Sec Form 4 Filing - AFEYAN NOUBAR @ Seres Therapeutics, Inc. - 2019-06-18

FORM 4 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 AFEYAN NOUBAR 2. Issuer Name and Ticker or Trading Symbol Seres Therapeutics, Inc. [ MCRB] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ 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) __ X __ Form filed by One Reporting Person _____ 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 06/18/2019 P 4,444,444 ( 1 ) A $ 2.25 4,444,444 I See Footnote ( 2 ) Common Stock 06/18/2019 P 4,444,444 ( 3 ) A $ 2.25 4,444,444 I See Footnote ( 4 ) Common Stock 8,022,420 I See Footnote ( 5 ) Common Stock 1,925,462 I See Footnote ( 6 ) Common Stock 2,734,994 I See Footnote ( 7 ) Common Stock 15,141 D

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 AFEYAN NOUBAR 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA02142 X X Signatures /s/ Noubar B. Afeyan 06/20/2019 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. Dr. Afeyan disclaims beneficial ownership of such shares except to the extent of his or 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. Dr. Afeyan disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. ( 5 )The shares are held directly by Flagship Ventures Fund IV, L.P. ("Flagship Fund IV"). Flagship Ventures Fund IV General Partner LLC ("Flagship Fund IV GP") is the general partner of Flagship Fund IV. Noubar B. Afeyan, Ph.D. and Edwin M. Kania, Jr. are the managers of Flagship Fund IV GP and each of these managers may be deemed to share voting and investment power with respect to all shares held by Flagship Fund IV. While Mr. Kania is retired from Flagship Pioneering, Inc., he continues to serve as a manager of Flagship Fund IV GP. Dr. Afeyan disclaims beneficial ownership of the shares except to the extent of his or its pecuniary interest therein. ( 6 )The shares are held directly by Flagship Ventures Fund IV-Rx, L.P. ("Flagship IV-Rx"). Flagship Fund IV GP is the general partner of Flagship IV-Rx. Dr. Afeyan disclaims beneficial ownership of the shares except to the extent of his or its pecuniary interest therein. ( 7 )The shares are held directly by Flagship VentureLabs IV LLC ("VentureLabs IV"). Flagship Fund IV is a member of VentureLabs IV and also serves as its manager. Dr. Afeyan disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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