GAMT is a methyltransferase which belongs to the class I-like SAM-binding methyltransferase superfamily. It contains one RMT2 (arginine N-methyltransferase 2-like) domain and is expressed in liver. GAMT converts guanidoacetate to creatine, using S-adenosylmethionine as the methyl donor. Defects in GAMT are the cause of guanidinoacetate methyltransferase deficiency, which is an autosomal recessive disorder characterized by developmental delay/regression, mental retardation, severe disturbance of expressive and cognitive speech, intractable seizures and movement disturbances, severe depletion of creatine/phosphocreatine in the brain, and accumulation of guanidinoacetic acid in brain and body fluids.