"Hartnup disease is caused by mutation to the SLC6A19 gene, inherited with an autosomal recessive pattern 5. SLC6A19 encodes for an important neutral amino acid (monoamino-monocarboxylic acids) transporter 5. Defects in this transporter leads to amino acid deficiencies, notably including tryptophan (precursor to niacin), resulting in the clinical syndrome 5."
"History and etymology"