It is worth noting that although the peptide transporters PEPT1 (encoded by Slc15a1) and PEPT2 (encoded by Slc15a2) normally contribute to tryptophan absorption (Nassl et al., 2011), and likely can compensate for amino acid depletion in Hartnup disorder (Broer, 2009; Daniel, 2004), our restricted diet only provides tryptophan as amino acid and not as protein. The gene discussed is SLC15A2; the disease is Hartnup disease.