PD-1 has two ligands: PD-L1 (B7-H1 or CD274) and PD-L2 (B7-DC or CD273), which are found on APC, endothelial cells, fibroblasts, and bone marrow-derived mast cells and it can also be expressed by tumor cells [34,35,36]. The gene discussed is PDCD1; the disease is neoplasm.