PDCD1 and cancer: The immune checkpoint proteins Programmed cell Death protein 1 (PD-1) and Cytotoxic T Lymphocyte Associated 4 (CTLA-4) are receptors expressed on the surface of cytotoxic T-lymphocytes that interact with their ligands Programmed Death Ligand 1 (PD-L1) and Cluster Differentiated 86 (CD86) on antigen presenting cells, which helps the cancer cells evade T cell-mediated death.