BRCA1 and cancer: PARPi resistance in HRD cancer cells can occur through a myriad of mechanisms, including restoration of homologous recombination (HR), DNA replication fork protection, diminished PARP trapping, PARPi efflux, chromatin remodelling, and, in the case of BRCA1-mutated cancers, 53BP1 mutation [16, 25–34].