IDO1 and central nervous system cancer: IDO2 exhibited much lower catalytic affinity toward Trp than IDO1/TDO, although IDO2 plays a nonredundant role in Trp metabolism.36 Given that IDO2 expression was not correlated with the pathologic grades of gliomas, overall survival, or Ki67 index, it can be concluded that the expression and activity of IDO1/TDO rather than IDO2 were positively correlated with the malignancy of gliomas.