PLX51107 is as a novel BET inhibitor with modest preference for bromodomain-1 (BD1) versus bromodomain-2 (BD2) within each BET protein (Kd = 1.6, 2.1, 1.7, and 5 nM for BD1 and 5.9, 6.2, 6.1 and 120 nM for BD2 of BRD2, BRD3, BRD4, and BRDT, respectively. Among non-BET proteins, PLX51107 shows significant interactions only with the bromodomains of CBP and EP300 (p300) (Kd in the 100 nM range).