Here is a talk taken from wikipedia explaining what Cysteine is and what is used for. Here is the text for initial part of the video. 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.