PDCD1 and neoplasm: Tumor microenvironment in HCC is strongly immunosuppressive, expressing a high level of immune checkpoint inhibitors(ICIs), such as programmed death-1 (PD-1), cytotoxic T-lymphocyte antigen 4 (CTLA-4), lymphocyte activating gene 3 protein (LAG-3), and mucin domain molecule 3 (TIM-3).