TNFRSF9 (Tumor necrosis factor receptor superfamily member 9/4-1BB/CD137) is induced when T cells receive antigen-specific stimuli. Its ligand is TNFSF9 (Tumor necrosis factor ligand superfamily member 9/ 4-1BBL/CD137L) and is induced on antigen-presenting cells, such as dendritic cells, macrophages, and B cells. TNFSF9 forms homotrimers like rest of the TNF ligands and it may interact with three TNFRSF9 receptors. Structure of TNFSF9/TNFRSF9 is not available but model has been proposed where one receptor interacts predominantly with just one ligand from the trimer and only slightly with the C-terminal tail of the adjacent subunit (Won et al. 2009). TNFRSF9/TNFSF9 pathway co-stimulates T cells to carry out effector functions such as eradication of established tumors (Halstead et al. 2002, Bertram et al. 2002) and the broadening of primary and memory CD8+ T cell responses (Won et al. 2010, Vinay et al. 2006).