PDCD1 and cancer: In recent years, immune checkpoint inhibitors (ICIs), such as anti-CTLA-4 (cytotoxic T lymphocyte associated antigen-4) inhibitors and anti-PD-1 (programmed cell death 1)/PD-L1 (programmed cell death ligand 1) inhibitors, as well as the combined treatment with chemotherapy and immunotherapy, have made significant progress in many types of cancers [12].