We analyzed six different HDAC inhibitors which have shown inhibitory activities on tumor cell proliferation and/or viability or entered in various clinical trials, including valproic acid (VPA), sodium butyrate (NaB), suberoylanilide hydroxamic acid (SAHA), PxD 101, MS-275 and AR-42 [6, 16, 20–29] on six AR/cPAcP-positive and four AR/cPAcP-negative PCa cell lines. The gene discussed is HDAC9; the disease is neoplasm.