In vitro experiments demonstrate that deficiency in HR by mutations in the MRN genes may sensitize endometrial [46] and microsatellite unstable colorectal cancer cells [47,48] to treatment with poly (ADP-ribose) polymerase (PARP) inhibitors and might therefore serve as a predictive biomarker of PARP inhibitor therapy. The gene discussed is PARP1; the disease is colorectal cancer.