IDO1 and cancer: On the basis of the co-expression of IDO1 and TDO2 in several cancer types and the potential for TDO2 to undergo compensatory upregulation in response to IDO1 inhibition, combining IDO1 inhibitors with TDO2 inhibitors or using dual IDO1–TDO2 inhibitors might provide an effective means of targeting Trp catabolism.