X0306 4 2021-02-08 0 0001829802 Sensei Biotherapeutics, Inc. SNSE 0001843594 Peyer James C/O SENSEI BIOTHERAPEUTICS, INC. 1405 RESEARCH BLVD, SUITE 125 ROCKVILLE MD 20850 1 0 1 0 Common Stock 2021-02-08 4 C 0 2407328 A 4690153 I See footnote Series AA Preferred Convertible Stock 2021-02-08 4 C 0 110729827 0.00 D Common Stock 2306870 0 I See footnote Series BB Preferred Convertible Stock 2021-02-08 4 C 0 4821996 0.00 D Common Stock 100458 0 I See footnotes The total represents shares received upon conversion of shares of Series AA Preferred Stock and Series BB Preferred Stock. Each share of the Series AA Preferred Stock and Series BB Preferred Stock converted to 0.0208333 shares of the Issuer's common stock, effective immediately prior to the closing of the Issuer's initial public offering of its common stock. The Series AA Preferred Stock and Series BB Preferred Stock haves no expiration date. These shares are held by Cambrian Biopharma, Inc., a Delaware corporation ("Cambrian").The Reporting Person is the Chief Executive Officer of Cambrian and in such capacity may direct the voting and disposition of the shares held by Cambrian, subject in certain instances to the approval of Cambrian's Board of Directors. Mr. Peyer disclaims beneficial ownership of such shares. By: /s/Mark Ballantyne, Attorney-in-Fact 2021-02-10