Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	Owned after	D/I
Series A Preferred Stock^F5,F1	—	Oct 10, 2017	C	7,651,831	D	—	—	Common Stock	834,441	0	I
Series A-1 Junior Preferred Stock^F6,F1	—	Oct 10, 2017	C	15,887,703	D	—	—	Common Stock	1,732,573	0	I

Explanation of responses

F1Each share of preferred stock converted into approximately 0.10905125 shares of common stock upon the closing of the issuer's initial public offering. The shares had no expiration date.
F2The shares are held as follows: 2,499,299 by MPM BioVentures V, L.P. ("BV V") and 97,093 by MPM Asset Management Investors BV5 LLC ("AM BV5"). MPM BioVentures V GP LLC and MPM BioVentures V LLC ("BV LLC") are the direct and indirect general partners of BV V and BV LLC is the manager of AM BV5. Ansbert Gadicke, Luke Evnin and Todd Foley are the members of BV LLC. Mr. Foley is a director of the Issuer and has separately filed a Form 4. Each Reporting Person disclaims beneficial ownership of the securities except to the extent of his or its respective pecuniary interest therein.
F3Shares were purchased as follows: 96,260 by BV V and 3,740 by AM BV5.
F4Shares are held as follows: 2,595,559 by BV V and 100,833 by AM BV5. Each Reporting Person disclaims beneficial ownership of the securities except to the extent of his or its respective pecuniary interest therein.
F5The shares of Series A Preferred Stock were held as follows: 7,365,688 by BV V and 286,143 by AM BV5.
F6The shares of Series A-1 Junior Preferred Stock were held as follows: 15,293,569 by BV V and 594,134 by AM BV5.