This data indicates that p27Kip1 deficiency promotes prostate cell proliferation and carcinogenesis but does not affect 9cRA's potential to suppress prostate carcinogenesis, suggesting that patients with PIN and carcinomas lacking or having a low level of p27Kip1 expression may also benefit from clinical trials with retinoids. This evidence concerns the gene CDKN1B and prostate intraepithelial neoplasia.