Previous studies revealed some potential mechanisms for the association between smoking, colorectal polyps and CRN, such as the reduced methylation of relevant genes26, genetic variants in carcinogen-metabolising enzymes12, the polymorphisms in DNA repair genes EXO1 and ATM27, the mutations in mismatch repair enzymes28, and XPC polymorphisms29, etc. In a word, tobacco contains many carcinogens that are thought to create no less than irreversible genetic damage to the colorectal mucosa, initiating the formation of colorectal polyps19. The gene discussed is XPC; the disease is polyp of large intestine.