Hypermethioninemia is caused by the deficiency of glycine N-methyltransferase (GNMT) activity (Augoustides-Savvopoulou et al., 2003; Luka and Wagner, 2003; Luka et al., 2006) and/or mutations of the MAT1A gene (Chou, 2000; Chamberlin et al., 2000), which resulted in the plasma concentration of methionine reaching as high as 1870 μM (normal range 5–35 μM). This evidence concerns the gene GNMT and disorder of methionine catabolism.