The SDI1/WAF1/CIP1 gene encodes a M(r) 21,000 protein (p21) that can arrest cell growth by associating with and inhibiting cyclin-dependent kinase complexes necessary for cells to exit G(1). It is a critical downstream effector in the p53 growth control pathway and can be transcriptionally upregulated by increasing levels of wild-type p53 protein. Somatic mutations in the p21 gene have been detected in 17% of primary prostate cancers. In the current study, we examined four prostate cancer cell lines for expression of and mutations in the p21 gene. Transcripts for p21 mRNA were present in all PC cell lines; p21 protein was detected in androgen-dependent LNCaP cells, as well as in androgen-independent DU-145 and TSU-Pr1 cells but not in androgen-independent PC-3 cells. Examination of the entire coding region of the p21 gene by SSCP analysis and direct DNA sequencing did not detect mutations in the coding domains of the p21 gene. These data indicate that mutations of the SDI1/WAF1/CIP1/p21 gene are not present in cultured PC cells and suggest that defects in the p21 gene are infrequent in prostate cancers.