XRCC1 and chronic obstructive pulmonary disease: No statistical difference was observed between COPD and control group for allelic frequency of 399Gln XRCC1 (0.343 vs 0.314), 326Cys OGG1 (0.166 vs 0.200), 241Met XRCC3 (0.461 vs 0.500), and 401Thr XRCC4 (0.132 vs 0.098) for homozygous genotype.