GNMT and disorder of methionine catabolism: It is a well-known phenomenon that (transient) elevation of methionine serum concentration could be observed secondary to liver disease while hypermethioninemia is a biochemical hallmark of some inborn errors of metabolism, including methionine adenosyltransferase I/III deficiency, glycine-N-methyltransferase (GNMT) deficiency, S-adenosylhomocysteine hydrolase (SAH hydrolase, AHCY) deficiency, cystathionine beta-synthase deficiency, fumarylacetoacetate hydrolase deficiency, and citrin deficiency (2, 3).