The Arg399Gln polymorphism is located in the region of the breast cancer susceptibility gene C terminus I (BRCT-I) interaction domain of XRCC1 with poly(ADP-ribose) polymerase, and the presence of the variant 399Gln allele has been shown to be associated with measurable reduced DRC, as assessed by the persistence of DNA adducts [18, 19], tumor-suppressor gene P53 mutations [20], increased red blood cell glycophorin A [18], elevated levels of sister chromatid exchanges [19], and prolonged cell-cycle delay [21]. The gene discussed is TP53; the disease is breast cancer.