XRCC1 and breast cancer: Numerous epidemiologic studies have been conducted to evaluate the association between XRCC1 and breast cancer.[18, 19] The findings have been inconsistent; the rare allele of each polymorphism (XRCC1 194W allele, XRCC1 280H allele, XRCC1 399Q allele) has been associated with either an increase or decrease in breast cancer risk depending upon the population or subgroup analyzed.