However, according to Spranger et al. [35] and Xu et al. [36], IFN-γ acts as an important cytokine in the tumor microenvironment that can potently induce the expression of PD-L1 and indoleamine-2,3-dioxygenase (IDO) in cancer cells to contribute to tumor immune evasion. This evidence concerns the gene IDO1 and cancer.