Viral infection by "(HBV)", or "(HCV)" causes an increase of reactive oxygen species (ROS). The increase in intracellular ROS is about 10,000-fold with chronic HBV infection and 100,000-fold following HCV infection. This increase in ROS causes inflammation and further increase in ROS. ROS cause more than 20 types of DNA damage. Oxidative DNA damage is mutagenic and also causes epigenetic alterations at the sites of DNA repair. Epigenetic alterations and mutations affect the cellular machinery that may cause the cell to replicate at a higher rate or result in the cell avoiding apoptosis, and thus contribute to liver disease. By the time accumulating epigenetic and mutational changes eventually cause hepatocellular carcinoma, epigenetic alterations appear to have an even larger role in carcinogenesis than mutations. Only one gene, TP53, is mutated in more than 20% of liver cancers while 41 genes each have hypermethylated promoters (repressing gene expression) in more than 20% of liver cancers.