A BET bromodomain inhibitor that interacts with BRD4 and BRD3 BET proteins and displaces them from chromatin; displays highefficacy against human and murine MLL-fusion leukemic cell lines, through the induction of early cell cycle arrest and apoptosis. The mode of action is, at least in part, due to suppressing the transcription of key genes (BCL2, C-MYC and CDK6) that are critical for MLL-fusion leukemia maintenance.