PDCD1 and cancer: Cancers employ numerous mechanisms of immune evasion (7) that dampen T-cell activity, which in some patients can be successfully reversed by monoclonal antibodies (mAbs) targeting immune checkpoints, such as cytotoxic T lymphocyte-associated protein 4 (CTLA-4), and the programmed cell death 1 (PD-1)/PD ligand 1 (PD-L1) axis.