The CTNS gene, also known as the cystinosin gene, is located on chromosome 17p 13.2 in humans and encodes the protein cystinosin, a receptor protein with seven transmembrane domains that is embedded in the lysosomal membrane of cells. Cystinosin is a transport protein.
Cystinosin acts as a symporter that actively transports protons and specifically the amino acid cystine out of the lysosome. This process is necessary to prevent cystine from accumulating in the lysosome, allowing the lysosome to continue to function. Cystinosin has also been discovered in melanosomes and is associated with the control and regulation of melanin.