['BRCA1 promotes homologous recombination-mediated DNA repair (HRR). However, HRR must be tightly regulated to prevent illegitimate recombination. We previously found that BRCA1 HRR function is regulated by the RAP80 complex, but the mechanism was unclear. We have now observed that PARP1 interacts with and poly-ADP-ribosylates (aka PARsylates) BRCA1. PARsylation is directed at the BRCA1 DNA binding domain and downmodulates its function. Moreover, RAP80 contains a poly-ADP-ribose-interacting domain that binds PARsylated BRCA1 and helps to maintain the stability of PARP1-BRCA1-RAP80 complexes. BRCA1 PARsylation is a key step in BRCA1 HRR control. When BRCA1 PARsylation is defective, it gives rise to excessive HRR and manifestations of genome instability. BRCA1 PARsylation and/or RAP80 expression is defective in a subset of sporadic breast cancer cell lines and patient-derived tumor xenograft models. These observations are consistent with the possibility that such defects, when chronic, contribute to tumor development in BRCA1+/+ individuals.'] Does pARP1-driven poly-ADP-ribosylation regulate BRCA1 function in homologous recombination-mediated DNA repair?