Biomarkers of Homologous Recombination Deficiency in the era of PARP inhibitors

Homologous Recombination Deficiency (HRD) was initially described in cancers with germline muta-tions of BRCA1 and BRCA2 and thereafter in both sporadic and hereditary cancers carrying muta-tions or epigenetic inactivation of other genes in-volved in HR. Since cancers harbouring HRD are particularly susceptible to PARP inhibitors (PARPi), identifying methods to detect HRD that can accu-rately predict clinical sensitivity to PARPi beyond BRCA1/2 mutations has been challenging. In this review, we describe the HRD biomarkers identified up to now, pointing out strengths and weakness-es of each associated assay.