PDCD1 and neoplasm: Cytotoxic T-lymphocyte antigen 4 (CTLA-4) and programmed cell death protein-1 (PD-1) are two major immune checkpoint receptors that bind to their ligands CD80(B7-1) and CD86(B7-2), as well as programmed death ligands 1 and 2 (PD-L1, PD-L2), respectively, leading to tumor cell tolerance and the downregulation of effector T cells [19].