Cystine is an amino acid that is synthesized in the liver and involved in multiple metabolic pathways. It is also a source of metabolic sulfur in the body.  Human hair and skin contain about 10-14% Cystine by mass. Cystine was discovered in 1810, but not recognized as a protein component until 1899 when it was extracted from an animal horn. It has been linked to aiding those with Cystic Fibrosis as well.