Sec Form 3 Filing - Atlas Venture Fund X, L.P. @ Q32 Bio Inc. - 2024-03-25

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 Atlas Venture Fund X, L.P. 2. Issuer Name and Ticker or Trading Symbol Q32 Bio Inc. [ QTTB] 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) 300 TECHNOLOGY SQUARE, 8TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 03/25/2024 (Street) CAMBRIDGE, MA02139 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 864,261 D ( 1 ) Common Stock 503,296 I By Atlas Venture Opportunity Fund I, L.P. ( 2 ) Common Stock 724,549 I By Atlas Venture Opportunity Fund II, L.P. ( 3 )

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 Atlas Venture Fund X, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X ATLAS VENTURE ASSOCIATES X, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates X, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Opportunity Fund I, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates Opportunity I, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates Opportunity I, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Opportunity Fund II, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates Opportunity II, LP 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates Opportunity II, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Signatures Atlas Venture Fund X, L.P., By: Atlas Venture Associates X, L.P., its general partner, by Atlas Venture Associates X, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer 03/27/2024 Signature of Reporting Person Date Atlas Venture Associates X, L.P., by Atlas Venture Associates X, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer 03/27/2024 Signature of Reporting Person Date Atlas Venture Associates X, LLC, By: /s/ Ommer Chohan, Chief Financial Officer 03/27/2024 Signature of Reporting Person Date Atlas Venture Opportunity Fund I, L.P., By: Atlas Venture Associates Opportunity I, L.P., its General Partner, By: Atlas Venture Associates Opportunity I, LLC, its General Partner, By: /s/ Ommer Chohan, Chief Financial Officer 03/27/2024 Signature of Reporting Person Date Atlas Venture Associates Opportunity I, L.P., By: Atlas Venture Associates Opportunity I, LLC, its General Partner, By: /s/ Ommer Chohan, Chief Financial Officer 03/27/2024 Signature of Reporting Person Date Atlas Venture Associates Opportunity I, LLC, By: /s/ Ommer Chohan, Chief Financial Officer 03/27/2024 Signature of Reporting Person Date Atlas Venture Opportunity Fund II, L.P., By: Atlas Venture Associates Opportunity II, L.P., its General Partner, By: Atlas Venture Associates Opportunity II, LLC, its General Partner, By: /s/ Ommer Chohan, Chief Financial Officer 03/27/2024 Signature of Reporting Person Date Atlas Venture Associates Opportunity II, L.P., By: Atlas Venture Associates Opportunity II, LLC, its General Partner, By: /s/ Ommer Chohan, Chief Financial Officer 03/27/2024 Signature of Reporting Person Date Atlas Venture Associates Opportunity II, LLC, By: /s/ Ommer Chohan, Chief Financial Officer 03/27/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Shares held by Atlas Venture Fund X, L.P. ("Atlas Venture Fund X"). The general partner of Atlas Venture Fund X is Atlas Venture Associates X, L.P. ("AVA X LP"). Atlas Venture Associates X, LLC ("AVA X LLC") is the general partner of AVA X LP. Each of AVA X LP and AVA X LLC disclaims beneficial ownership of these shares except to the extent of its pecuniary interest therein, if any, and this report shall not be deemed an admission that it is the beneficial owner of such shares for purposes of Section 16 of the Exchange Act or for any other purpose. ( 2 )Shares held by Atlas Venture Opportunity Fund I, L.P. ("AVOF I"). Atlas Venture Associates Opportunity I, L.P. ("AVAO LP") is the general partner of AVOF I. Atlas Venture Associates Opportunity I, LLC ("AVAO LLC") is the general partner of AVAO LP. Each of AVAO LP and AVAO LLC disclaims beneficial ownership of these shares except to the extent of its pecuniary interest therein, if any, and this report shall not be deemed an admission that it is the beneficial owner of such shares for purposes of Section 16 of the Exchange Act or for any other purpose. ( 3 )The shares are owned directly by Atlas Venture Opportunity Fund II, L.P. ("AVOF II"). Atlas Venture Associates Opportunity II, L.P. ("AVAO II LP") is the general partner of AVOF II. Atlas Venture Associates Opportunity II, LLC ("AVAO II LLC") is the general partner of AVAO II LP. Each of AVAO II LP and AVAO II LLC disclaims beneficial ownership of these shares except to the extent of its pecuniary interest therein, if any, and this report shall not be deemed an admission that it is the beneficial owner of such shares for purposes of Section 16 of the Exchange Act or for any other purpose.

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

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