Ctns encodes the lysosomal membrane protein cystinosin. Cystinosin has 367 amino acids and seven transmembrane domains, and its key function is to transport cystine out of lysosomes. Mutations in Ctns lead to cystinosis, an autosomal recessive lysosomal storage disease [1,2,3,4]. Cystinosis is the most common hereditary cause of renal Fanconi syndrome in children [2].

To date, more than 140 CTNS mutations have been reported worldwide [3]. Different mutations are associated with various forms of cystinosis. The nephropathic or infantile type, the most common form, is characterized by renal failure at around 10 years old and other systemic complications. Intermediate cystinosis has a later onset of renal disease, while benign or non-nephropathic cystinosis mainly presents with corneal crystals and photophobia [1]. In general, certain splicing or missense mutations are related to milder cystinosis phenotypes [1]. Some studies have also discovered that cystinosin has functions beyond cystine transport, such as regulating the oxidative state, lysosomal dynamics, and autophagy [3].

In conclusion, Ctns is crucial for normal cystine transport out of lysosomes. Mutations in Ctns lead to cystinosis, a disease with various clinical phenotypes depending on the type of mutation. Understanding Ctns and its associated mutations helps in comprehending the molecular basis of cystinosis, which may potentially guide the development of new therapeutic strategies for this rare disease.