A significant positive correlation between TRP and KYN was observed only in patients with major depression, suggesting increased IDO1-, IDO2- or TDO activity only in patients with major depression, while no correlative relationship was found in the healthy controls (major depression: Spearman rank correlation coefficient: r = 0.253, p = 0.035; healthy controls: Spearman rank correlation coefficient: r =  − 0.005, p = 0.975). This evidence concerns the gene TDO2 and major depressive disorder.