Methionine

(CC) Image: David E. Volk
Methionine, a common amino acid.

Methionine, abbreviated Met or M, is one of the twenty common $\alpha$-amino acids used by living organisms to build proteins. Methionine and cysteine are the only amino acids that contain sulphur. The DNA codon for methionine is the "start" codon for protein synthesis, so all protein synthesis starts with a methionine on the N-terminus of proteins. Methionine also plays a role in the transfer of methyl groups within cells.