PDCD1 and hepatocellular carcinoma: Programmed death 1 (PD-1) and its ligand PD-L1 inhibitors and cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) monoclonal antibodies have been approved for the treatment of advanced hepatocellular carcinoma (HCC), but the response rates of these immunotherapy are not high, and they are easy to be resistant.