PARP1 and cancer: Mutations in BRCA1 and BRCA2 are susceptible to breast and ovarian cancers, and DSBs are not easily repaired in BRCA-mutant tumor cells.573 Therefore, PARP inhibition in BRCA-mutant cancers can induce synthetic lethality due to the simultaneous blockade of both DSB and SSB repair pathways (Fig. 7).