As a well known risk factor for CC [50], exposure to tobacco smoke, or to its key ingredients (such as nicotine or its derivative), is followed by overexpression of DNA methyltransferases 1, 3A or 3B [51, 52], which has been reported to cause hypermethylation of P16INK4a promoter in mice and cancer patients [53]. The gene discussed is DNMT1; the disease is cancer.