Cysteine is a nonessential amino acid. Cysteine is called a semi-essential amino acid in children because adults generally produce adequate amounts, but children may not. As a key constituent of glutathione, cysteine has many important physiological functions. Glutathione, formed from cysteine, glutamic acid, and glycine, is found in all human tissues. The antioxidant activity of glutathione is attributed specifically to the presence of cysteine in the compound.