A broad-spectrum phenyl hydroxamic acid inhibitor of histone deacetylase (HDAC) with potential antineoplastic activity. HDAC inhibitor PCI-24781 inhibits several isoforms of HDAC, resulting in an accumulation of highly acetylated histones, followed by the induction of chromatin remodeling; the selective transcription of tumor suppressor genes; and the tumor suppressor protein-mediated inhibition of tumor cell division and induction of tumor cell apoptosis. In addition, this agent has been shown to inhibit homologous recombination (HR) activity by inhibiting the expression of RAD51. RAD51, a homologue of bacterial RecA, assists in the repair of DNA double-strand breaks (DSBs).