X0306 4 2020-07-31 0 0001818794 Dyne Therapeutics, Inc. DYN 0001702233 Stehman-Breen Catherine C/O DYNE THERAPEUTICS, INC. 830 WINTER STREET WALTHAM MA 02451 1 0 0 0 Common Stock 2020-09-21 4 C 0 10725 A 115713 D Stock Option (right to buy) 5.54 2020-07-31 4 A 0 11156 0.00 A 2030-07-30 Common Stock 11156 11156 D Stock Option (right to buy) 5.54 2020-07-31 4 A 0 14444 0.00 A 2030-07-30 Common Stock 14444 14444 D Series B Preferred Stock 2020-08-07 4 P 0 35574 2.811 A Common Stock 10725 35574 D Series B Preferred Stock 2020-09-21 4 C 0 35574 0.00 D Common Stock 10725 0 D On September 21, 2020, the Series B Preferred Stock automatically converted into Common Stock on a 3.3169-for-one basis without payment of further consideration upon closing of the initial public offering of the Issuer's common stock. The shares had no expiration date. The option was granted on July 31, 2020. The shares underlying the option are scheduled to vest over four years in equal quarterly installments through July 31, 2024. The option was granted on July 31, 2020. The shares underlying the option are scheduled to vest over four years in equal quarterly installments through July 31, 2024. The Series B Preferred Stock was convertible into common stock on a 3.3169-for-one basis into the number of shares of common stock shown in Column 7 without payment of further consideration at the holder's election or upon closing of the initial public offering of the Issuer's common stock. The shares had no expiration date. /s/ Richard Scalzo, Attorney-in-Fact 2020-09-23