Hartnup disease is an autosomal recessive metabolic disorder characterized by dysfunction in the neutral amino acid transporter. This causes decreased absorption of nonpolar amino acids, specifically tryptophan. Since tryptophan is important for the synthesis of niacin, patients with Hartnup disease will also present with signs and symptoms consistent with niacin deficiency. Pellagra is a disease caused by a lack of niacin, so Hartnup disease is also known as pellagra-like dermatosis. Since the signs and symptoms are primarily due to niacin deficiency, supplementation with niacin and/or a high-protein diet can be used to treat the disease.