Cystine is an amino acid consisting of two molecules of cysteine joined together. Cysteine is very unstable and is easily converted to L-cystine; however, each form is capable of converting into the other as needed. Both are sulfur-containing amino acids that aid in the formation of skin and are important in detoxification. 

Cystine or the N-acetyl form of cysteine (N-acetylysteine or NAC) may be used in place of L-cystine. NAC aids in preventing side effects from chemotherapy and radiation therapy. Because it increases glutathione levels in the lungs, kidneys, liver, and bone marrow, it has an anti-aging effect on the body--reducing the accumulation of age spots, for example.