ITSA-1 is a novel, potent, cell-membrane permeable and specific HDAC (Histone deacetylase) inhibitor that suppresses TSA inhibition of HDAC (histone deacetylase), but not other HDAC inhibitors. ITSA-1 may prove to be valuable probes of many biological processes. HDAC inhibitors are being developed as new clinical agents in cancer therapy, in part because they interrupt cell cycle progression in transformed cell lines.