Sec Form 3 Filing - Angelino Mark D. @ Generation Bio Co. - 2020-06-11

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 Angelino Mark D. 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 Operating Officer (Last) (First) (Middle) C/O GENERATION BIO CO.,, 301 BINNEY STREET 3. Date of Earliest Transaction (MM/DD/YY) 06/11/2020 (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 283,077 D Common Stock 169,846 I See Footnote ( 1 )

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) $ 0.6 ( 2 ) 01/04/2028 Common Stock 100,096 D Stock Option (right to buy) $ 4.6 ( 3 ) 04/01/2029 Common Stock 123,761 D Stock Option (right to buy) $ 5.16 ( 4 ) 03/04/2030 Common Stock 84,597 D Stock Option (right to buy) $ 5.16 ( 5 ) 03/04/2030 Common Stock 84,596 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Angelino Mark D. C/O GENERATION BIO CO., 301 BINNEY STREET CAMBRIDGE, MA02142 Chief Operating Officer Signatures /s/ Jennifer Elliott, Attorney-in-Fact 06/11/2020 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The securities are directly held by the Mark D. Angelino 2020 Qualified Annuity Trust, or the Trust. Mark D. Angelino is the settlor and trustee of the Trust, and the Trust is for the benefit of Elizabeth I., Emily A., John P., and Matthew R. Angelino. The reporting person may be deemed to beneficially own the shares of common stock owned by the Trust. ( 2 )The option was granted on January 5, 2018. The shares underlying the option vest over four years, with 25% of the shares vested on January 5, 2019 and the remaining shares vesting in equal quarterly installments thereafter. ( 3 )The option was granted on April 2, 2018. The shares underlying the option vest over four years, with 25% of the shares vested on April 2, 2019 and the remaining shares vesting in equal quarterly installments thereafter. ( 4 )The option was granted on March 5, 2020. The shares underlying the option vest in full upon the date on which the Board of Directors of the Issuer determines that the Issuer has dosed the initial patient in its first clinical trial, provided that such event occurs on or before July 1, 2023. ( 5 )The option was granted on March 5, 2020. The shares underlying the option vest over four years, with 25% of the shares vesting on March 5, 2021 and the remaining shares vesting in equal quarterly installments thereafter.

Remarks: Exhibit Index: 24.1 Power of Attorney

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