Cystine is a neutral sulfur-containing amino acid involved in a variety of important cellular functions, including detoxification, metabolism, and protein synthesis. Cystine is excreted through the kidneys, with the epithelial cells of the renal proximal tubules absorbing ∼99% of filtered cystine through a high-affinity luminal transport system in the proximal renal tubule, which also carries the dibasic amino acids lysine, arginine, and ornithine. Genetic mutations in the genes encoding this transport system can cause cystinuria, characterized by excessive amounts of cystine, arginine, lysine, and ornithine in the urine (1) and decreased absorption of cystine through the intestine (2).