More recently, inflammation has been implicated in major depressive disorder (MDD) and postulated to induce a serotonin deficiency by accelerating tryptophan (Trp) degradation along the kynurenine (Kyn) pathway (KP) by induction of the extrahepatic Trp-degrading enzyme indoleamine 2,3-dioxygenase (IDO1). This evidence concerns the gene IDO1 and major depressive disorder.