L-Homoserine ((S)-(-)-2-AMINO-4-HYDROXYBUTYRIC ACID) is a more reactive variant of the amino acid serine. In this variant, the hydroxyl side chain contains an additional CH2 group which brings the hydroxyl group closer to its own carboxyl group, allowing it to chemically react to form a five-membered ring. This occurs at the point that amino acids normally join to their neighbours in a peptide bond. L-Homoserine is therefore unsuitable for forming proteins and has been eliminated from the repertoire of amino acids used by living things. L-Homoserine is the final product on the C-terminal end of the N-terminal fragment following a cyanogen bromide cleavage.