4-1BB is a member of the tumor necrosis factor (TNF) receptor family. Its alternative names are tumor necrosis factor receptor superfamily member 9 (TNFRSF9), CD137 and induced by lymphocyte activation (ILA). It is currently of interest to immunologists as a co-stimulatory immune checkpoint molecule. 4-1BB can be expressed by activated T cells, but to a larger extent on CD8 than on CD4 T cells. In addition, 4-1BB expression is found on dendritic cells, follicular dendritic cells, natural killer cells, granulocytes and cells of blood vessel walls at sites of inflammation.