X0306 4 2021-04-07 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 Common Stock 2021-04-07 4 S 0 37970 28.48 D 229807 D Common Stock 2021-04-07 4 S 0 1894 29.19 D 227913 D Common Stock 2021-04-08 4 S 0 40359 28.47 D 187554 D Common Stock 2021-04-08 4 S 0 3191 28.90 D 184363 D Common Stock 2021-04-09 4 S 0 41549 27.57 D 142814 D Common Stock 2021-04-09 4 S 0 2132 28.35 D 140682 D Common Stock 9111939 I See footnote Shares were sold pursuant to a Rule 10b5-1 trading plan dated January 14, 2021. The shares reported herein give effect to the pro rata distribution of shares by Atlas Venture Fund X, L.P. ("Atlas Venture Fund X") for no additional consideration to its limited partners and its and general partner, Atlas Venture Associates X, L.P. ("AVA X LP"), subsequent to the Reporting Persons' last Section 16 filing reporting ownership of the Issuer's Common Stock. As the distribution of such shares constituted only a change in the form of the Reporting Persons' ownership in such shares, the Reporting Persons were not required to report the distribution pursuant to Section 16. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $28.10 to $29.07 inclusive. The Reporting Person 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 (3), (5), (6), (7), (8) and (9). The shares are held directly by 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 price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $29.12 to $29.30 inclusive. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $27.79 to $28.785 inclusive. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $28.79 to $29.00 inclusive. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $27.18 to $28.17 inclusive. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $28.18 to $28.63 inclusive. These shares are held directly by Atlas Venture Fund X. The general partner of Atlas Venture Fund X is AVA X LP and the general partner of AVA X LP is AVA X LLC. Each of AVA X LP and AVA LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Fund X, except to the extend of its respective 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 2021-04-09 Atlas Venture Associates X, LLC, By: /s/ Ommer Chohan, Chief Financial Officer 2021-04-09