PDCD1 and hepatocellular carcinoma: At the same time, in order to prevent excessive immune response from injuring normal hepatocytes, there are some immunoregulatory proteins on the surface of many immune cells, such as programmed cell death protein 1 (PD-1), which can transmit inhibitory downstream signals after binding with programmed death protein-ligand-1 (PD-L1) expressed on HCC cells or programmed death protein-ligand-2 (PD-L2) expressed on immune cells, resulting in a immunosuppression and immune tolerance environment.