Cystinosis was the first documented genetic disease belonging to the group of lysosomal storage disease disorders. Cystinosis is caused by mutations in the "CTNS" gene that codes for cystinosin, the lysosomal membrane-specific transporter for cystine. Intracellular metabolism of cystine, as it happens with all amino acids, requires its transport across the cell membrane. After degradation of endocytosed protein to cystine within lysosomes, it is normally transported to the cytosol. But if there is a defect in the carrier protein, cystine is accumulated in lysosomes. As cystine is highly insoluble, when its concentration in tissue lysosomes increases, its solubility is immediately exceeded and crystalline precipitates are formed in almost all organs and tissues.