It has been argued that IFN-γ induced depression is associated with induction of IDO that increases levels of kynurenine (KYN), which in turn leads to formation of neurotoxic metabolites, 3-hydroxykynurenine and quinolinic acid, rather than just TRP degradation by itself [13], [39], [44], [47], [48], [49]. Here, IDO1 is linked to major depressive disorder.