In addition to breast and ovarian cancer in women, recent evidence suggests that germ-line mutations of the breast cancer susceptibility gene-1 (BRCA1) also confer an increased life-time risk for prostate cancer in male probands. However, it is not known if and how BRCA1 functions in prostate cancer. We stably expressed wild-type (wt) and tumor-associated mutant BRCA1 transgenes in DU-145, a human prostate cancer cell line with low endogenous expression of BRCA1. As compared with parental cells and vector transfected clones, wtBRCA1 clones exhibited: (1) a slightly decreased proliferation rate (doubling time = 25 h as compared with 22 h for control cells); (2) a (3-6)-fold increase in sensitivity to chemotherapy drugs (adriamycin, camptothecin, and taxol); (3) increased susceptibility to drug-induced apoptosis; (4) reduced repair of single-strand DNA strand breaks; and (5) alterations in expression of key cellular regulatory proteins (including BRCA2, p300, Mdm-2, p21(WAF1/CIP1), Bcl-2 and Bax). Clones transfected with the 5677insA breast cancer-associated mutant BRCA1 (insBRCA1) displayed a similar phenotype to wtBRCA1 clones, except that insBRCA1 clones had a significantly decreased proliferation rate (doubling time = 42 h). On the other hand, cells transfected with with 185delAG mutant BRCA1 showed no obvious phenotype as compared with parental or vector transfected cells. These findings suggest that BRCA1 may function as a human prostate tumor suppressor by virtue of its ability to modulate proliferation and various components of the cellular damage response. They also suggest several potential target gene products for a BRCA1 prostate tumor suppressor function.