origin. Budd–Chiari syndrome is the clinical picture caused by occlusion of the hepatic vein. Mechanisms Liver diseases can develop through several mechanisms: DNA damage One general mechanism, increased DNA damage, is shared by some of the major liver diseases, including infection by hepatitis B virus or hepatitis C virus, heavy alcohol consumption, and obesity. Viral infection by hepatitis B virus, or hepatitis C virus causes an increase of reactive oxygen species. The increase in intracellular reactive oxygen species is about 10,000-fold with chronic hepatitis B virus infection and 100,000-fold following hepatitis C virus infection. This increase in reactive oxygen species causes inflammation and 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. Alcohol consumption in excess causes a build-up of acetaldehyde. Acetaldehyde and free radicals generated by metabolizing alcohol induce DNA damage and oxidative stress. In addition, activation of neutrophils in alcoholic liver disease contributes to the pathogenesis of hepatocellular damage by releasing reactive oxygen species (which can damage DNA). The level of oxidative stress and acetaldehyde-induced DNA adducts due to alcohol consumption does not appear sufficient to cause increased mutagenesis. However, as reviewed by Nishida et al., alcohol exposure, causing oxidative DNA damage (which is repairable), can result in epigenetic alterations at the sites of DNA repair. Alcohol-induced epigenetic alterations of gene expression appear to lead to liver injury and ultimately carcinoma. Obesity is associated with a higher risk of primary liver cancer. As shown with mice, obese mice are prone to liver cancer, likely due to two factors. Obese mice have increased pro-inflammatory cytokines. Obese mice also have higher levels of deoxycholic acid, a product of bile acid alteration by certain gut microbes, and these microbes are increased with obesity. The excess deoxycholic acid causes DNA damage and inflammation in the liver, which, in turn, can lead