X0306 4 2021-08-03 0 0001850838 Omega Therapeutics, Inc. OMGA 0001400240 Flagship Pioneering Inc. 55 CAMBRIDGE PARKWAY, SUITE 800 CAMBRIDGE MA 02142 0 0 1 0 0001724957 Flagship Pioneering Fund VI General Partner LLC 55 CAMBRIDGE PARKWAY, SUITE 800 CAMBRIDGE MA 02142 0 0 1 0 0001716926 Flagship Pioneering Fund VI, L.P. 55 CAMBRIDGE PARKWAY, SUITE 800 CAMBRIDGE MA 02142 0 0 1 0 0001761283 Flagship Pioneering Special Opportunities Fund II, L.P. 55 CAMBRIDGE PARKWAY, SUITE 800 CAMBRIDGE MA 02142 0 0 1 0 0001826217 Flagship Pioneering Special Opportunities Fund II General Partner LLC 55 CAMBRIDGE PARKWAY, SUITE 800 CAMBRIDGE MA 02142 0 0 1 0 0001780956 Nutritional Health LTP Fund, L.P. 55 CAMBRIDGE PARKWAY, SUITE 800 CAMBRIDGE MA 02142 0 0 1 0 0001780951 Nutritional Health LTP Fund General Partner LLC 55 CAMBRIDGE PARKWAY, SUITE 800 CAMBRIDGE MA 0 0 1 0 0001860059 FPN General Partner LLC GENERAL PARTNER, LLC 55 CAMBRIDGE PARKWAY, SUITE 800 CAMBRIDGE MA 02142 0 0 1 0 0001860062 FPN, L.P. GENERAL PARTNER, LLC 55 CAMBRIDGE PARKWAY, SUITE 800 CAMBRIDGE MA 02142 0 0 1 0 0001222012 AFEYAN NOUBAR C/O OMEGA THERAPEUTICS, INC. 20 ACORN PARK DRIVE CAMBRIDGE MA 02140 1 0 1 0 Common Stock 2021-08-03 4 C 0 1088470 A 1088470 I See Footnotes Common Stock 2021-08-03 4 C 0 5896386 A 5896386 I See Footnotes Common Stock 2021-08-03 4 C 0 8396825 A 8396825 I See Footnotes Common Stock 2021-08-03 4 P 0 220588 17.00 A 8617413 I See Footnotes Common Stock 2021-08-03 4 C 0 970588 A 970588 I See Footnotes Common Stock 2021-08-03 4 C 0 4852943 A 4852943 I See Footnotes Common Stock 2021-08-03 4 P 0 661764 17.00 A 5514707 I See Footnotes Common Stock 2021-08-03 4 C 0 1323529 A 1323529 I See Footnotes Common Stock 2021-08-03 4 P 0 294117 17.00 A 1617646 I See Footnotes Common Stock 2197059 I See Footnotes Series A Convertible Preferred Stock 2021-08-03 4 C 0 4112000 D Common Stock 1088470 0 I See Footnotes Series A Convertible Preferred Stock 2021-08-03 4 C 0 22275232 D Common Stock 5896386 0 I See Footnotes Series A Convertible Preferred Stock 2021-08-03 4 C 0 27388000 D Common Stock 7249767 0 I See Footnotes Series A Convertible Preferred Stock 2021-08-03 4 C 0 3000000 D Common Stock 794118 0 I See Footnotes Series B Convertible Preferred Stock 2021-08-03 4 C 0 2666667 D Common Stock 705882 0 I See Footnotes Series B Convertible Preferred Stock 2021-08-03 4 C 0 666667 D Common Stock 176470 0 I See Footnotes Series B Convertible Preferred Stock 2021-08-03 4 C 0 13333333 D Common Stock 3529413 0 I See Footnotes Series B Convertible Preferred Stock 2021-08-03 4 C 0 3333333 D Common Stock 882353 0 I See Footnotes Series C Convertible Preferred Stock 2021-08-03 4 C 0 1666667 D Common Stock 441176 0 I See Footnotes Series C Convertible Preferred Stock 2021-08-03 4 C 0 5000000 D Common Stock 1323530 0 I See Footnotes Series C Convertible Preferred Stock 2021-08-03 4 C 0 1666666 D Common Stock 441176 0 I See Footnotes The shares of Issuer's preferred stock have no expiration date and are convertible at the holder's election into Issuer's common stock at a conversion ratio of 1-to-0.264706. The preferred stock will automatically convert into shares of the Issuer's common stock, for no additional consideration, upon the closing of the Issuer's initial public offering. Represents shares held directly by Flagship V VentureLabs Rx Fund, L.P. ("Flagship Fund V Rx"). Represents shares held directly by Flagship Ventures Fund V, L.P. ("Flagship Fund V"). Represents shares held directly by Flagship Pioneering Fund VI, L.P. ("Flagship Pioneering VI"). Represents shares held directly by Nutritional Health LTP Fund, L.P. ("Nutritional LTP"). Represents shares held directly by Flagship Pioneering Special Opportunities Fund II, L.P. ("Flagship