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        <BookValue>16149873</BookValue>
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      <TotalBookValueOtherAssets>16158274</TotalBookValueOtherAssets>
      <OtherLiabilities>
        <Description>ACCRUED PAYROLL LIABILITIES</Description>
        <Amount>1052608</Amount>
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      <TotalOfAmounts>1052608</TotalOfAmounts>
      <FootnoteInPartXIII>X</FootnoteInPartXIII>
      <TotRevenueEtcAuditedFinclStmt>34502852</TotRevenueEtcAuditedFinclStmt>
      <NetUnrealizedGainsInvestments>271526</NetUnrealizedGainsInvestments>
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      <RevenueNotRptdOnFinStmt>2555</RevenueNotRptdOnFinStmt>
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        <Identifier>DESCRIPTION OF UNCERTAIN TAX POSITIONS UNDER FIN 48:</Identifier>
        <ReturnReference>PART X, LINE 2:</ReturnReference>
        <Explanation>PLOS HAS RECEIVED NOTIFICATION FROM THE INTERNAL REVENUE SERVICE AND THE STATE OF CALIFORNIA THAT IT QUALIFIES FOR TAX-EXEMPT STATUS UNDER SECTION 501(C)(3) OF THE INTERNAL REVENUE CODE AND SECTION 27301D OF THE CALIFORNIA REVENUE AND TAXATION CODE. THE EXEMPTIONS ARE SUBJECT TO PERIODIC REVIEW BY THE FEDERAL AND STATE TAXING AUTHORITIES AND MANAGEMENT IS CONFIDENT THAT PLOS CONTINUES TO SATISFY ALL FEDERAL AND STATE STATUTES IN ORDER TO QUALIFY FOR CONTINUED TAX EXEMPTION STATUS. PLOS MAY PERIODICALLY RECEIVE UNRELATED BUSINESS INCOME (SUCH AS SUBLEASE RENTAL INCOME, ADVERTISING INCOME, ETC.) REQUIRING PLOS TO FILE SEPERATE TAX RETURNS UNDER FEDERAL AND STATE STATUTES. PLOS ALSO HAS CERTAIN TRANSACTIONS REQUIRING THE PAYMENT OF VALUE ADDED TAXES. UNDER SUCH CONDITIONS, PLOS CALCULATES AND ACCRUES THE APPLICABLE TAXES. PLOS ASSESSES TAX POSITIONS TAKEN OR EXPECTED TO BE TAKEN AGAINST MORE-LIKELY-THAN-NOT RECOGNITION THRESHOLD AND MEASUREMENT ATTRIBUTES FOR FINANCIAL STATEMENT RECOGNITION. BASED ON AN ANALYSIS PREPARED BY PLOS, IT WAS DETERMINED THAT PLOS BELIEVES THAT IT HAS APPROPRIATE SUPPORT FOR ANY TAX POSITIONS TAKEN, AND AS SUCH, DOES NOT HAVE ANY UNCERTAIN TAX POSITIONS THAT ARE MATERIAL TO THE FINANCIAL STATEMENTS.</Explanation>
      </Form990ScheduleDPartXIII>
      <Form990ScheduleDPartXIII>
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        <Explanation>GAIN / LOSS ON EXCHANGE RATE 2,555.</Explanation>
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      <Form990ScheduleDPartXIII>
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        <Explanation>GAIN / LOSS ON EXCHANGE RATE 2,555.</Explanation>
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        <NumberOfEmployees>36</NumberOfEmployees>
        <TypeOfActivitiesConducted>PROGRAM SERVICES</TypeOfActivitiesConducted>
        <SpecificServicesProvided>EDITORIAL SUPPORT TO SERVICE US OPERATIONS.</SpecificServicesProvided>
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        <NamePerson>RICHARD CAVE</NamePerson>
        <Title>DIRECTOR OF INFORMATION TECHNOLOGY</Title>
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        <OtherCompensationRelatedOrgs>0</OtherCompensationRelatedOrgs>
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        <NamePerson>KATIE SHARABATI</NamePerson>
        <Title>DIRECTOR OF HUMAN RESOURCES</Title>
        <BaseCompensationFilingOrg>144035</BaseCompensationFilingOrg>
        <CompBasedOnRelatedOrgs>0</CompBasedOnRelatedOrgs>
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        <NamePerson>VIRGINIA BARBOUR</NamePerson>
