Cystinuria is an autosomal recessive disorder characterized by impaired transport of cystine, lysine, ornithine, and arginine in the proximal renal tubule and in epithelial cells of the gastrointestinal tract, and resulting in elevated urinary concentrations of these amino acids [1]. Transport of these amino acids is mediated by the rBAT/b^0,+AT transporter [2], whose subunits are encoded by the genes SLC3A1, located on chromosome 2p16.3-21 [3], and SLC7A9, located on chromosome 19q12-13.1 [4,5].

Based on the urinary cystine excretion patterns of obligate heterozygotes, cystinuria is classified into type I and non type I [6]. Heterozygotes of type I show a normal amino acid urinary pattern in heterozygotes, whereas those of non type I show a variable degree of urinary hyper excretion of cystine and dibasic amino acids. Mutations in the SLC3A1 gene cause type I, while mutations in the SLC7A9 gene cause non type I [3,7]. However, an SLC7A9 mutation carrier status was found to be compatible with a normal amino aciduria in 14% of cases [8]. On the basis of genetic aspects, cystinuria is classified into type A, caused by mutations in both alleles of SLC3A1; type B, caused by mutations in both alleles of SLC7A9 and type AB, caused by one mutation in SLC3A1 and one mutation in SLC7A9 derived from the two parents, respectively [8]. The type AB is extremely rare [9]. More than 112 mutations in the SLC3A1 gene and 69 in the SLC7A9 gene have been described [10].