Mocetinostat selectively inhibits histone deacetylases (HDACs) 1-3 and 11 at submicromolar concentrations. It blocks cancer cell proliferation in vitro and has significant antitumor activity in vivo. Mocetinostat induces hyperacetylation of histones, causes expression of the tumor suppressor protein p21(WAF1/CIP1), and inhibits proliferation of human cancer cells growing in culture.