Hartnup Disease: An autosomal recessive disorder due to defective absorption of NEUTRAL AMINO ACIDS by both the intestine and the PROXIMAL RENAL TUBULES. The abnormal urinary loss of TRYPTOPHAN, a precursor of NIACIN, leads to a NICOTINAMIDE deficiency, PELLAGRA-like light-sensitive rash, CEREBELLAR ATAXIA, emotional instability, and aminoaciduria. Mutations involve the neurotransmitter transporter gene SLC6A19.Tryptophan: An essential amino acid that is necessary for normal growth in infants and for NITROGEN balance in adults. It is a precursor of INDOLE ALKALOIDS in plants. It is a precursor of SEROTONIN (hence its use as an antidepressant and sleep aid). It can be a precursor to NIACIN, albeit inefficiently, in mammals.