This approach has been applied to numerous cancers, including BRCA1/2-deficient ovarian and breast cancers, where the inhibition of poly(ADP-ribose) polymerase 1 (PARP) has been proven to be synthetically lethal in other tumors with PTEN deficiency, and, more recently, in patients with homologous recombination (HR) repair gene-mutated metastatic castration-resistant PC (mCRPC) [8,12,13]. This evidence concerns the gene PARP1 and breast cancer.