Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	D/I
Series A Preferred Stock^F2,F1	—	Jul 23, 2018	C	11,000,000	D	—	—	Common Stock	999,180	D
Series B Preferred Stock^F2,F1	—	Jul 23, 2018	C	10,650,000	D	—	—	Common Stock	967,389	D
Series D Preferred Stock^F2,F1	—	Jul 23, 2018	C	1,086,411	D	—	—	Common Stock	98,683	D
Series E Preferred Stock^F2,F1	—	Jul 23, 2018	C	2,285,640	D	—	—	Common Stock	266,934	D
Series E-1 Preferred Stock^F2,F1	—	Jul 23, 2018	C	3,107,930	D	—	—	Common Stock	282,308	D
Series F Preferred Stock^F2,F1	—	Jul 23, 2018	C	1,000,000	D	—	—	Common Stock	90,840	D

Explanation of responses

F1On July 23, 2018, the Series A, Series B, Series D, Series E-1 and Series F Preferred Stock converted into Common Stock on a 11.009-for-one basis, and the Series E Preferred Stock converted into Common Stock on a 8.56256-for-one basis, upon the closing of the Issuer's initial public offering without payment of consideration. The Series A, Series B, Series C, Series E, Series E-1 and Series F Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial pubic offering. The shares had no expiration date.
F2These securities are directly held by Third Rock Ventures, L.P. ("TRV"). The general partner of TRV is Third Rock Ventures GP, L.P. ("TRV GP"). The general partner of TRV GP is TRV GP, LLC ("TRV GP LLC"). The individual managers of TRV GP LLC are Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("Tepper"). Each of TRV GP, TRV GP LLC, Levin, Starr and Tepper disclaims beneficial ownership of the shares except to the extent of its or his pecuniary interest therein, if any, and this report shall not be deemed an admission that it or he is the beneficial owner of such shares