A BET family protein inhibitor; inhibits binding of BRD2, BRD3, and BRD4 to an acetylated histone H4 peptide in vitro; decreases Myc levels in, and inhibits proliferation of, AML, DLBCL, and MM cells; reduces tumor growth in AML, DLBCL, and MM mouse xenograft models; reduces tumor growth when administered alone or in combination with docetaxel or MDV 3100 in a 22Rv1 CRPC mouse xenograft model