Cysteine is an amino acid found in proteins. In mammals, cysteine is a non-essential amino acid, meaning it does not need to be present in the diet.

The hydroxy and sulfur-containing amino acids are generally more hydrophilic than their aliphatic analogs, although methionine is fairly hydrophobic. Cysteine can ionize at high pH (see here) Cysteine, together with another cysteine in a protein, can form disulfide bonds (see here). Disulfide bonds help to stabilize the structure of some proteins.

Some proteins interconvert between disulfides and thiols. Glutathione provides a major protective mechanism against oxidative stress. For example, it helps keep cysteine thiol groups in proteins in the reduced state (see here). If two thiol groups become oxidized, they can be reduced nonenzymatically by glutathione.

One Letter Code	Three Letter Code	Molecular Wt. (Daltons)	Genetic Code Codons
C	CYS	103.14	UGU, UGC