Cysteine

Cysteine (abbreviated as Cys or C) is an α-amino acid. It has the chemical formula HO2CCH(NH2)CH2SH. 
Cysteine is a semi-essential amino acid, which means that humans can make it. The codons UGU and UGC code for cysteine. The thiol side chain in cysteine often does enzyme reactions as a nucleophile. The thiol oxidises to give the disulfide derivative cystine, which is important in many protein structures. When used as a food additive, cysteine has the E number E920.