Methionine is an important amino acid that helps to initiate translation of messenger RNA by being the first amino acid incorporated into the N-terminal position of all proteins. This sulfur-containing amino acid is also the source of sulfur for cysteine in animals and man. In that regard, methionine is considered an essential amino acid whereas cysteine is not, so cysteine is nonessential only as long as the diet contains adaquate amounts of methionine. The terminal methyl group of the methionine side chain often participates in biochemical methyl transfer reactions making methionine a member of the "methyl donor" class of biochemicals. On a molar basis, methionine is incorporated into proteins and enzymes at the rate of 1.7 percent, but this is partially due to posttranslational protein-modifying events that often occur where methionine and several other N-terminal amino acids are removed from the protein.