•  A novel bromodomain inhibitor CG13250 suppresses MM cell proliferation.•  CG13250 decreases C-MYC expression, resulting in the induction of apoptosis.•  CG13250 prolongs the survivals of MM-bearing mice.In text: JQ-1 [12] and I-BET762 [13] were the first BET inhibitors to be described and these compounds suppress MM cell proliferation [14] ;  [15]. Other BET inhibitors described to date include PFI-1, RVX-208, and OTX015 [9] ;  [16]. By conducting chemical screening against BRD4 and subsequent chemical optimization, we developed a novel bromodomain inhibitor CG13250. Unlike previously described BET inhibitors, it has a quinolinone core and displayed high affinity and specificity to BET proteins. In this study, we investigated the inhibitory effects of CG13250 on the proliferation of MM cells.