XRCC1 and cancer: Although many SNPs in NQO1, CYP1A1, ERCC4, EXO1, MSH2, XRCC1 and hOGG1 have been identified, only some of them have been extensively investigated in epidemiological studies [34], SNPs for which potential functional evidence in the development, progression and metastasis of cancer remains unknown, especially for Exo1 gene.