Security	Date	Code	Shares	Price	A/D	Owned after	D/I	Nature of ownership
Common Stock^F5,F1	Jul 6, 2016	C	213,333	—	A	213,333	I	See Footnote
Common Stock^F6,F1	Jul 6, 2016	C	53,333	—	A	53,333	I	See Footnote
Common Stock^F5,F2	Jul 6, 2016	C	1,066,666	—	A	1,279,999	I	See Footnote
Common Stock^F6,F2	Jul 6, 2016	C	266,666	—	A	319,999	I	See Footnote
Common Stock^F5,F3	Jul 6, 2016	C	1,600,000	—	A	2,879,999	I	See Footnote
Common Stock^F6,F3	Jul 6, 2016	C	400,000	—	A	719,999	I	See Footnote
Common Stock^F5,F4	Jul 6, 2016	C	474,661	—	A	3,354,660	I	See Footnote
Common Stock^F6,F4	Jul 6, 2016	C	118,665	—	A	838,664	I	See Footnote
Common Stock^F5	Jul 6, 2016	P	320,000	$12.50	A	3,674,660	I	See Footnote
Common Stock^F6	Jul 6, 2016	P	80,000	$12.50	A	918,664	I	See Footnote
Common Stock^F7	holding	—	—		—	213,332	I	See Footnote

Table II — derivative securities

Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	D/I
Series A-1 Preferred Stock^F5,F1	—	Jul 6, 2016	C	800,000	D	—	—	Common Stock	213,333	I
Series A-1 Preferred Stock^F6,F1	—	Jul 6, 2016	C	200,000	D	—	—	Common Stock	53,333	I
Series A-2 Preferred Stock^F5,F2	—	Jul 6, 2016	C	4,000,000	D	—	—	Common Stock	1,066,666	I
Series A-2 Preferred Stock^F6,F2	—	Jul 6, 2016	C	1,000,000	D	—	—	Common Stock	266,666	I
Series A-3 Preferred Stock^F5,F3	—	Jul 6, 2016	C	6,000,000	D	—	—	Common Stock	1,600,000	I
Series A-3 Preferred Stock^F6,F3	—	Jul 6, 2016	C	1,500,000	D	—	—	Common Stock	400,000	I
Series B Preferred Stock^F5,F4	—	Jul 6, 2016	C	1,779,981	D	—	—	Common Stock	474,661	I
Series B Preferred Stock^F6,F4	—	Jul 6, 2016	C	444,995	D	—	—	Common Stock	118,665	I

Explanation of responses

F1The Series A-1 Preferred Stock converted into Common Stock on a 3.75-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A-1 Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.
F2The Series A-2 Preferred Stock converted into Common Stock on a 3.75-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A-2 Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.
F3The Series A-3 Preferred Stock converted into Common Stock on a 3.75-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A-3 Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.
F4The Series B Preferred Stock converted into Common Stock on a 3.75-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series B Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.
F5Held by Flagship Ventures Fund IV, L.P. ("Flagship IV"). Flagship Ventures Fund IV General Partner LLC ("Flagship IV LLC") is the general partner of Flagship IV. Noubar B. Afeyan, Ph.D. and Edwin M. Kania, Jr. are the managers of Flagship IV LLC. Flagship IV LLC and each of these individuals may be deemed to share voting and investment power with respect to all shares held by Flagship IV. Each of the filing persons other than Flagship IV disclaims beneficial ownership of the shares except to the extent of his or its pecuniary interest therein. Stephane Bancel, a director of the Issuer, is a limited partner of Flagship IV and a member of Flagship IV LLC. Mr. Bancel disclaims beneficial ownership of the shares except to the extent of his pecuniary interest therein.
F6Held by Flagship Ventures Fund IV-Rx, L.P. ("Flagship IV-Rx"). Flagship IV LLC is the general partner of Flagship IV-Rx. Noubar B. Afeyan, Ph.D. and Edwin M. Kania, Jr. are the managers of Flagship IV LLC. Flagship IV LLC and each of these individuals may be deemed to share voting and investment power with respect to all shares held by Flagship IV-Rx. Each of the filing persons other than Flagship IV-Rx disclaims beneficial ownership of the shares except to the extent of his or its pecuniary interest therein. Stephane Bancel, a director of the Issuer, is a member of Flagship IV LLC. Mr. Bancel disclaims beneficial ownership of the shares except to the extent of his pecuniary interest therein.
F7Held by Flagship VentureLabs IV, LLC ("VentureLabs IV"). Flagship IV is the manager of VentureLabs IV. Flagship IV LLC is the general partner of Flagship IV. Noubar B. Afeyan, Ph.D. and Edwin M. Kania, Jr. are the managers of Flagship IV LLC. Flagship IV, Flagship IV LLC and each of these individuals may be deemed to share voting and investment power with respect to all shares held by VentureLabs IV. Each of the filing persons other than VentureLabs IV disclaims beneficial ownership of the shares except to the extent of his or its pecuniary interest therein. Stephane Bancel, a director of the Issuer, is a limited partner of Flagship IV and a member of Flagship IV LLC. Mr. Bancel disclaims beneficial ownership of the shares except to the extent of his pecuniary interest therein.