Hartnup disease (also known as "pellagra-like dermatosis"[1] and "Hartnup disorder"[2]) is an autosomal recessive[3] 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, a necessary component of NAD+.[4]:541
The causative gene, SLC6A19, is located on chromosome 5.[