An isoxazole class pan-BET family inhibitor, blocking BRD2, BRD3, and BRD4 with IC50 values of 0.5, 0.25, and 0.79 µM, respectively); induces apoptosis or triggers G0/G1 cell cycle arrest in MLL-fusion cell lines; effective in vivo, suppressing MLL leukemia progression in two different mouse models.