However, early studies have suggested that, for unknown reasons, only 30–40% of BRCA1/2-mutated ovarian and breast cancers respond to PARP inhibitors.29, 30, 31 The demonstration that functional NHEJ contributes to the cytotoxicity of such inhibitors suggests that HR-deficient cancers with diminished NHEJ will be relatively resistant. Here, BRCA1 is linked to breast carcinoma.