L-Cysteine: Cysteine is a major biological source of sulfur and is one of the two common sulfur-containing amino acids. The biosynthesis of cysteine occurs through the initial condensation of homocysteine and serine via cystathionine synthase to form cystathionine, which in turn undergoes cleavage by cystathionase to give cysteine and a-ketobutyrate. Cysteine can readily dimerize to form cystine via the oxidation of the thiol side chain residues to give a disulfide covalent bond. The formation of such cystine links between cysteine residues in proteins is an important part of the stabilization of the three-dimensional structure of proteins. An investigation into cysteine and cystine levels in normal and malignant cells with a relationship to g-cystathionase levels and tumor sensitivity to L-cysteine and cystine depletion has been reported. The toxicity of human neuronal cell lines to cysteine and its metabolites has been investigated. Cysteine can inhibit the activity of enzymes such as lecithincholesterolacyltransferase. A protocol for the use of cysteine in pulse-chase experiments to study protein expression from an inducible promoter has been reported.