This information is provided by the National Institutes of Health (NIH) Genetic and Rare Diseases Information Center (GARD).
Hartnup disease is a metabolic disorder characterized by abnormal transport of certain amino acids in the kidney and gastrointestinal system. It is a type of aminoaciduria. The condition may be diagnosed based on the results of newborn screening tests. Most people with the condition have no symptoms (asymptomatic). For those who do show symptoms, the onset of the disease is usually between the ages of 3 and 9; occasionally the disease may present in adulthood. Mental development is usually normal, though a few cases with intellectual impairment have been reported. The signs and symptoms of Hartnup disease incude skin photosensitivity, neurologic findings, psychiatric symptoms, and ocular (eye) findings. Hartnup disease is caused by mutations in the SLC6A19 gene and is inherited in an autosomal recessive manner.
People with Hartnup disease may benefit from a high-protein diet, protection from sunlight, vitamin supplementation, and avoidance of certain drugs/medications. In some cases, treatment with nicotinamide supplements and tryptophan ethyl ester may be indicated. 
For more information, visit GARD.