TDO2 and neoplasm: Block of IDO1 and TDO2 may therefore have a two-fold beneficial effect on tumour progression: (i) to down-regulate Treg expression, while promoting CD8+ cell activation to engage an immune response to the tumour, and (ii) to block kynurenine-AhR-induced Pol κ, attenuating TMZ-induced drug resistance and improving prognosis in response to chemotherapy (Fig. 8).