<prompt> Answer this question truthfullyCould you please provide me with the source sentence so that I can better understand the context?  <response>: <reason> 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, a necessary component of NAD. : 541 The causative gene, SLC6A19, is located on chromosome 5. 