PDCD1 and neoplasm: Programmed cell death protein-1 (PD-1) is expressed in various cell types, including activated T cells, B cells, NK cells, and myeloid cells, and when it binds to programmed death-ligand 1 (PD-L1) or programmed death-ligand 2 (PD-L2) in tumor cells, this molecule can reduce the immune response [91].