Further causes of hypermethioninemia include IEM of Met, S-adenosylmethionine and S-adenosylhomocysteine, such as deficiencies of Met adenosyltransferase I and III (MAT I/III), glycine N-methyltransferase (GNMT), and s-adenosylhomocysteine (AdoHcy) hydrolase, and citrin deficiency [14,27,34]. Here, GNMT is linked to disorder of methionine catabolism.