Indoleamine 2,3-dioxygenase 1 (IDO1), an Trp dioxygenase extensively detected in mammalian tissues except the liver [211–213], usually degrades Trp into L- kynurenine that subsequently activates aryl hydrocarbon receptor (AhR) via the Kyn–AhR–AQP4 signaling pathway, which promotes cell motility and increases malignancy of gliomas. Here, IDO1 is linked to central nervous system cancer.