Since immunosuppression could be induced by Trp depletion and Kyn activation that are initiated by three enzymes namely IDO1, IDO2 and TDO while IDO1 and TDO are more vital than IDO2 in pathologic grade of gliomas [214], the application of IDO1 and TDO inhibitors simultaneously, excluding IDO2, is advised. Here, IDO2 is linked to central nervous system cancer.