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). Here, IDO1 is linked to glioma.