In a clinical study, patients with IFN-α therapy show increases in the total Montgomery-Asberg Depression Rating Score (MADRS), an index of depressive symptoms, as well as in the KYN/TRP ratio (reflecting IDO1 activity) and the KYN/kynurenic acid (KA) ratio (reflecting the neurotoxic challenge)29, although the TRP/competing amino acid (CAA) ratio (reflecting TRP availability to the brain) does not significantly change during therapy. Here, IDO1 is linked to depressive disorder.