RAD51 and breast cancer: PARP inhibitors (PARPi) are particularly toxic in homologous recombination (HR)-deficient cells, notably in breast cancer (BC) cells mutated in HR genes BRCA. This is due to the trapping effect of the PARP enzymes on chromatin leading to a restricted accessibility to the repair factors and consequently to the accumulation of lesions that generate replication-coupled double-stranded DNA breaks (DSBs)/single-stranded DNA (ss DNA)-gaps, which need homology-directed repair by BRCA1/2 and RAD51.