Security	Date	Code	Shares	Price	A/D	Owned after	D/I	Nature of ownership
Common Stock^F5,F1	Dec 11, 2018	C	7,292,145	—	A	7,292,145	I	By: Flagship Ventures Fund IV, LP
Common Stock^F5,F2	Dec 11, 2018	C	1,823,036	—	A	1,823,036	I	By: Flagship Ventures Fund IV-Rx, LP
Common Stock^F6,F1	Dec 11, 2018	C	24,782,551	—	A	32,074,696	I	By: Flagship Ventures Fund IV, LP
Common Stock^F6,F2	Dec 11, 2018	C	6,195,632	—	A	8,018,668	I	By: Flagship Ventures Fund IV-Rx, LP
Common Stock^F7,F1	Dec 11, 2018	C	5,713,739	—	A	37,788,435	I	By: Flagship Ventures Fund IV, LP
Common Stock^F7,F2	Dec 11, 2018	C	1,428,432	—	A	9,447,100	I	By: Flagship Ventures Fund IV-Rx, LP
Common Stock^F8,F1	Dec 11, 2018	C	85,989	—	A	37,874,424	I	By: Flagship Ventures Fund IV, LP
Common Stock^F8,F2	Dec 11, 2018	C	21,496	—	A	9,468,596	I	By: Flagship Ventures Fund IV-Rx, LP
Common Stock^F3	holding	—	—		—	11,460,435	I	By: Flagship Venture Labs IV LLC
Common Stock^F4	holding	—	—		—	3,924	I	By: Flagship Pioneering, Inc.

Table II — derivative securities

Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	D/I
Series A Preferred Stock^F5,F1	—	Dec 11, 2018	C	7,292,145	D	—	—	Common Stock	7,292,145	I
Series A Preferred Stock^F5,F2	—	Dec 11, 2018	C	1,823,036	D	—	—	Common Stock	1,823,036	I
Series B Preferred Stock^F6,F1	—	Dec 11, 2018	C	24,782,551	D	—	—	Common Stock	24,782,551	I
Series B Preferred Stock^F6,F2	—	Dec 11, 2018	C	6,195,632	D	—	—	Common Stock	6,195,632	I
Series C Preferred Stock^F7,F1	—	Dec 11, 2018	C	5,713,739	D	—	—	Common Stock	5,713,739	I
Series C Preferred Stock^F7,F2	—	Dec 11, 2018	C	1,428,432	D	—	—	Common Stock	1,428,432	I
Series D Preferred Stock^F8,F1	—	Dec 11, 2018	C	85,989	D	—	—	Common Stock	85,989	I
Series D Preferred Stock^F8,F2	—	Dec 11, 2018	C	21,496	D	—	—	Common Stock	21,496	I

Explanation of responses

F1Shares held by Flagship Ventures Fund IV, L.P. ("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. and Edwin M. Kania, Jr. are the managers of Flagship Fund IV GP. While Mr. Kania is retired from Flagship Pioneering, Inc. ("Pioneering"), he continues to serve as a 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.
F2Shares 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. While Mr. Kania is retired from Pioneering he continues to serve as a manager of the 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.
F3Shares 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. Noubar B. Afeyan, Ph.D. and Edwin M. Kania, Jr. are the managers of Flagship Fund IV GP. While Mr. Kania is retired from Pioneering he continues to serve as a manager of the 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.
F4Securities held by Pioneering. Noubar B. Afeyan, Ph.D. is the CEO and a stockholder of Pioneering. Each of the reporting persons except for Pioneering disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein.
F5The Series A Preferred Stock converted into shares of the Issuer's common stock, $0.0001 par value per share ("Common Stock") based on an applicable conversion ratio of 0.45576 upon the closing of the Issuer's initial public offering on December 11, 2018. The Series A Preferred Stock had no expiration date.
F6The Series B Preferred Stock converted into Common Stock based on an applicable conversion ratio of 0.45457 upon the closing of the Issuer's initial public offering on December 11, 2018. The Series B Preferred Stock had no expiration date.
F7The Series C Preferred Stock converted into Common Stock based on an applicable conversion ratio of 0.45853 upon the closing of the Issuer's initial public offering on December 11, 2018. The Series C Preferred Stock had no expiration date.
F8The Series D Preferred Stock converted into Common Stock based on an applicable conversion ratio of 0.45854 upon the closing of the Issuer's initial public offering on December 11, 2018. The Series D Preferred Stock had no expiration date.