X0306 4 2020-01-14 0 0001609727 Synthorx, Inc. THOR 0001376355 Lichter Jay C/O SYNTHORX, INC. 11099 N. TORREY PINES ROAD, SUITE 190 LA JOLLA CA 92037 1 0 0 0 Common Stock 2020-01-14 4 J 0 4598796 D 150000 I By Avalon Ventures X, L.P. Common Stock 2020-01-14 4 J 0 448567 A 448567 I By Avalon Ventures X GP, LLC Common Stock 1682086 I By Avalon X SPV I, L.P. These shares were distributed on a pro rata basis to the partners of Avalon Ventures X, L.P. without the receipt of consideration. The Reporting Person is a managing member of Avalon Ventures X GP LLC and Avalon Ventures X SPV GP LLC, the general partners of Avalon Ventures X, L.P. and Avalon X SPV I, L.P., and shares voting and investment power with respect to the shares held by Avalon Ventures X, L.P. and Avalon X SPV I, L.P. The Reporting Person disclaims beneficial ownership of the shares held by Avalon Ventures X, L.P., Avalon Ventures X GP LLC and Avalon X SPV I, L.P., except to the extent of his actual pecuniary interest therein if any. Shares received in connection with the distribution by Avalon Ventures X L.P. for no consideration. /s/ Jay Lichter 2020-01-16