Monarch Disease Ontology identifier MONDO_0011698 (glycine N-methyltransferase deficiency) (GNMT deficiency) is a very rare condition characterized by persistent and isolated excess levels of methionine in the blood (hypermethioninemia). The only clinical abnormalities are mild increase of the liver size (hepatomegaly) and chronic elevation of the transaminase levels in the blood without liver disease. Methionine may also be increased in urine. However, because elevated levels of methionine in the blood itself is a risk factor for development of neurological signs and symptoms, people with GNMT deficiency can have neurological problems when methionine levelsare greater than 800μmol/L. GNMT deficiency is caused by mutations in the GNMT gene. Inheritance is autosomal recessive. Treatment is not needed in most cases. Also known as: GNMT deficiency, Glycine N-methyltransferase deficiency, hypermethioninemia due to GNMT deficiency, hypermethioninemia due to glycine N-methyltransferase deficiency.