Cysteine (abbreviated as Cys or C) is a semi-essential proteinogenic amino acid with the formula HO₂CCH(NH₂)CH₂SH>. It is encoded by the codons UGU and UGC. The thiol side chain in cysteine often participates in enzymatic reactions, as a nucleophile. The thiol is susceptible to oxidization to give the disulfide derivative cystine, which serves an important structural role in many proteins. When used as a food additive, it has the E number E920.