XY221 (Compound 16o) selectively inhibits BRD4 BD2, with an IC50 of 5.8 nM. XY221 demonstrates high pan-BD2 selectivity (667-fold over BRD4 BD1) and BRD4 BD2 domain selectivity (9-32-fold over BRD2/3/T BD2). XY221 induce Apoptosis in MV4-11 cells and shows anticancer activity [1].