On the contrary, ectopic overexpression of HNF4α could significantly inhibit the cell proliferation of prostate cancer cells, induce cell-cycle arrest at G2/M phase and trigger the cellular senescence in prostate cancer cells by activation of p21 signal pathway in a p53-independent manner via its direct transactivation of CDKN1A. Together, our results show that HNF4α performs a tumor suppressor function in prostate cancer via a mechanism of p21-driven cellular senescence. The gene discussed is CDKN1A; the disease is prostate carcinoma.