Cystinuria is an inherited form of nephrolithiasis due to failure of reabsorptive transport in the proximal tubule. Patients with classical recessive cystinuria have inherited two mutations of the SLC3A1 gene, encoding a subunit of the transport mechanism. Patients with the dominant form of cystinuria have inherited two mutations of the SLC7A9 gene, encoding the transport channel itself. A smaller subset of patients have mixed-type cystinuria, combining recessive and dominant mutant alleles. Children at risk for nephrolithiasis can be identified by the level of urinary cystine only after tubular transport has matured (age 2 years). Conservative therapy with high urine volume and urinary alkalinization is sufficient for some, but recurrent stone formation may cause renal damage and warrants prophylaxis with agents that form mixed disulfides with cystine.