Cysteamine (trade name CYSTAGON) is a cystine-depleting agent indicated for the treatment of corneal cystine crystal accumulation in patients with cystinosis. Cystinosis is an autosomal recessive inborn error of metabolism in which the transport of cystine out of lysosomes is abnormal; in the nephropathic form, accumulation of cystine and formation of crystals damage various organs, especially the kidney, leading to renal tubular Fanconi Syndrome and progressive glomerular failure, with end-stage renal failure by the end of the first decade of life. In four studies of cystinosis patients before cysteamine was available, renal death (need for transplant or dialysis) occurred at the median age of fewer than 10 years. Patients with cystinosis also experience growth failure, rickets, and photophobia due to cystine deposits in the cornea. With time most organs are damaged, including the retina, muscles and central nervous system. Cysteamine is an aminothiol that participates within lysosomes in a thiol-disulfide interchange reaction converting cystine into cysteine and cysteine-cysteamine mixed disulfide, both of which can exit the lysosome in patients with cystinosis.