Hartnup disease is an autosomal recessive metabolic disorder resulting from mutations in SLC6A19 gene on chromosome 5. This gene encodes a sodium-dependent neutral amino acid transporter that is predominantly expressed in the kidneys and intestine. Defects in these transporters lead to impaired absorption of nonpolar amino acids, particularly tryptophan, which is necessary for the production of serotonin, melatonin, and niacin.

Clinical symptoms are highly variable but may appear in infancy with failure to thrive, intermittent ataxia, tremor, and photosensitive dermatitis. Seizures have also been reported.

Others may remain asymptomatic or have symptoms that occur intermittently as "attacks" triggered by poor nutrition, sunlight, illness, stress, and certain medications.

Patients may also have short stature, psychiatric illness, and/or intellectual disability.