SLC7A9 belongs to the amino acid-polyamine-organocation (APC) superfamily. It is a disulfide linked heterodimer with the amino acid transport protein SLC3A1. SLC7A9 is involved in the high affinity, sodium independent transport of cystine and neutral and dibasic amino acids (system b(0,+)-like activity). Thought to be responsible for the high affinity reabsorption of cystine in the kidney tubule. Defects in SLC7A9 are a cause of non type I cystinuria (CSNU). CSNU arises from impaired transport of cystine and dibasic amino acids through the epithelial cells of the renal tubule and gastrointestinal tract. Three types of cystinuria have been described: type I (fully recessive or silent); type II (high excretor); type III (moderate excretor). Defects in SLC7A9 are associated with type II and type III cystinuria. They also might account for some non classic type I cystinuria cases.