'''Cystein''' (viết tắt là '''Cys''' orhoặc '''C''')<ref>{{IUPAC-IUB amino acids 1983}}</ref> là một α-[[axit amin]] với [[công thức hóa học]] HO<sub>2</sub>CCH(NH<sub>2</sub>)CH<sub>2</sub>SH. Đây là axit amin không thiết yếu, nghĩa là cơ thể người tự tổng hợp được. <!--Its [[codons]] are UGU and UGC. The side chain on cysteine is [[thiol]], which is nonpolar and thus cysteine is usually classified as a [[hydrophobic]] amino acid. The thiol side chain often participates in [[enzymatic]] reactions, serving as a [[nucleophile]]. The thiol is susceptible to oxidization to give the [[disulfide bond|disulfide]] derivative [[cystine]], which serves an important structural role in many [[protein]]s. Cysteine is named after cystine.-->
