Some of the psychobiotics also enhance the activity of the enzyme indoleamine-2,3-dioxygenase (IDO), which is employed in the catabolism of tryptophan and formation of kynurenic and quinolinic acid—the neuroactive compounds that are a means to affect the neuronal pathways to the brain and correlate positively with the symptoms of depression [33]. The gene discussed is IDO1; the disease is major depressive disorder.