XY221 (Compound 16o) selectively inhibits BRD4 BD2 with an IC50 of 5.8 nM. It demonstrates high selectivity for pan-BD2 and BRD4 BD2 domains, being 667 times more selective than for BRD4 BD1, and 9-32 times more selective than for BRD2/3/T BD2. XY221 can induce apoptosis in MV4-11 cells and exhibits anti-cancer activity.