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Homocysteine is a chemical compound with the formula HSCH<sub>2</sub>CH<sub>2</sub>CH(NH<sub>2</sub>)CO<sub>2</sub>H. It is a homologue of the naturally-occurring amino acid cysteine, differing in that its side-chain contains an additional methylene (-CH<sub>2</sub>-) group before the thiol (-SH) group. Alternatively, homocysteine can be derived from methionine by removing the latter's terminal C<sup>ε</sup> methyl group.