X0306 4 2021-09-22 0 0001733294 Generation Bio Co. GBIO 0001577014 Rhodes Jason P C/O GENERATION BIO CO. 301 BINNEY STREET CAMBRIDGE MA 02142 1 0 0 0 Common Stock 2021-09-22 4 S 0 30560 25.891 D 185126 I See footnote Common Stock 2021-09-23 4 S 0 75324 26.133 D 109802 I See footnote Common Stock 2021-09-23 4 S 0 1 26.55 D 109801 I See footnote Common Stock 2021-09-24 4 S 0 28862 25.839 D 80939 I See footnote Common Stock 2021-09-22 4 S 0 57 25.891 D 951 I See footnote Common Stock 2021-09-23 4 S 0 140 26.133 D 811 I See footnote Common Stock 2021-09-24 4 S 0 54 25.839 D 757 I See footnote Common Stock 7111939 I See footnote Common Stock 1166937 I See footnote Shares were sold pursuant to a Rule 10b5-1 trading plan dated April 20, 2021. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $25.54 to $26.21 inclusive. The Reporting Persons undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in footnotes (2), (4) and (5). The shares are held directly by 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. The Reporting Person is a member of AVA X LLC and disclaims Section 16 beneficial ownership of such securities held by AVA X LP, except to the extent of his pecuniary interest therein, if any. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $25.53 to $26.518 inclusive. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $25.40 to $26.21 inclusive. The shares are held directly by Atlas Venture Associates Opportunity I, LP ("AVAO LP"). Atlas Venture Associates Opportunity I, LLC ("AVAO LLC") is the general partner of AVAO LP. The Reporting Person is a member of AVAO LLC and disclaims Section 16 beneficial ownership of such securities held by AVAO LP, except to the extent of his pecuniary interest therein, if any. These shares are held directly by Atlas Venture Fund X, L.P. ("Atlas Venture Fund X"). The general partner of Atlas Venture Fund X is AVA X LP. AVA X LLC is the general partner of AVA X LP. The Reporting Person is a member of AVA X LLC and disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Fund X, except to the extend if its pecuniary interest therein, if any. The shares are owned directly by Atlas Venture Opportunity Fund I, L.P. ("AVOF I"). AVAO LP is the general partner of AVOF I. AVAO LLC is the general partner of AVAO LP. The Reporting Person is a member of AVAO LLC and disclaims Section 16 beneficial ownership of such securities held by AVOF I, except to the extent of his pecuniary interest therein, if any. /s/ Ommer Chohan, Attorney-in-Fact 2021-09-24