Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	Owned after	D/I
Series A Preferred Stock^F2,F4	—	Jun 29, 2018	C	45,000,000	D	—	—	Common Stock	8,999,999	0	D
Series B Preferred Stock^F2,F4	—	Jun 29, 2018	C	1,708,185	D	—	—	Common Stock	341,637	0	D

Explanation of responses

F1Represents the total number of shares of Common Stock received by Third Rock Ventures III, L.P. ("TRV III") upon the conversion of the Issuer's Series A Preferred Stock in connection with the closing of the Issuer's initial public offering.
F2All series of Preferred Stock automatically converted into the Issuer's Common Stock on a 5-for-1 basis upon the closing of the Issuer's initial public offering on June 29, 2018 and had no expiration date.
F3Represents the total number of shares of Common Stock received by TRV III upon the conversion of the Issuer's Series B Preferred Stock in connection with the closing of the Issuer's initial public offering.
F4These shares are directly held by TRV III. The general partner of TRV III is Third Rock Ventures GP III, L.P. ("TRV GP III"). The general partner of TRV GP III is TRV GP III, LLC ("TRV GP III LLC"). The individual managers of TRV GP III LLC are Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("Tepper"). Each of TRV GP III, TRV GP III 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.