According to the neurodegeneration hypothesis of depression [16], an inflammatory state can induce IDO activity, and increase the kynurenine/tryptophan ratio associated with a shift in the kynurenine pathway, resulting in an imbalance between neuroprotective (kynurenic acid) and neurodegenerative metabolites (3-hydroxyanthranilic acid, 3-hydroxy-kynurenine, and quinolinic acid). This evidence concerns the gene IDO1 and major depressive disorder.