

<!-- Created by Merrill Corporation -->
<!--XBRL document created with Merrill Bridge  5.13.77.0 -->
<!--Based on XBRL 2.1-->
<!--Created on: 8/13/2015 8:23:33 AM-->
<!--Modified on: 8/13/2015 8:23:33 AM-->
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  <element id="catb_StockIncentivePlan2008Member" name="StockIncentivePlan2008Member" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_EmployeeStockPurchasePlan2015Member" name="EmployeeStockPurchasePlan2015Member" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_AnnualIncreaseMember" name="AnnualIncreaseMember" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_ScheduleOfSharesReservedForFutureIssuanceTableTextBlock" name="ScheduleOfSharesReservedForFutureIssuanceTableTextBlock" nillable="true" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_SeriesConvertibleApreferredStockMember" name="SeriesConvertibleApreferredStockMember" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_SeriesConvertibleBpreferredStockMember" name="SeriesConvertibleBpreferredStockMember" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_NumberOfPerferredStockSeries" name="NumberOfPerferredStockSeries" nillable="true" type="xbrli:pureItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_WarrantsTextBlock" name="WarrantsTextBlock" nillable="true" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_ScheduleOfWarrantsFairValueActivityTableTextBlock" name="ScheduleOfWarrantsFairValueActivityTableTextBlock" nillable="true" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_ScheduleOfWeigthedAverageAssupmtionsFairValueTableTextBlock" name="ScheduleOfWeigthedAverageAssupmtionsFairValueTableTextBlock" nillable="true" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_FairValueAssetsAndLiabilitiesTransfersBetweenLevelsAmount" name="FairValueAssetsAndLiabilitiesTransfersBetweenLevelsAmount" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="credit" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_ScheduleOfFuturePrincipalPaymentsNotesTableTextBlock" name="ScheduleOfFuturePrincipalPaymentsNotesTableTextBlock" nillable="true" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_LongTermDebtOutstandingAmountDuePercentage" name="LongTermDebtOutstandingAmountDuePercentage" nillable="true" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_WarrantsPreferredStock2014Member" name="WarrantsPreferredStock2014Member" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_WarrantsPreferredStock2015Member" name="WarrantsPreferredStock2015Member" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_TermLoanB2015Member" name="TermLoanB2015Member" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_TermLoanA2014Member" name="TermLoanA2014Member" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_ClassOfWarrantOrRightExercisablePeriod" name="ClassOfWarrantOrRightExercisablePeriod" nillable="true" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_DebtCostsDiscounts" name="DebtCostsDiscounts" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="debit" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_LongTermNotesPayableNet" name="LongTermNotesPayableNet" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="credit" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_WarrantsDisclosuresAbstract" name="WarrantsDisclosuresAbstract" nillable="true" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_WarrantsAndRightsIssued" name="WarrantsAndRightsIssued" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="credit" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_WarrantsOrRightsReclassifiedToEquity" name="WarrantsOrRightsReclassifiedToEquity" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="debit" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_MinimumNumberOfDiseasePathwaysModulatedSimultaneously" name="MinimumNumberOfDiseasePathwaysModulatedSimultaneously" nillable="true" type="xbrli:pureItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_NumberOfMoleculesUsedToCreateProductCandidates" name="NumberOfMoleculesUsedToCreateProductCandidates" nillable="true" type="xbrli:pureItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_NumberOfLinkers" name="NumberOfLinkers" nillable="true" type="xbrli:pureItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_ReclassificationNonCurrentAssetsToEquity" name="ReclassificationNonCurrentAssetsToEquity" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="credit" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_ReclassificationStockWarrantsToEquity" name="ReclassificationStockWarrantsToEquity" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="credit" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_OtherIncomeLossFairValueAdjustmentToWarrants" name="OtherIncomeLossFairValueAdjustmentToWarrants" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="credit" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_TermLoanOtherThanAndBMember" name="TermLoanOtherThanAndBMember" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_AntidilutiveCommonStockEquivalentsExcludedFromComputationOfDilutedNetLossPerShareAbstract" name="AntidilutiveCommonStockEquivalentsExcludedFromComputationOfDilutedNetLossPerShareAbstract" nillable="true" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
  <element id="catb_SecondLeaseAmendmentMember" name="SecondLeaseAmendmentMember" nillable="true" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xmlns="http://www.w3.org/2001/XMLSchema" />
</xsd:schema>