PDCD1 and cancer: However, cancer cells can escape the various pathways that provide immunologic antitumor responses, and such pathways include immune system inhibitory pathways such as cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), programmed cell death ligand 1 (PD-L1), and programmed cell death 1 (PD-1) (natural checkpoints that dampen the antitumor responses of T cells).