Kynurenine-3-monooxygenase (just as indoleamine-2,3-dioxygenase 1, IDO1, does) is a rate-limiting enzyme of the kynurenine pathway of tryptophan metabolism in the body and is activated in response to immunostimulation caused by external factors (e.g., infection, inflammation, dietary changes, various stressors, and peripheral exposure to lipopolysaccharide) [61,62]. Here, IDO1 is linked to infection.