PDCD1 and neoplasm: Based on the understanding of cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) and the programmed cell death protein 1/ programmed death-ligand 1 (PD-1/PD-L1) and other pathways in tumor immune microenvironment, immune checkpoint inhibitors (ICIs) can enhance the intrinsic immune response against tumor antigens by promoting T cell activation and function, and have been approved for the treatment of a variety of tumors [3].