S-Methylmethionine, or S-methyl-L-methionine, is a derivative of methionine with chemical formula C6H15NO2S. In plants, it is produced from methionine by the enzyme methionine S-methyltransferase. S-Methylmethionine is sometimes called vitamin U in naturopathic medicine, but it is not recognized as a vitamin by mainstream nutrition science. Methionine in itself has not been demonstrated as effective for treating peptic and duodenal ulcers. Its proponents claim that sources of methionine are limited, or claim it can be found only in raw cabbage; however, these claims are incorrect. Methionine is a common amino acid found in a wide variety of fruits, vegetables, and legumes.