TDO2 and central nervous system cancer: IDO1 and TDO, which are both positively related with the glioma grade, could promote the migration and invasion of glioma cells via the Kyn/AHR/APQ4 signaling pathway (49).The IDO1 expression level is elevated in glioma stem cells compared to GBM cells, and IDO1 leads to therapeutic resistance through the promotion of immunosuppression (50).