PARP1 and breast cancer: Furthermore, when BRCA1 and BRCA2 were down-regulated using shRNA in wild-type BRCA breast cancer cell lines, PARP inhibitors were highly effective in promoting the apoptosis of MDA-MB-231 and BT-20 triple-negative breast cancer cell lines, proving the concept that dual PARP1- and BRCA-inhibition is lethal for cells [45].