PDCD1 and neoplasm: The immune checkpoint pathways that are currently targeted include: 1) the PD1 pathway with its ligands PDL1 and PDL2 which are expressed on lymphoid, myeloid, epithelial cells and tumor cells; 2) the cytotoxic T-lymphocyte antigen 4 (CTLA4) pathway and its ligands CD80/86 which are expressed on myeloid and lymphoid cells, and 3) the lymphocyte-activation gene 3 pathway [15–18].