PDCD1 and hepatocellular carcinoma: Immune checkpoints such as cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), PD-1, PD-L1, V-domain Ig suppressor of T cell activation (VISTA), T cell immunoglobulin and mucin domain containing-3 (TIM-3), lymphocyte-activation gene-3 (LAG-3), and OX40 are reported to be prominently activated in HCC cells and involved in its prognosis.