The activity of rate-limiting enzyme of tryptophan-kynurenine, indoleamine-2,3-dioxygenase (IDO), was enhanced significantly in T2D patients[23], thus downstream metabolites such as kynurenine, kynurenic acid, xanthurenic acid and hydroxykynurenine, were higher in T2D than in non-diabetic subjects[24, 25], although inconsistent observations of tryptophan levels were reported[25, 26]. The gene discussed is IDO1; the disease is type 2 diabetes mellitus.