PARP1 and breast cancer: Though PARP1 cleavage has been used primarily as a marker of apoptosis, the recent use of PARP1 inhibitors for therapy of homologous recombination-dependent DNA repair-deficient (constitutionally Breast Cancer gene [BRCA]1/2-deficient) cancers has shed new light on the mechanistic significance of differences in the sensitivity to PARP1-cleavage between cancer cells of differing sensitivity to therapy.