DNMT1 and animal viral hepatitis: Since DNA methylation alterations at the precancerous stage are stably maintained on the DNA double strand by covalent bonding through a maintenance methylation mechanism involving DNA methyltransferase 1 (DNMT1) [17, 18], we have successfully established carcinogenic risk estimation criteria for viral hepatitis-related HCC [19, 20] and urothelial carcinoma [21, 22] based on DNA methylation profiles.