Therefore, the combination therapy with the inhibitor of IDO1 and the agent targeting PD-1/PD ligand 1 (PD-L1) was expected to be an effective option for overcoming the immunosuppressive tumor microenvironment in anti-PD1-resistant tumors, such as melanoma, glioblastoma multiforme (GBM), or non-small cell lung cancer (Figure 3) [113,146]. The gene discussed is IDO1; the disease is melanoma.