PARP1 and breast cancer: siRNA directed or chemical inhibition of PARP profoundly inhibited clonogenicity of BRCA1-deficient mouse embryonic stem cells as compared to BRCA1-proficient isogenic cell lines; similar results were obtained upon simultaneous inhibition of BRCA1 and PARP in MCF7 breast cancer cell line. This effect was attributed to the massive growth arrest and subsequent apoptosis.