)	Mar. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option expiration period	10 years
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%
Estimated fair value of the shares on the date of grant	110.00%
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%
2015 Plan \| Vesting per Month After First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent	2.08%

Stock Option Plans - Schedule of Activity Under Company's Stock Option Plans (Details)	3 Months Ended
	Mar. 31, 2016 $ / shares shares
Number of Options
Balances, beginning of the period \| shares	5,270,751
Options granted \| shares	905,246
Options exercised \| shares	(41,600)
Option forfeited \| shares	(8,743)
Balances, end of the period \| shares	6,125,654
Options exercisable, end of the period \| shares	2,156,260
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period \| $ / shares	$ 3.694
Options granted \| $ / shares	14.459
Options exercised \| $ / shares	1.116
Option forfeited \| $ / shares	3.904
Balances, end of the period \| $ / shares	5.302
Options exercisable, end of the period \| $ / shares	$ 2.191

Stock Based Compensation - Total Stock-based Compensation Recorded Related to Option Granted to Employees and Nonemployees (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Stock-based compensation expense:
Total stock-based compensation expense	$ 2,521	$ 175
Research and development
Stock-based compensation expense:
Total stock-based compensation expense	1,349	79
General and administrative
Stock-based compensation expense:
Total stock-based compensation expense	$ 1,172	$ 96

Related Party Transactions - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Collaborative Arrangement \| Pfizer Inc
Related Party Transaction [Line Items]
Revenue recognized	$ 400,000	$ 300,000
Deferred revenue	4,700,000		$ 4,900,000
Amount due from agreement	200,000		400,000
Third Rock Ventures
Related Party Transaction [Line Items]
General and administrative expenses from transactions with related party	12,000	$ 8,400
Accounts payable, related parties	12,000		0
Principal loan amount, remaining	$ 78,000		$ 78,000

Employee Benefit Plans- Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Company contributions to defined contribution plan	$ 141,000	$ 18,000
Employee Stock Purchase Plan
Defined Contribution Plan Disclosure [Line Items]
Discount percentage of common stock purchase price	15.00%
Estimated fair value of the shares on the date of grant	85.00%
Number of shares issued during period	0		0

Net Loss Per Share Attributable to Common Stockholders - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	5,951,381	21,502,045
Redeemable convertible preferred stock (on an as-converted basis)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		18,612,177
Convertible preferred stock (on an as-converted basis)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		244,782
Convertible preferred stock warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		81,620
Options 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	5,951,381	2,563,466

Net Loss Per Share Attributable to Common Stockholders - Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Numerator:
Net loss	$ (16,029)	$ (6,211)
Accretion to redemption value and cumulative dividends on preferred stock		(1,432)
Net loss attributable to common stockholders	$ (16,029)	$ (7,643)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted	36,063,425	996,551
Net loss per share attributable to common stockholders, basic and diluted	$ (0.44)	$ (7.56)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Subsequent Events - Additional Information (Details) - Subsequent Event $ in Millions	Apr. 21, 2016 USD ($) Target Agreement
Subsequent Event [Line Items]
Number of collaboration agreements \| Agreement	2
Co-Development Agreement
Subsequent Event [Line Items]
Percentage of global development costs	35.00%
Profit and loss sharing percentage	35.00%
Co-Development Agreement \| Abb Vie
Subsequent Event [Line Items]
Upfront payment received	$ 20
Reduction in percentage of upfront payment	25.00%
Profit and loss sharing percentage	65.00%
Agreement termination description	AbbVie may terminate the agreement in its entirety or on a country-by-country basis after April 21, 2018 for no reason or at any time for certain development, regulatory or commercialization reasons.
Co-Development Agreement \| Maximum \| Abb Vie
Subsequent Event [Line Items]
Milestone payments	$ 470
Discovery Collaboration Agreement \| Abb Vie
Subsequent Event [Line Items]
Upfront payment received	$ 10
Agreement termination description	AbbVie may terminate the agreement in its entirety or on a country-by-country or target-by-target basis for no reason after April 21, 2017 or at any time for certain development, regulatory or commercialization reasons.
Discovery Collaboration Agreement \| Maximum
Subsequent Event [Line Items]
Number of targets \| Target	2

License Agreement - Schedule of Future Minimum Royalty Obligations Due Under Terms of Certain Exclusive Licensed Patent Rights (Details) $ in Thousands	Mar. 31, 2016 USD ($)
Research And Development [Abstract]
2017	$ 150
Total minimum royalty obligations	$ 150