L-Cysteine ni imwe muri 20 za aside amine karemano kandi, usibye methionine, imwe yonyine irimo sulfure.Ni thiol irimo aside amine idakenewe ya okiside kugirango ikore Cystine.Nibintu bidakenewe cyane birimo sulfure irimo aside amine mu bantu, ifitanye isano na cystine, Cysteine ni ingenzi mu gusanisha poroteyine, kwangiza, n'imikorere itandukanye ya metabolike.Cysteine iboneka muri beta-keratin, poroteyine nyamukuru mu nzara, ku ruhu, no ku musatsi, Cysteine ni ingenzi mu musaruro wa kolagen, ndetse no kuba uruhu rworoshye ndetse n’imiterere.