<?xml version="1.0" encoding="US-ASCII"?>
<!-- Generated by Broadridge Transform (tm) - http://www.broadridge.com -->
<!-- Created: Mon Apr 20 20:15:26 UTC 2026 -->
<linkbase xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:xbrldt="http://xbrl.org/2005/xbrldt">
  <roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#AwardTypeOnly" roleURI="http://xbrl.sec.gov/ecd/AwardTypeOnly"/>
  <roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#IndividualsOnly" roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly"/>
  <arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default"/>
  <arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain"/>
  <arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension"/>
  <arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member"/>
  <arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" arcroleURI="http://xbrl.org/int/dim/arcrole/all"/>
  <definitionLink xlink:role="http://xbrl.sec.gov/ecd/AwardTypeOnly" xlink:type="extended">
    <loc xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <loc xlink:type="locator" xlink:label="ctmx_Tranche1Member" xlink:href="ctmx-20260617.xsd#ctmx_Tranche1Member"/>
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="ctmx_Tranche1Member" order="5" xbrldt:closed="true"/>
    <loc xlink:type="locator" xlink:label="ctmx_Tranche2Member" xlink:href="ctmx-20260617.xsd#ctmx_Tranche2Member"/>
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="ctmx_Tranche2Member" order="7" xbrldt:closed="true"/>
  </definitionLink>
  <definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <loc xlink:type="locator" xlink:label="ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#ecd_AllIndividualsMember"/>
    <loc xlink:type="locator" xlink:label="ctmx_SeanAMcCarthyDPhilMember" xlink:href="ctmx-20260617.xsd#ctmx_SeanAMcCarthyDPhilMember"/>
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ecd_AllIndividualsMember" xlink:to="ctmx_SeanAMcCarthyDPhilMember" order="2" xbrldt:closed="true"/>
    <loc xlink:type="locator" xlink:label="ctmx_ChrisOgdenMember" xlink:href="ctmx-20260617.xsd#ctmx_ChrisOgdenMember"/>
<!-- [WMV7][9ryu8RkpWk2KJc7jZtMwJeuQsYkVNjmnV4kdtm4tgG+IyddSGm01gYQA6p2czYyMNuwt2feIsZ/AIWcIgTpqObfvqLnz++ZlvOlmGkZcJHShckSoXFIRlGZtP+YiY+MTsrHOxCRx5ivZQ5xXpYtzKb15ZWR4F1WMe93ZYvNiFvscXHKKem1zpTVpwumwzFVa4ZDyO9RxAX1vSjRY793nBEMG4eANzsGVyrrEdZ4KL6naREbxUntH/ZN2mqC8qsL1avJsEudpesEzQ9fFX7ipZpUh41Ll1/tjtInDTnDhUaLVXhaJgoHTGS3dzcGNrwLrtx9ak6IqvZVWAcIznFo80lkyiolMVGNNOku37xMSt5tT+8CZ4vNZstOAhwE=] CSR-->
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ecd_AllIndividualsMember" xlink:to="ctmx_ChrisOgdenMember" order="4" xbrldt:closed="true"/>
  </definitionLink>
</linkbase>
