X0306 4 2020-09-21 1 0001818794 Dyne Therapeutics, Inc. DYN 0001824569 VV Manager II, LLC 40 BROAD STREET, SUITE 201 BOSTON MA 02109 0 0 1 0 0001776851 Vida Ventures II, LLC 40 BROAD STREET, SUITE 201 BOSTON MA 02109 0 0 1 0 0001781930 VIDA VENTURES II-A, LLC 40 BROAD STREET, SUITE 201 BOSTON MA 02109 0 0 1 0 Common Stock 2020-09-21 4 C 0 2087130 A 2087130 I See footnotes Common Stock 2020-09-21 4 P 0 792995 19.00 A 2880125 I See footnotes Common Stock 2020-09-21 4 C 0 57916 A 57916 I See footnote Common Stock 2020-09-21 4 P 0 22005 19.00 A 79921 I See footnote Series B Preferred Stock 2020-09-21 4 C 0 6922803 0.00 D Common Stock 2087130 0 I See footnotes Series B Preferred Stock 2020-09-21 4 C 0 192102 0.00 D Common Stock 57916 0 I See footnote 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 shares are held directly by Vida Ventures II, LLC ("Vida II"). VV Manager II, LLC ("VV Manager II") is the manager of Vida II. Arie Belldegrun, Fred Cohen, and Leonard Potter are the members of the management committee of VV Manager II (the "Management Committee") and Stefan Vitorovic, Arjun Goyal, Helen Kim, Rajul Jain, and Joshua Kazam are the members of the investment committee of VV Manager II (the "Investment Committee"). (Continued from Footnote 2) Each of the Management Committee, the Investment Committee and the respective members thereof may be deemed to share voting and dispositive power over the shares held by Vida II. VV Manager II, the Management Committee, the Investment Committee and each member of each of the Management Committee and Investment Committee disclaims beneficial ownership over the securities held of record by Vida II and this report shall not be deemed an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interest therein. The shares are held directly by Vida Ventures II-A, LLC ("Vida II-A"). VV Manager II is the manager of Vida II-A. Each of the Management Committee ,the Investment Committee and the respective members thereof may be deemed to share voting and dispositive power over the shares held by Vida II-A. VV Manager II, the Management Committee, the Investment Committee and each member of each of the Management Committee and Investment Committee disclaims beneficial ownership over the securities held of record by Vida II-A and this report shall not be deemed an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interest therein. Vida Ventures II, LLC By: VV Manager II, LLC, its Manager, By: /s/ Stefan Vitorovic, its Managing Director 2020-09-23 Vida Ventures II-A, LLC By: VV Manager II, LLC, its Manager, By: /s/ Stefan Vitorovic, its Managing Director 2020-09-23 VV Manager II, LLC, By: /s/ Stefan Vitorovic, its Managing Director 2020-09-23