In total, 1-2% of adults and 6-8% of children suffering from kidney stones have cystinuria, a defect in the transport of amino acids, which leads to high concentrations of cystine in the urine. Two genes have been implicated, solute carrier family 3 (cystine, basic and) neutral amino acid transporter, member 1 (SLC3A1) coding for the protein related to the system bo,+ amino-acid transporter, and solute carrier family 7, member 9 (SLC7A9), coding for the protein bo,+AT. There are three clinically defined types of cystinuria. Type I (autosomal recessive) is characterised by mutations in the SLC3A1 gene; carriers have normal cystine levels in urine. In total 80% of non-type-I patients have a detectable mutation in the SLC7A9 gene. First-degree relatives of a patient with type 2 (dominant) or type 3 (incomplete recessive) cystinuria may have slightly elevated cystine levels in the urine. The early recognition and treatment of cystinuria is important because recurrent stone formation may lead to urinary tract infection and, eventually, renal failure.