        <Title>MEDICINE EDITORIAL DIRECTOR</Title>
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    <IRS990ScheduleO documentId="RetDoc1044400001">
      <GeneralExplanation>
        <Identifier>NUMBER OF EMPLOYEES</Identifier>
        <ReturnReference>FORM 990, PART V, LINE 2A</ReturnReference>
        <Explanation>THE NUMBER OF EMPLOYEES REPORTED IN PART V AND PART I OF THE FORM 990 INCLUDES 126 W-2 EQUIVALENT EMPLOYEES IN THE US AS PART OF TRINET PEO AND 36 EMPLOYEES IN THE UK AT THE END OF 2012.</Explanation>
      </GeneralExplanation>
      <GeneralExplanation>
        <ReturnReference>FORM 990, PART VI, SECTION A, LINE 4</ReturnReference>
        <Explanation>THE BYLAWS OF PLOS WERE UPDATED TO REFLECT THE CURRENT ORGANIZATIONAL STRUCTURE AT PLOS SINCE IT WAS NOT CHANGED SINCE THE INCEPTION OF THE ORGANIZATION. THE AMENDED BYLAWS WERE RATIFIED IN MAY 2013, WHICH WE ARE INCLUDING AS PART OF OUR 2012 TAX FILING DISCLOSURE, RATHER THAN IN OUR 2013 TAX FILING PERIOD.</Explanation>
      </GeneralExplanation>
      <GeneralExplanation>
        <ReturnReference>FORM 990, PART VI, SECTION B, LINE 11</ReturnReference>
        <Explanation>FORM 990 IS PREPARED BY AN OUTSIDE TAX PROFESSIONAL WITH ASSISTANCE FROM THE ORGANIZATION. THE FORM IS THEN REVIEWED BY THE CFO AND MEMBERS FROM THE AUDIT COMMITTEE. AFTER REVIEW AND MODIFICATIONS WHERE NECESSARY, THE FINAL VERSION OF THE TAX RETURN IS PROVIDED TO THE BOARD OF DIRECTORS. THE CFO SIGNS AND FILES ALL REQUIRED TAX FILINGS.</Explanation>
      </GeneralExplanation>
      <GeneralExplanation>
        <ReturnReference>FORM 990, PART VI, SECTION B, LINE 12C</ReturnReference>
        <Explanation>A COMMITTEE OF THE BOARD OF DIRECTORS REVIEWS ALL POTENTIAL CONFLICTS OF INTEREST AT LEAST ANNUALLY. THE CEO AND ALL BOARD MEMBERS ARE REQUIRED TO DISCLOSE (IN WRITING) POTENTIAL CONFLICTS AND ANY RELATED PARTY AFFILIATIONS. LOANS BETWEEN THE ORGANIZATION AND MEMBERS OF MANAGEMENT AND THE BOARD ARE STRICTLY PROHIBITED. THE ORGANIZATION SEEKS FULL TRANSPARENCY ON ALL RELATIONSHIPS. ANY POTENTIAL CONFLICTS (IN FACT OR APPEARANCE) ARE DISCUSSED OPENLY AND RESOLVED IN ACCORDANCE WITH THE ORGANIZATION'S POLICIES AND PROCEDURES.</Explanation>
      </GeneralExplanation>
      <GeneralExplanation>
        <ReturnReference>FORM 990, PART VI, SECTION B, LINE 15</ReturnReference>
        <Explanation>THE COMPENSATION COMMITTEE OF THE THE BOARD OF DIRECTORS REVIEWS THE COMPENSATION OF ALL HIGH-LEVEL PERSONNEL ANNUALLY IN ACCORDANCE WITH IRS RULES AND REGULATIONS. EFFORTS ARE MADE TO SECURE COMPENSATION DATA FROM INDUSTRY SOURCES IN ORDER TO DETERMINE COMPETITIVENESS AND APPROPRIATENESS OF SALARIES AND BENEFITS. EVERY EFFORT IS MADE TO ENSURE THAT THE PROCESS IS THOROUGH AND TRANSPARENT IN ACCORDANCE WITH APPROPRIATE GOVERNANCE AND REGULATORY COMPLIANCE. THIS IS PERFORMED ANNUALLY AND THE PROCESS IS MANAGED BY THE HR DIRECTOR. REVIEW ENCOMPASSES CEO, CFO, EXECUTIVES AND KEY EMPLOYEES.</Explanation>
      </GeneralExplanation>
      <GeneralExplanation>
        <ReturnReference>FORM 990, PART VI, SECTION C, LINE 19</ReturnReference>
        <Explanation>PLOS MAKES ITS GOVERNING DOCUMENTS, CONFLICT OF INTEREST POLICY, AND FINANCIAL STATEMENTS AVAILABLE TO THE PUBLIC UPON REQUEST. THESE DOCUMENTS ARE ALSO POSTED ON OUR WEBSITE.</Explanation>
      </GeneralExplanation>
      <GeneralExplanation>
        <Explanation>FORM 990, PART XII, LINE 2C: THE PROCESS HAS NOT CHANGED FROM PRIOR YEAR.</Explanation>
      </GeneralExplanation>
    </IRS990ScheduleO>
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</Return>