Lysinuric protein intolerance (LPI; OMIM 222700) is a rare, recessive disorder with a worldwide distribution, but with a high prevalence in the Finnish population(1); symptoms include failure to thrive, growth retardation, muscle hypotonia and hepatosplenomegaly. A defect in the plasma membrane transport of dibasic amino acids has been demonstrated at the basolateral membrane of epithelial cells in small intestine and in renal tubules(2-4) and in plasma membrane of cultured skin fibroblasts(5) from LPI patients. The gene causing LPI has been assigned by linkage analysis to 14q11-13. Here we report mutations in SLC7A7 cDNA (encoding y(+)L amino acid transporter-1, y(+)LAT-1), which expresses dibasic amino-acid transport activity and is located in the LPI region, in 31 Finnish LPI patients and 1 Spanish patient. The Finnish patients are homozygous for a founder missense mutation leading to a premature stop codon. The Spanish patient is a compound heterozygote with a missense mutation in one allele and a frameshift mutation in the other. The frameshift mutation generates a premature stop codon, eliminating the last one-third of the protein. The missense mutation abolishes y(+)LAT-1 amino-acid transport activity when co-expressed with the heavy chain of the cell-surface antigen 4F2 (4F2hc, also known as CD98) in Xenopus laevis oocytes. Our data establish that mutations in SLC7A7 cause LPI.