Sec Form 3 Filing - Flagship Pioneering Inc. @ Seres Therapeutics, Inc. - 2019-06-18

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Flagship Pioneering Inc. 2. Issuer Name and Ticker or Trading Symbol Seres Therapeutics, Inc. [ MCRB] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 55 CAMBRIDGE PARKWAY 3. Date of Earliest Transaction (MM/DD/YY) 06/18/2019 (Street) CAMBRIDGE, MA02142 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 4,444,444 ( 1 ) I See Footnote ( 2 ) Common Stock 4,444,444 ( 3 ) I See Footnote ( 4 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Flagship Pioneering Inc. 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA02142 X Flagship Pioneering Fund VI General Partner LLC 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA02142 X Flagship Pioneering Fund VI, L.P. 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA02142 X Nutritional Health LTP Fund General Partner LLC 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA02142 X Nutritional Health LTP Fund, L.P. 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA02142 X Signatures Flagship Pioneering, Inc., By: /s/ Noubar B. Afeyan, Name: Noubar B. Afeyan, Ph.D., Title: CEO 06/28/2019 Signature of Reporting Person Date Flagship Pioneering Fund VI General Partner LLC, By: Flagship Pioneering, Inc., its manager, By: /s/ Noubar B. Afeyan, Name: Noubar B. Afeyan, Ph.D., Title: CEO 06/28/2019 Signature of Reporting Person Date Flagship Pioneering Fund VI, L.P., By: Flagship Pioneering Fund VI General Partner LLC, its general partner, By: Flagship Pioneering, Inc., its manager, By: /s/ Noubar B. Afeyan, Name: Noubar B. Afeyan, Ph.D., Title: CEO 06/28/2019 Signature of Reporting Person Date Nutritional Health LTP Fund General Partner LLC, By: /s/ Noubar B. Afeyan, Name: Noubar B. Afeyan, Ph.D., Title: Manager 06/28/2019 Signature of Reporting Person Date Nutritional Health LTP Fund, L.P., By: Nutritional Health LTP Fund General Partner LLC, its general partner, By: /s/ Noubar B. Afeyan, Name: Noubar B. Afeyan, Ph.D., Title: Manager 06/28/2019 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On June 18, 2019, Flagship Pioneering Fund VI, L.P. ("Flagship Fund VI") acquired 4,444,444 shares pursuant to an underwritten public offering of common stock by the issuer (the "Offering"). ( 2 )The shares are held by Flagship Fund VI. Flagship Pioneering Fund VI General Partner LLC ("Fund VI GP") is the general partner of Flagship Fund VI. Flagship Pioneering, Inc. ("Flagship Pioneering") is the manager of Fund VI GP. Noubar B. Afeyan Ph.D. is the CEO and sole shareholder of Flagship Pioneering and may be deemed to have sole voting and investment power with respect to all shares held by Flagship Fund VI. Each of the reporting persons disclaims beneficial ownership of such shares except to the extent of its pecuniary interest therein. ( 3 )On June 18, 2019, Nutritional Health LTP Fund, L.P. ("Nutritional LTP") acquired 4,444,444 shares pursuant to the Offering. ( 4 )The shares are held by Nutritional LTP. Nutritional Health LTP Fund General Partner LLC ("Nutritional LTP GP") is the general partner of Nutritional LTP. Noubar B. Afeyan Ph.D. is the sole member and manager of Nutritional LTP GP and may be deemed to have sole voting and investment power with respect to all shares held by Nutritional LTP. Each of the reporting persons disclaims beneficial ownership of such shares except to the extent of its pecuniary interest therein.

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