Hypoxia, metabolic competition with HCC cells, lack of CD4+ T cells, and high expression of several regulatory molecules, including VEGF, CXCL17, IL-10, and IDO, cause restriction of tumor-associated antigen (TAA)-specific response and poor production of IFN-γ by CTLs [67,82,83,84]. This evidence concerns the gene IDO1 and hepatocellular carcinoma.