PARP1 and cancer: PARP inhibitors prevent the repair of SSBs by trapping the inactivated enzyme onto DNA, creating a block to replication and promoting the collapse of replication forks at SSBs.7 A further DNA damage tolerance function is attributed to PARP-1 in the rescue of stalled replication forks,8,9 which may contribute to the mechanism of synthetic lethality with BRCA1 and BRCA2. Niraparib inhibits both PARP-1 and PARP-2 with low nanomolar half-maximal inhibitory concentration (IC50) values, selectively kills BRCA1/2 mutant cancer cells and traps PARP-1 more efficiently than olaparib.10,11