PDCD1 and neoplasm: Different from traditional anti-tumor drugs that directly attack cancer cells, immune checkpoint inhibitors (ICIs) enhance the tumor-killing response mediated by CD8-positive T cells through blocking the programmed cell death 1 (PD-1), programmed cell death-ligand 1 (PD-L1), or cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) pathways (1, 2).