CPI203 is a novel potent, selective and cell permeable inhibitor of the bromodomain and extra terminal (BET) family protein BRD4 with an IC50 of ~37 nM (BRD4 α-screen assay). It has an IC50 of ~99 nM in inhibiting Myc expression in MV4-11 cells and an IC50 of ~30 nM in inhibiting IL-6 production in THP-1 cells stimulated with LPS, decreases specific Ser2 phosphorylation of the carboxyl-terminal domain (CTD) of the RNA polymerase II (Pol II) by either endogenous BRD4 or a BRD4 mutant, BRD4 FEE-AAA, that is incapable of binding PTEFb. CPI203 is an analog of JQ-1 but has shown superior bioavailability with oral or intraperitoneal (IP) administration. When mice that were transplanted with primary mouse T-ALL cells, either Fbxw7+/+ or Fbxw7mut/+, were treated with CPI203 at 5 mg/Kg orally twice per day, a significant and rapid reduction in leukemia burden was observed. CPI203 is a useful chemical probe to study the suppression of Myc-dependent cancer development.