Alanine aminotransferase (ALT) measurements are useful in the diagnosis and treatment of certain liver diseases (e.g., viral hepatitis and cirrhosis) and heart disease. ALT is an enzyme involved in amino acid metabolism and is, therefore, found in many tissues. The highest levels of ALT are found in the liver and kidney tissues. Tissue destruction leads to the release of the intracellular enzyme into the circulating blood. Markedly elevated serum ALT levels may be found in a variety of diseases which involve the liver, such as hepatitis, mononucleosis and cirrhosis. These very high levels of ALT are not usually observed in other disease processes, e.g. myocardial infarction, thus ALT is regarded as a reasonably specific indicator of liver disease.