Methionine is an amino acid found in proteins. In mammals, methionine is an essential amino acid, meaning it must be present in the diet.

The hydroxy- and sulfur-containing amino acids are generally more hydrophilic than their aliphatic analogs, although methionine is fairly hydrophobic.

In the genetic code, methionine is coded for by the codon AUG. This codon is called the start codon because methionine is the first amino acid used to build a protein chain. Methionine forms the so-called amino terminus of a protein. In prokaryotes, a modified form of methionine, formyl-methionine is used as the first (but not subsequent) amino acid in proteins. Formyl-methionine is carried by a modified tRNA from the tRNA that carries unmodified methionine.

One Letter Code	Three Letter Code	Molecular Wt. (Daltons)	Genetic Code Codons
M	MET	131.21	AUG