X0206 3 2024-07-02 0 0000907654 ARCA biopharma, Inc. ABIO 0001274173 JANUS HENDERSON GROUP PLC 201 BISHOPGATE LONDON X0 EC2M 3AE UNITED KINGDOM 1 0002030299 Janus Henderson Biotech Innovation Master Fund Ltd 201 BISHOPGATE LONDON X0 EC2M 3AE UNITED KINGDOM 1 Common Stock 1482535 I By Fund The securities reported are beneficially owned directly by Janus Henderson Biotech Innovation Master Fund Ltd. (the "Fund"). Janus Henderson Group plc has a 100% ownership stake in Janus Henderson Investors US LLC, which serves as the investment advisor to the Fund, and may therefore be deemed a beneficial owner of the securities reported herein. Each Reporting Person disclaims beneficial ownership of the securities reported herein except to the extent of its pecuniary interest therein, if any. /s/ Michelle Rosenberg, CAO, General Counsel & Company Secretary 2024-07-12 /s/ Janus Henderson Biotech Innovation Master Fund Ltd., By: Janus Henderson Investors US LLC, By: Peter Falconer, Assistant Secretary 2024-07-12