Sec Form 3 Filing - Dooley Mary Beth @ SYNLOGIC, INC. - 2024-03-18

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 Dooley Mary Beth 2. Issuer Name and Ticker or Trading Symbol SYNLOGIC, INC. [ SYBX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Head of Finance (Last) (First) (Middle) C/O SYNLOGIC, INC., 301 BINNEY STREET, SUITE 402 3. Date of Earliest Transaction (MM/DD/YY) 03/18/2024 (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 15,419 ( 1 ) 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 Stock Option (right to buy) $ 1.85 ( 2 ) 06/29/2028 Common Stock 333 D Stock Option (right to buy) $ 1.85 ( 2 ) 11/30/2028 Common Stock 200 D Stock Option (right to buy) $ 1.85 ( 2 ) 03/05/2029 Common Stock 466 D Stock Option (right to buy) $ 1.85 ( 3 ) 03/10/2030 Common Stock 625 D Stock Option (right to buy) $ 1.85 ( 2 ) 03/10/2030 Common Stock 500 D Stock Option (right to buy) $ 1.85 ( 4 ) 03/10/2031 Common Stock 2,083 D Stock Option (right to buy) $ 1.85 ( 5 ) 02/04/2032 Common Stock 3,255 D Stock Option (right to buy) $ 1.85 ( 6 ) 09/30/2032 Common Stock 3,916 D Stock Option (right to buy) $ 1.85 ( 7 ) 03/16/2033 Common Stock 3,612 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Dooley Mary Beth C/O SYNLOGIC, INC. 301 BINNEY STREET, SUITE 402 CAMBRIDGE, MA02142 Head of Finance Signatures /s/ Mary Beth Dooley 03/21/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Includes 14,485 restricted stock awards subject to time-based vesting. ( 2 )The shares subject to this option are fully vested as of the date hereof. ( 3 )The shares subject to this option vest in approximately equal monthly installments through April 1, 2024. ( 4 )The shares subject to this option vest in approximately equal monthly installments through April 1, 2025. ( 5 )The shares subject to this option vest in approximately equal monthly installments through February 4, 2026. ( 6 )The shares subject to this option vest in approximately equal monthly installments through September 16, 2026. ( 7 )The shares subject to this option vest in approximately equal monthly installments through April 1, 2027.

Remarks: Exhibit 24.1 - Power of Attorney

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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