Cysteine (CYS) is a non-essential, sulfur containing amino acid in humans, related to cystine. Cysteine is important for protein synthesis, detoxification, and diverse metabolic functions. It is found in beta-keratin, which comprises the main protein in nails, skin, and hair. Cysteine is also important in collagen production, as well as skin elasticity and texture. A component of the antioxidant, glutathione, cysteine plays a role in the metabolism of essential biochemicals such as coenzyme A, heparin, and biotin. An elevated level of total cysteine can predict cardiovascular disease and metabolic syndromes. Cysteine deficiency is known to be one of the consequences of aging.