In this study, we validated the association between 6 SNPs in the drug metabolizing enzyme genes, SLC22A2 (rs316019) & EPHX1 (rs1051740), and the DNA repair genes, ERCC1 (rs11615 & rs3212986) & ERCC2 (rs13181 & rs1799793), and CIN in the 169 Thai patients with head and neck, lung, or esophageal cancer. This evidence concerns the gene EPHX1 and cervical squamous intraepithelial neoplasia.