X0306 4 2021-09-27 0 0001733294 Generation Bio Co. GBIO 0001742766 ATLAS VENTURE ASSOCIATES X, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001742765 Atlas Venture Associates X, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001759241 Atlas Venture Opportunity Fund I, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001780066 Atlas Venture Associates Opportunity I, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 Common Stock 2021-09-27 4 S 0 51294 25.70 D 29645 D Common Stock 2021-09-27 4 S 0 95 25.70 D 662 I See footnote Common Stock 2021-09-28 4 S 0 13089 25.24 D 16556 D Common Stock 2021-09-28 4 S 0 24 25.24 D 638 I See footnote Common Stock 2021-09-29 4 S 0 16556 24.59 D 0 D Common Stock 2021-09-29 4 S 0 30 24.59 D 608 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 $25.15 to $26.07 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), (5) and (6). 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. AVA X LLC disclaims Section 16 beneficial ownership of the securities held by AVA X LP except to the extent of its pecuniary interest therein, if any. 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. AVAO LLC disclaims Section 16 beneficial ownership of the securities held by AVAO LP except to the extent of its 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 $24.93 to $25.79 inclusive. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $24.46 to $25.36 inclusive. The 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. Each of AVA X LP and AVA X LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Fund X, except to the extent of its pecuniary interest therein, if any. The shares are held directly by Atlas Venture Opportunity Fund I, L.P. ("Atlas Venture Opportunity Fund I"). The general partner of Atlas Venture Opportunity Fund I is AVAO I LP. AVAO I LLC is the general partner of AVAO I LP. Each of AVAO I LP and AVAO I LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Opportunity Fund I, except to the extent of its pecuniary interest therein, if any. Atlas Venture Associates X, L.P., By: Atlas Venture Associates X, LLC, Its: General Partner, By: /s/ Ommer Chohan, Chief Financial Officer 2021-09-29 Atlas Venture Associates X, LLC, By: /s/ Ommer Chohan, Chief Financial Officer 2021-09-29 Atlas Venture Associates Opportunity I, L.P., By: Atlas Venture Associates Opportunity I, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 2021-09-29 Atlas Venture Associates Opportunity I, LLC, By: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 2021-09-29