YSR734 (Compound 21) is a covalent HDAC inhibitor with IC 50 values of 110 nM, 154 nM, and 143 nM for HDAC1 , HDAC2 , and HDAC3 , respectively. YSR734 can induce apoptosis in leukemia cells. YSR734 can induce myoblast differentiation and is used in the study of Duchenne muscular dystrophy.