Our phenotypic data indicate that individuals carrying the variant genotypes (CT and CC) at XRCC6 rs5751129, a SNP in the promoter region, had remarkably reduced expression levels of both mRNA and protein, resulting in a significantly lower NHEJ repair capacity, which could explain the increased NPC risk conferred by the variant genotypes. This evidence concerns the gene XRCC6 and nasopharyngeal carcinoma.