Gene expression of tryptophan catabolism limiting enzymes to kynurenine, such as indolamine 2,3-dioxygenase 1 (IDO1), indolamine 2,3-dioxygenase 2 (IDO2), and tryptophan 2,3-dioxygenase (TDO2), is shown to increase in patients with T2D [134]. This evidence concerns the gene TDO2 and type 2 diabetes mellitus.