BRCA1 and breast cancer: Although BRCA1 and BRCA2 gene products are considered the major players in the HR system, cancers harboring a pathological variant of BRCA1/2 genes represent only the tip of the iceberg of the overall BC characterized by a homologous recombination deficiency, due to the emerging subgroup of tumors that share clinico–biological features of BRCA-mutant tumors in the absence of a BRCA1 or BRCA2 mutation, a condition known as BRCAness [63].