SLC3A1 and cystinuria: Cystinuria is an autosomal recessive disorder caused by mutations in either the SLC3A1 or the SLC7A9 genes, resulting in a disordered amino acid transport in the proximal tubule,28 Cystinuria is characterized by urinary hyperexcretion of cystine and the dibasic amino acids lysine, ornithine, and arginine.