Disorder of amino acid transport characterized by the childhood (or rarely adult) onset of photosensitive dermatitis and intermittent neurologic symptoms; ataxia, personality changes, migraine headaches, and photophobia may occur periodically; results from impaired sodium-dependent transport of neutral amino acids across the brush border membrane of the small intestine and renal tubular epithelium.

Hartnup disease (also known as "pellagra-like dermatosis" and "Hartnup disorder") is an autosomal recessive metabolic disorder affecting the absorption of nonpolar amino acids (particularly tryptophan that can be, in turn, converted into serotonin, melatonin, and niacin). Niacin is a precursor to nicotinamide (both are forms of vitamin B3), a necessary component of NAD+.: 541  The causative gene, SLC6A19, is located on chromosome 5...