The only known cause of cystine stones is a hereditary defect of renal tubule transport that permits large amounts of cystine to pass into the urine. The defects, which include three separate varieties,51 involve not only cystine but also the other dibasic amino acids—ornithine, lysine, and arginine. Only the cystinuria has consequences that are clinically important. One might think that cystine loss would cause an amino-acid deficiency state, but this is not a recognized problem. Stone formation primarily is caused by the low solubility of cystine: 300 mg/L urine. Normal people excrete < 60 mg per day of cystine, a level below supersaturation. Heterozygotes for the so-called type 1 form of cystinuria excrete normal amounts of cystine; heterozygotes for types 2 and 3 excrete abnormal amounts, in the range of 80 to 300 mg per day, along with an excess of the other three dibasic amino acids. However, heterozygotes do not form stones. Homozygous cystinurics, of all three types, excrete more than 400 mg per day of cystine. Patients with cystinuria may have other causes of stones as well, including those associated with calcium salts.