PDCD1 and cancer: In the TME, the overexpression and secretion of certain immune checkpoint proteins (ICPs) such as programmed cell death 1 (PD-1) and its ligand, programmed cell death ligand 1 (PD-L1), as well as cytotoxic T lymphocyte antigen 4 (CTLA-4) have led to the establishment of a highly immunosuppressive environment that allows cancer cells to resist and evade immune annihilation.