The effects of HDAC inhibitors on cell viability were measured by MTT assay in three human neuroblastoma cell lines SK-N-DZ, SK-N-SH and SH-SY-5Y and a normal cell line HS-68 by exposing them to 0.03–10 μM of PCI-24781 and 0.03–30 μM of CI-994 for 24 h, respectively. The gene discussed is HDAC9; the disease is neuroblastoma.