Cystinosis is an autosomal recessive lysosomal storage disease caused by mutations in the gene CTNS. The CTNS gene product, cystinosin, has 367 amino acids and seven transmembrane domains and is thought to transport cystine out of lysosomes. The most common form of cystinosis, the nephropathic or infantile type, is characterized by renal failure at 10 years of age and other systemic complications. To date, 32 different CTNS mutations have been described in nephropathic cystinosis patients. Intermediate cystinosis, with later onset of renal disease, has been associated with three different CTNS mutations. Benign or nonnephropathic cystinosis, with symptoms related only to corneal crystals and photophobia, has been associated with two other CTNS mutations. In general, only certain splicing or missense mutations are associated with milder cystinosis phenotypes. Hum Mutat 14:454-458, 1999. Published 1999 Wiley-Liss, Inc.