TDO2 and neoplasm: In gliomas, kynurenine, the product of the enzymatic reaction catalyzed by Indoleamine 2,3-Dioxygenase 1 (IDO1), Indoleamine 2,3-Dioxygenase 2 (IDO2) and Tryptophan 2,3-Dioxygenase (TDO), was shown to be an endogenous ligand of human AhR and to suppress antitumor immune responses and to promote tumor cell survival and motility through AhR in an autocrine/paracrine fashion [8].