Cystinuria is a hereditary disorder of renal function in which defective tubular reabsorption of the amino acids cystine, lysine, arginine, and ornithine leads to the excretion of large amounts of these substances in the urine. The only reported clinical consequence has been formation of renal stones composed of cystine. As a result of the limited solubility of cystine, precipitation may occur when the concentration in urine exceeds the solubility. Cystine stones are generally found only in persons excreting at least 250μg. of cystine per milligram of creatinine.1
Two patterns of inheritance have been noted.2 All homozygous persons excrete cystine, lysine, arginine, and ornithine. In Type I families the trait is completely recessive, and the heterozygous parents show no defect in excretion of cystine. In Type II families the condition is "incompletely recessive," and the trait is manifested in the heterozygous person by increased excretion of cystine and lysine