Sec Form 3 Filing - Conway Kevin John @ Generation Bio Co. - 2025-01-10

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 Conway Kevin John 2. Issuer Name and Ticker or Trading Symbol Generation Bio Co. [ GBIO] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Chief Financial Officer (Last) (First) (Middle) C/O GENERATION BIO CO., 301 BINNEY ST, SUITE 401 3. Date of Earliest Transaction (MM/DD/YY) 01/10/2025 (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 17,062 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) $ 6.3587 ( 1 ) 08/11/2029 Common Stock 11,927 D Stock Option (right to buy) $ 5.1576 ( 2 ) 03/04/2030 Common Stock 1,132 D Stock Option (right to buy) $ 5.1576 ( 3 ) 03/04/2030 Common Stock 1,700 D Stock Option (right to buy) $ 19 ( 4 ) 06/10/2030 Common Stock 10,621 D Stock Option (right to buy) $ 30.25 ( 5 ) 01/21/2031 Common Stock 6,210 D Stock Option (right to buy) $ 31.69 ( 6 ) 05/31/2031 Common Stock 14,000 D Stock Option (right to buy) $ 6.49 ( 7 ) 01/20/2032 Common Stock 25,856 D Stock Option (right to buy) $ 8.18 ( 8 ) 03/31/2032 Common Stock 28,000 D Stock Option (right to buy) $ 4.705 ( 9 ) 01/19/2033 Common Stock 18,781 D Stock Option (right to buy) $ 5.06 ( 10 ) 08/31/2033 Common Stock 5,500 D Stock Option (right to buy) $ 1.39 ( 11 ) 11/30/2033 Common Stock 17,634 D Stock Option (right to buy) $ 1.86 ( 12 ) 01/01/2034 Common Stock 59,400 D Stock Option (right to buy) $ 4.29 ( 13 ) 03/31/2034 Common Stock 2,000 D Stock Option (right to buy) $ 3.16 ( 14 ) 07/30/2034 Common Stock 2,500 D Restricted Stock Units ( 16 ) ( 15 ) ( 15 ) Common Stock 5,281 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Conway Kevin John C/O GENERATION BIO CO. 301 BINNEY ST, SUITE 401 CAMBRIDGE, MA02142 Chief Financial Officer Signatures /s/ Shawna-Gay White, Attorney-in-Fact 01/14/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The option to purchase 15,852 shares was granted on August 12, 2019. The shares underlying the option vest over four years, with 25% of the shares vested on July 22, 2020 and the remaining shares vesting in equal quarterly installments thereafter. ( 2 )The option to purchase 2,264 shares was granted on March 5, 2020. The shares underlying the option vested over two years, with 25% of the shares vested on August 24, 2020 and the remaining shares vesting in equal quarterly installments thereafter. ( 3 )The option to purchase 2,264 shares was granted on March 5, 2020. The shares underlying the option vested over four years, with 25% of the shares vested on March 5, 2021 and the remaining shares vesting in equal quarterly installments thereafter. ( 4 )The option to purchase 10,621 shares was granted on June 11, 2020. The shares underlying the option vested over four years, with 25% of the shares vested on April 1, 2021 and the remaining shares vesting in equal quarterly installments thereafter. ( 5 )The option to purchase 6,210 shares was granted on January 22, 2021. The shares underlying the option vested over four years, with 25% of the shares vested on January 22, 2022 and the remaining shares vesting in equal quarterly installments thereafter. ( 6 )The option to purchase 14,000 shares was granted on June 1, 2021. The shares underlying the option vested over four years, with 25% of the shares vested on May 1, 2022 and the remaining shares vesting in equal quarterly installments thereafter. ( 7 )The option to purchase 25,856 shares was granted on January 21, 2022. The shares underlying the option vested over four years, with 25% of the shares vested on January 21, 2023 and the remaining shares vesting in equal quarterly installments thereafter. ( 8 )The option to purchase 28,000 shares was granted on April 1, 2022. The shares underlying the option vested over four years, with 25% of the shares vested on March 1, 2023 and the remaining shares vesting in equal quarterly installments thereafter. ( 9 )The option to purchase 18,781 shares was granted on January 20, 2023. The shares underlying the option vested over four years, with 25% of the shares vested on January 20, 2024 and the remaining shares vesting in equal quarterly installments thereafter. ( 10 )The option to purchase 5,500 shares was granted on September 1, 2023. The shares underlying the option vested over two years, with 25% of the shares vested on March 1, 2024 and the remaining shares vesting in equal quarterly installments thereafter. ( 11 )The option to purchase 28,215 shares was granted on December 1, 2023. The shares underlying the option vested over two years, with 25% of the shares vested on June 1, 2024 and the remaining shares vesting in equal quarterly installments thereafter. ( 12 )The option to purchase 59,400 shares was granted on January 2, 2024. The shares underlying the option vested over four years, with 25% of the shares vesting on January 2, 2025 and the remaining shares vesting in equal quarterly installments thereafter. ( 13 )The option to purchase 2,000 shares was granted on April 1, 2024. The shares underlying the option vested over two years, with 25% of the shares vested on October 1, 2024 and the remaining shares vesting in equal quarterly installments thereafter. ( 14 )The option to purchase 2,500 shares was granted on August 1, 2024. The shares underlying the option vested over two years, with 25% of the shares vesting on February 1, 2025 and the remaining shares vesting in equal quarterly installments thereafter. ( 15 )The grant of 9,390 restricted stock units was made on January 20, 2023. The shares underlying the restricted stock units vest over four years, with 25% of the shares vested on January 15, 2024 and the remaining shares vesting in equal quarterly installments thereafter. ( 16 )Each restricted stock unit represents the right to receive one share of the company's common stock.

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).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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