Tumor necrosis factor receptor superfamily member 9(TNFRSF9), also known as CD137 and 4-1BB, is an inducible T cell surface protein belonging to the tumor necrosis factor receptor superfamily. It is a single-pass type I membrane protein which contains 4 TNFR-Cys repeats. The human and mouse proteins share 60% amino acid sequence identity. CD137 is expressed by mesenchymal cells, including endothelial cells, chondrocytes, and cells of the central nervous system. CD137 is also broadly expressed by cells of the human immune system, is broadly expressed by cells of the human immune system, including activated CD8+ and CD4+ T cells, activated natural killer (NK) cells, follicular dendritic cells (FDCs) and monocytes. CD137 has diverse roles in the immune response, the one key function is to promote the survival of both T cells and dendritic cells by binding the cognate ligand CD137L (4-1BBL).