X0306 4 2015-10-16 0 0000907654 ARCA biopharma, Inc. ABIO 0001290482 Keuer Thomas A ARCA BIOPHARMA, INC. 11080 CIRCLEPOINT ROAD, SUITE 140 WESTMINSTER CO 80020 0 1 0 0 Chief Operating Officer Common Stock 2015-10-16 4 S 0 496 5.14 D 16938 D Represents shares of common stock sold pursuant to a Rule 10b5-1 Plan adopted by the Reporting Person for satisfaction of their tax withholding obligation with respect to the vesting of previously reported Restricted Stock Units ("RSUs") granted to the Reporting Person under the Issuer's 2013 Equity Incentive Plan (the "Plan"). Includes (i) remaining 2,143 previously reported shares issued as RSUs under the Plan that vests on 9/17/16, (ii) remaining 1,429 previously reported shares issued as RSUs under the Plan that vests on 10/15/16, (iii) 3,664 previously reported shares issued as RSUs under the Plan that vest in three equal annual installments beginning on 2/27/16, and (iv) 3,621 previously reported shares issued as RSUs under the Plan that vest in three equal annual installments beginning April 2, 2016, provided that the Reporting Person's continuous service to the Issuer has not been terminated as defined under the Plan; and 6,081 previously reported shares of common stock owned. /s/ Thomas A. Keuer 2015-10-19