XRCC4 and chronic obstructive pulmonary disease: In addition, the induced DNA damage by MMS increases in COPD patients with variant genotypes in XRCC1 (Arg399Gln), OGG1 (Ser326Cys), XRCC3 (Thr241Met) and XRCC4 (Ile401Thr) showing impairment of DNA repair.