Some genetic diseases exhibit hypermethioninemia secondary to generalized hepatic dysfunction; examples include citrin deficiency, fumaryl acetoacetate hydrolase deficiency and mitochondrial depletion syndromes due to mutations in MPV17 or DGUOK. Other causes of hypermethioninemia involve primary defects in methionine metabolism and the transsulfuration pathway; they include deficiencies of cystathionine β synthase (CBS), methionine adenosyltransferase (MAT) type I and III, glycine N-methyltransferase (GNMT), and S adenosylhomocysteine hydrolase (SAHHD). The gene discussed is GNMT; the disease is hyperinsulinemic hypoglycemia, familial, 4.