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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2021-11-08T23:15:43.6385474+00:00 -->
<!-- Version            : 5.0.1.702 -->
<!-- Package ID         : 60eea23e33e74f9d927d497d1e2f95ee -->
<!-- Copyright (c) 2021 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="cccc_LeadSeriesIdentificationAchievementMember" name="LeadSeriesIdentificationAchievementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_GoodLaboratoryPracticeGlpToxicologyToxStudyPhaseMember" name="GoodLaboratoryPracticeGlpToxicologyToxStudyPhaseMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_ResearchDevelopmentAndCommercialMilestonePaymentsMember" name="ResearchDevelopmentAndCommercialMilestonePaymentsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_OneTimeSalesBasedPaymentsMember" name="OneTimeSalesBasedPaymentsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_BiogenLicenseAgreementMember" name="BiogenLicenseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_ResearchAndDevelopmentMilestonesMember" name="ResearchAndDevelopmentMilestonesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_CalicoLicenseAgreementMember" name="CalicoLicenseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_PotentialResearchDevelopmentAndCommercialMilestonePaymentsMember" name="PotentialResearchDevelopmentAndCommercialMilestonePaymentsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_RestatedRocheAgreementBiogenLicenseAgreementAndCalicoLicenseAgreementMember" name="RestatedRocheAgreementBiogenLicenseAgreementAndCalicoLicenseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_UpfrontFees" name="UpfrontFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="cccc_AdditionalUpfrontConsideration" name="AdditionalUpfrontConsideration" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="cccc_AnnualResearchPlanPaymentsReceivables" name="AnnualResearchPlanPaymentsReceivables" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="cccc_OptionExerciseFee" name="OptionExerciseFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="cccc_AnnualResearchPlanPaymentsPeriods" name="AnnualResearchPlanPaymentsPeriods" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_CollaborationDescription" name="CollaborationDescription" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_AgreementDate" name="AgreementDate" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_NumberOfTargetedProteinDegradation" name="NumberOfTargetedProteinDegradation" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_ResearchAndDevelopmentAgreementPeriod" name="ResearchAndDevelopmentAgreementPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_AdditionalPaymentOnExtensionOfContract" name="AdditionalPaymentOnExtensionOfContract" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="cccc_AdditionalTargetsForDevelopment" name="AdditionalTargetsForDevelopment" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_NonRefundableUpfrontPayment" name="NonRefundableUpfrontPayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="cccc_NoticePeriodForTerminationOfAgreement" name="NoticePeriodForTerminationOfAgreement" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_NumberOfPerformanceObligation" name="NumberOfPerformanceObligation" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="cccc_NumberOfAdditionalTargets" name="NumberOfAdditionalTargets" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_LeadFeeAchieved" name="LeadFeeAchieved" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="cccc_ResearchTerm" name="ResearchTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_NumberOfTargetProteins" name="NumberOfTargetProteins" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="cccc_UpfrontPayment" name="UpfrontPayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="cccc_AnnualPayments" name="AnnualPayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="cccc_ResearchExtendedTerm" name="ResearchExtendedTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_AmountPayableForExtendResearchTermOption" name="AmountPayableForExtendResearchTermOption" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="cccc_ContractualTerm" name="ContractualTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_InitialContractualTermOnStraightLineBasis" name="InitialContractualTermOnStraightLineBasis" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_MilestonePaymentReceived" name="MilestonePaymentReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="cccc_RocheAgreementMember" name="RocheAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_BiogenAgreementMember" name="BiogenAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_CalicoAgreementMember" name="CalicoAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_TransactionPriceAllocatedToPerformanceObligation" name="TransactionPriceAllocatedToPerformanceObligation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="cccc_CashBalanceToBeMaintainedInBankAccountWhileDebtIsOutstanding" name="CashBalanceToBeMaintainedInBankAccountWhileDebtIsOutstanding" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="cccc_LongTermDebtAndWarrantLiabilityAbstract" name="LongTermDebtAndWarrantLiabilityAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_LongTermDebtAndWarrantLiabilityRelatedPartyLineItems" name="LongTermDebtAndWarrantLiabilityRelatedPartyLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_LongTermDebtAndWarrantLiabilityTable" name="LongTermDebtAndWarrantLiabilityTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_CreditAgreementWithPerceptiveLifeSciencesMasterFundLTDMember" name="CreditAgreementWithPerceptiveLifeSciencesMasterFundLTDMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_TermLoanMember" name="TermLoanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_TrancheOneMember" name="TrancheOneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_TrancheTwoMember" name="TrancheTwoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_DebtInstrumentPercentageOfPrincipalPaymentUntilMaturity" name="DebtInstrumentPercentageOfPrincipalPaymentUntilMaturity" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_DateUntilWhichInterestOnlyPaymentsAreMade" name="DateUntilWhichInterestOnlyPaymentsAreMade" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="cccc_StockholdersEquityLineItems" name="StockholdersEquityLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_StockholdersEquityTable" name="StockholdersEquityTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_UnderwritersMember" name="UnderwritersMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_FollowOnOfferingMember" name="FollowOnOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_NumberOfSharesWithholdEquivalentToAggregateExercisePrice" name="NumberOfSharesWithholdEquivalentToAggregateExercisePrice" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="cccc_TwoThousandFifteenIncentiveStockOptionAndGrantPlanMember" name="TwoThousandFifteenIncentiveStockOptionAndGrantPlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_TwoThousandAndTwentyStockOptionAndIncentivePlanMember" name="TwoThousandAndTwentyStockOptionAndIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_PaymentsForRelinquishmentOfRightToPurchaseCommonStockUponExerciseOfStockOptions" name="PaymentsForRelinquishmentOfRightToPurchaseCommonStockUponExerciseOfStockOptions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="cccc_WarrantToPurchaseCommonStockMember" name="WarrantToPurchaseCommonStockMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_OptionsToPurchaseCommonStockMember" name="OptionsToPurchaseCommonStockMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="cccc_SeriesSeedRedeemableConvertiblePreferredStockMember" name="SeriesSeedRedeemableConvertiblePreferredStockMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
