Breast cancers with loss-of-function mutations in BRCA1 or BRCA2 genes are deficient in the HR (Homologous Recombination) pathway that manages the repair of DNA DSBs (Double Strand Breaks); hence, they are exquisitely sensitive to poly(ADP ribose) polymerase (PARP) inhibitors (37). The gene discussed is PARP1; the disease is breast cancer.