PDCD1 and cancer: Immune checkpoint inhibitors (ICIs), a type of drug that increase the cytotoxicity of T cells by blocking intrinsic downregulators of immunity such as cytotoxic T-lymphocyte antigen 4 (CTLA-4) and programmed cell death 1 (PD-1) or programmed cell death ligand 1 (PD-L1), have become a new dawn in cancer treatment recently [5].