The tumor microenvironment (TME) of HCC is characterized by immune suppression via a variety of mechanisms, including the recruitment of suppressive immune cells [tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs), and regulatory T cells (Tregs)], the reduction of antitumor effector cells [natural killer cells (NK cells) and dendritic cells (DCs)], the change of cytokine level, and the increase of immune checkpoint proteins (PD-1/PD-L1 and CTLA-4), and these mechanisms are interactive (18) (Figure 1). The gene discussed is PDCD1; the disease is hepatocellular carcinoma.