Cysteine (CYS, C) is a sulfhydryl-containing amino acid, and is an important structural and functional part of proteins. In animals, cysteine is synthesized from trans-sulfuration of homocysteine (HCY), which is itself derived from metabolism of the amino acid methionine. The enzyme Cystathionine β-Synthase catalyzes condensation of homocysteine with serine to form cystathionine, which is deaminated and hydrolyzed by Cystathionine β-lyase to form cysteine and α-ketobutyrate. Due to its nucleophilic nature, the thiol group of cysteine has numerous biological functions. The formation of disulfide linkages between the thiol groups of cysteine residues helps to stabilize the tertiary and quaternary structure of proteins. Cysteine, homocysteine and other aminothiols exist in plasma in reduced, oxidized, and protein-bound forms, interacting with each other through redox pathways.

Cysteine is the limiting precursor of the major intracellular antioxidant glutathione. The individuals with lower cysteine levels are more prone to damage from reactive oxygen species, which are generally removed either by thiols or by glutathione-linked enzymes. An elevated level of total cysteine also predicts adverse outcomes such as cardiovascular diseases and metabolic syndromes.