siRNA directed or chemical inhibition of PARP profoundly inhibited clonogenicity of BRCA2-deficient mouse embryonic stem cells as compared to BRCA2-proficient isogenic cell lines; similar results were obtained on BRCA2-deficient Chinese hamster ovarian cancer cell line. This effect was attributed to the massive growth arrest and subsequent apoptosis. Chemical PARP inhibitor suppressed growth of BRCA2-deficient xenografts in athymic mice. The gene discussed is PARP1; the disease is ovarian carcinoma.