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 transpor