The current findings suggest that the IDO enzyme and production of neurotoxic TRYCATs are not upregulated in patients with mood disorders, which partially contradicts the IDO theory of affective disorders, postulating that IDO stimulation with decreased TRP and elevated neurotoxic TRYCATs is involved in the pathophysiology of MDD/BD (Maes et al., 2011d). This evidence concerns the gene IDO1 and mood disorder.