AbstractThis review covers briefly the major conditions, genetic and non‐genetic, sometimes leading to abnormally elevated methionine, with emphasis on recent developments. A major aim is to assist in the differential diagnosis of hypermethioninemia. The genetic conditions are: (1) Homocystinuria due to cystathionine β‐synthase (CBS) deficiency. At least 150 different mutations in theCBSgene have been identified since this deficiency was established in 1964. Hypermethioninemia is due chiefly to remethylation of the accumulated homocysteine. (2) Deficient activity of methionine adenosyltransferases I and III (MAT I/III), the isoenzymes the catalytic subunit of which are encoded byMAT1A. Methionine accumulates because its conversion toS‐adenosylmethionine (AdoMet) is impaired. (3) GlycineN‐methyltrasferase (GNMT) deficiency. Disruption of a quantitatively major pathway for AdoMet disposal leads to AdoMet accumulation with secondary down‐regulation of methionine flux into AdoMet. (4)S‐adenosylhomocysteine (AdoHcy) hydrolase (AHCY) deficiency. Not being catabolized normally, AdoHcy accumulates and inhibits many AdoMet‐dependent methyltransferases, producing accumulation of AdoMet and,
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