PDCD1 and neoplasm: Additionally, immune checkpoint proteins, including cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell death 1 (PD-1), and programmed cell death-ligand 1 (PD-L1), are expressed on Tregs, TAMs, MDSCs, and hepatoma cells and inhibit activation of effector immune cells, such as cytotoxic T cells (CTLs) and natural killer (NK) cells, in tumor tissue [6].