Opportunities Fund II") Represents shares held directly by FPN, L.P. ("FPN Fund"). Represents shares held directly by Flagship VentureLabs V, LLC ("VentureLabs V"). Flagship Fund V is a member of VentureLabs V. VentureLabs V Manager LLC ("VentureLabs V Manager") is the manager of VentureLabs V. Flagship Pioneering, Inc. ("Flagship Pioneering") is the manager of VentureLabs V Manager. The General Partner of Flagship Fund V and Flagship Fund V Rx is Flagship Ventures Fund V General Partner LLC ("Flagship V GP"). The General Partner of Flagship Pioneering VI is Flagship Pioneering Fund VI General Partner LLC ("Flagship Pioneering VI GP"). The General Partner of Flagship Opportunities Fund II is Flagship Pioneering Special Opportunities Fund II General Partner LLC ("Flagship Opportunities Fund II GP"). (continued from above) The general partner of FPN Fund is FPN General Partner LLC ("FPN GP"). The manager of Flagship Pioneering VI GP, Flagship Opportunities Fund II GP, and FPN GP is Flagship Pioneering. The general partner of Nutritional LTP is Nutritional Health LTP Fund General Partner LLC ("Nutritional LTP GP" and, together with VentureLabs V Manager, Flagship Pioneering, Flagship V GP, Flagship Pioneering VI GP, Flagship Opportunities Fund II GP and FPN GP, the "Flagship General Partners"). (continued from above) Noubar B. Afeyan, Ph.D. ("Dr. Afeyan") is the sole director of Flagship Pioneering and may be deemed to have beneficial ownership of all the shares held by VentureLabs V, Flagship Pioneering VI, Flagship Opportunities Fund II and FPN Fund. In addition, Dr. Afeyan serves as the sole manager of Flagship V GP and is the sole member and manager of Nutritional LTP GP and may be deemed to have beneficial ownership of all the shares held by Flagship Fund V, Flagship Fund V Rx and Nutritional LTP. None of the Flagship General Partners nor Dr. Afeyan directly own any of the shares held by the Reporting Persons, and each of the Flagship General Partners and Dr. Afeyan disclaims beneficial ownership of such shares except to the extent of its or his pecuniary interest therein. Due to the limitations of the electronic filing system, each of Flagship Ventures Fund V General Partner LLC, Flagship V VentureLabs RX Fund, L.P., Flagship Ventures Fund V, L.P., Flagship VentureLabs V Manager LLC and Flagship VentureLabs V LLC are filing a separate Form 4. Each Form 4 will be filed by Designated Filer Flagship Pioneering, Inc. Flagship Pioneering, Inc., By: /s/ Noubar B. Afeyan, Director 2021-08-05 Flagship Pioneering Fund VI General Partner LLC, By: Flagship Pioneering, Inc., its Manager, By: /s/ Noubar B. Afeyan, Director 2021-08-05 Flagship Pioneering Fund VI, L.P., By: Flagship Pioneering Fund VI General Partner LLC, its general partner, By: Flagship Pioneering, Inc., its Manager, By: /s/ Noubar B. Afeyan, Director 2021-08-05 Flagship Pioneering Special Opportunities Fund II, L.P., By: Flagship Pioneering, Inc. its manager, By: /s/ Noubar B. Afeyan Ph.D., CEO 2021-08-05 Flagship Pioneering Special Opportunities Fund II General Partner LLC, By: /s/ Noubar B. Afeyan Ph.D., Manager 2021-08-05 Nutritional Health LTP Fund, L.P., By: Nutritional Health LTP Fund General Partner LLC, its general partner, By: /s/ Noubar B. Afeyan, Ph.D., Manager 2021-08-05 Nutritional Health LTP Fund General Partner LLC, By: /s/ Noubar B. Afeyan, Ph.D., Manager 2021-08-05 FPN, L.P., By: FPN General Partner LLC, its general partner, By: Flagship Pioneering, Inc., its manager, By: /s/ Noubar B. Afeyan, Ph.D., its Chief Executive Officer 2021-08-05 FPN General Partner LLC, By: Flagship Pioneering, Inc., its manager, By: /s/ Noubar B. Afeyan, Ph.D., its Chief Executive Officer 2021-08-05 /s/ Noubar B. Afeyan, Ph.D. 2021-08-05