L-cysteine is a semi-essential amino acid that is important for a number of biologic functions. It is a constituent in the production of the anti-oxidant glutathione and in healthy skin, hair and nails .
Cysteine (symbol Cys or C ;  / ˈ s ɪ s t i iː n / )  is a semi- essential  proteinogenic amino acid with the formula HO 2 CCH(NH 2 )CH 2 SH. The thiol side chain in cysteine often participates in enzymatic reactions, as a nucleophile . The thiol is susceptible to oxidation 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. It is encoded by the codons UGU and UGC.