PDCD1 and cancer: Immune checkpoint inhibitors (ICIs) regulate the dysfunctional immune system by blocking the interaction between cytotoxic T lymphocyte-associated protein 4 (CTLA-4) or targeting programmed cell death 1 (PD-1) and its ligand programmed cell death-ligand 1 (PD-L1), thereby inducing CD8-positive T cell killing of cancer cells (9).