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Background: Cystinuria is a heritable disorder of amino acid transport characterized by the defective transport of cystine and the dibasic amino acids through the brush border epithelial cells of the renal tubule and intestine tract. Three types of cystinuria (I, II, and III) have been described based on the urinary excretion of cystine and dibasic amino acids in obligate heterozygotes. The SLC3A1 gene coding for an amino acid transporter named rBAT is responsible for type I cystinuria, whereas the SLC7A9 gene coding for a subunit (b0,+AT) of rBAT is involved in determining non-type I (types II and III) cystinuria.Methods: The SLC3A1 gene sequence was investigated in a sample of seven type I/type I, three type I/non-type I, six type I/untyped, and four untyped unrelated cystinuric patients by RNA single-strand conformation polymorphism (RNA-SSCP).Results: Eight new point mutations (S168X, 765+1G>T, 766-2A>G, R452Q, Y461X, S547W, L564F, and C673W) and seven previously reported mutations were detected. These new mutations increase the number of mutated alleles so far characterized in SLC3A1 to 62.Conclusions: We have found SLC3A1 mutations in 0.739 of the type I chromosomes studied. The relatively high proportion of uncharacterized type I chromosomes suggests either that there may be mutations not yet found in SLC3A1 or that many of the assigned type I chromosomes in mixed type I/non-type I patients may have mutations in SLC7A9. If the hypothesis is excluded in the future, we believe that a third gene may be involved in cystinuria. [ABSTRACT FROM AUTHOR]