IDO2 plays a nonredundant role in Trp metabolism36 and a nonredundant immunosuppressive role different from that of IDO1 and TDO.37,38 The function of IDO2 within the central nervous system is still not clear, but it seems to be different from that of IDO1 and TDO.39 Therefore, it is not unexpected to find that IDO2 is not correlated with the pathologic grade of gliomas. Here, IDO1 is linked to central nervous system cancer.