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.