XRCC1 and lip and oral cavity carcinoma: For the gene–environment interaction, the estimated OR of oral cancer from drinking–betel quid was 32.9 (95% CI = 14.1–76.9), XRCC1–XRCC2–betel quid was 31.0 (95% CI = 14.0–64.7), XRCC1–XRCC2–age–betel quid was 49.8 (95% CI = 21.0–117.7) and XRCC1–XRCC2–age–drinking–betel quid was 82.9 (95% CI = 31.0–221.5) [20].