TDO2 and central nervous system cancer: Invasive migration of glioma cells requires water channels called aquaporins for the redistribution of water.15 Aquaporin 4 (AQP4), one of the aquaporins, is highly expressed in the most malignant gliomas and participates in maintaining the water and ion balance.16,17 The knockdown of AQP4 impaired cell migration and invasion and induced glioblastoma cell apoptosis.18,19 Hence, it is interesting to address whether the TDO–Kyn–AhR signaling pathway modulates AQP4, thus affecting the motility of glioma cells.