In HCC, the dysfunction of these cells is caused by factors such as severe hypoxia, accumulation of the end products of tumor cell metabolism, inhibitory receptor switches or NK inhibitors (NKRs) as well as by the presence of molecules such as TGF-β, PGE2 or IDO1, and immunosuppressive cytokines such as IL-4, IL-10, and IL-13. The gene discussed is IDO1; the disease is hepatocellular carcinoma.