X0306 4 2021-03-19 0 0001709401 Rubius Therapeutics, Inc. RUBY 0001508051 Flagship Ventures Fund IV General Partner LLC 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE MA 02142 0 0 1 0 0001503559 Flagship Ventures Fund IV, L.P. 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE MA 02142 0 0 1 0 0001724575 Flagship Ventures Fund V General Partner LLC 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE MA 02142 0 0 1 0 0001627639 Flagship Ventures Fund V, L.P. 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE MA 02142 0 0 1 0 0001725004 Flagship Ventures Opportunities Fund I General Partner LLC 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE MA 02142 0 0 1 0 0001677345 Flagship Ventures Opportunities Fund I, L.P. 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE MA 02142 0 0 1 0 0001222012 AFEYAN NOUBAR 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE MA 02142 1 0 1 0 Common Stock 2021-03-19 4 P 0 70000 29.00 A 15393593 I See Footnote Common Stock 2021-03-19 4 P 0 70000 29.00 A 5859414 I See Footnote Common Stock 2021-03-19 4 P 0 70000 29.00 A 2633703 I See Footnote Common Stock 5000000 I See Footnote Common Stock 3830402 I See Footnote Common Stock 5789414 I See Footnote On March 19, 2021, Flagship Ventures Fund IV, L.P. ("Flagship Fund IV") acquired 70,000 shares pursuant to an underwritten public offering of common stock by the Issuer (the "Offering"). Shares held by Flagship Fund IV. Flagship Ventures Fund IV General Partner LLC ("Flagship Fund IV GP") is the general partner of Flagship Fund IV. Noubar B. Afeyan, Ph.D. ("Dr. Afeyan") is the sole manager of Flagship Fund IV GP. Each of the reporting persons except for Flagship Fund IV disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. Edwin M. Kania, Jr. is retired both from Flagship Pioneering, Inc. and as a manager of Flagship Fund IV GP. On March 19, 2021, Flagship Ventures Fund V, L.P. ("Flagship Fund V") acquired 70,000 shares pursuant to the Offering. Shares held by Flagship Fund V. Flagship Ventures Fund V General Partner LLC ("Flagship Fund V GP") is the general partner of Flagship Fund V. Dr. Afeyan is the sole manager of Flagship Fund V GP. Each of the reporting persons except Flagship Fund V disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. On March 19, 2021, Flagship Ventures Opportunities Fund I, L.P. ("Flagship Opportunities I") acquired 70,000 shares pursuant to the Offering. Shares held by Flagship Opportunities I. Flagship Ventures Opportunities Fund I General Partner LLC ("Flagship Opportunities GP") is the general partner of Flagship Opportunities I. Dr. Afeyan is the sole manager of Flagship Opportunities GP. Each of the reporting persons except for Flagship Opportunities I disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. Shares held by Flagship VentureLabs IV, LLC ("VentureLabs IV"). Flagship Fund IV is a member of VentureLabs IV and also serves as its manager. Flagship Fund IV GP is the general partner of Flagship Fund IV. Dr. Afeyan is the sole manager of Flagship Fund IV GP. Each of the reporting persons except for VentureLabs IV disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. Shares held by Flagship Ventures Fund IV-Rx, L.P. ("Flagship Fund IV-Rx"). Flagship Fund IV GP is the general partner of Flagship Fund IV-Rx. Dr. Afeyan is the sole manager of Flagship Fund IV GP. Each of the reporting persons except for Flagship Fund IV-Rx disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. Shares held by Flagship V VentureLabs Rx Fund, L.P. ("VentureLabs Rx V"). Flagship Fund V GP is the general partner of VentureLabs Rx V. Dr. Afeyan is the sole manager of Flagship Fund V GP. Each of the reporting persons except VentureLabs Rx V disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. Flagship Ventures Fund IV General Partner LLC, By: /s/ Noubar B. Afeyan, Ph.D., Title: Manager 2021-03-23 Flagship Ventures Fund IV, L.P., By: Flagship Ventures Fund IV General Partner LLC, its general partner, By: /s/ Noubar B. Afeyan, Ph.D., Title: Manager 2021-03-23 Flagship Ventures Fund V General Partner LLC, By: /s/ Noubar B. Afeyan, Ph.D., Title: Manager 2021-03-23 Flagship Ventures Fund V, L.P., By: Flagship Ventures Fund V General Partner LLC, its general partner, By: /s/ Noubar B. Afeyan, Ph.D., Title: Manager 2021-03-23 Flagship Ventures Opportunities Fund I General Partner LLC, By: /s/ Noubar B. Afeyan, Ph.D., Title: Manager 2021-03-23 Flagship Ventures Opportunities Fund I, L.P. By: Flagship Ventures Opportunities Fund I General Partner LLC, its general partner, By: /s/ Noubar B. Afeyan, Ph.D., Title: Manager 2021-03-23 Noubar B. Afeyan, Ph.D. By: /s/ Noubar B. Afeyan, Ph.D. 2021-03-23