Entinostat (MS-275) is a potent and selective HDAC inhibitor. It inhibits HDAC1 and HDAC3 with IC50 of 0.51 μM and 1.7 μM, but not the other HDACs 4, 6, 8, and 10 (IC50 > 100 μM). MS-275 induces accumulation of p21WAF1/CIP1 and gelsolin in K562 cell. It was shown to reduce S-phase cells and induce G1-phase cells in A2780 cells. MS-275 shows great inhibition to human leukemia and lymphoma cells, decreases expression of cyclin D1 and the anti-apoptotic proteins Mcl-1 and XIAP. In vivo MS-275 exhibits great antitumor activity against human tumor xenografts. Currently it is in Phase II/III clinical trials for Hodgkin's lymphoma and advanced breast cancer.