In summary, the data demonstrate that RNAi-mediated down-regulation of transcription factor AP-4 effectively inhibited the cell proliferation, indicated cell cycle arrest, triggered apoptosis and enhanced chemo-sensitivity of human gastric cancer cells with the decreased expression of cyclin D1, Bcl-2 and Bcl-xL and activated p21, p53 and Caspase-9 expression, which suggested AP-4 may be a oncogene playing an important role in tumorigenesis. The gene discussed is BCL2; the disease is gastric cancer.