HDAC4 down-regulation in SGC-7901 cells suppressed the cell proliferation and the colony formation numbers, arrested cell cycle and induced cell apoptosis dependent on the activation of caspases 3 and 9, which is consistent with the anti-proliferative and proapoptotic effects of HDAC inhibitors in human cancer cell lines [15] and with the previously reported observation that HDAC4 down-regulation reduces clonogenic survival and induces apoptosis in HeLa cells [5]. This evidence concerns the gene HDAC9 and cancer.