Hartnup disorder is an autosomal recessive disease that can be associated with neurological, psychiatric and dermatological abnormalities or be asymptomatic. Excessive intestinal and urinary loss of neutral amino acids is an essential feature of this disorder, which had been presumed to be due to hereditary abnormalities in an apical membrane-situated amino acid transporter. As anticipated, recently, mutations in the cytoplasmic and transmembrane domains of SLC6A19, the recently cloned neutral amino acid transporter, were detected in members of families with Hartnup disorder. Presumably, deficiency in neutral amino acid absorption and consequential hypoaminoacidemia is the cause of the symptoms of the disease because SLC6A19 is not expressed in the organs affected.