IDO1 and cancer: Both in the context of virus infection and cancer, sustained IFN-I signature is associated with chronic inflammation along with increases in immune checkpoint molecules, such as PD-1, PD-L1, IL-10 and IDO1 (indoleamine 2,3 dioxygenase 1), resulting in immunosuppression [37,38,39,40,41,42,43,44].