4-1BB, also known as CD137, is a type I transmembrane protein belonging to the TNF receptor superfamily (TNFRSF9). It is expressed on T cells, NK cells, monocytes, neutrophils, dendritic cells, and eosinophils. The ligand for 4-1BB, 4-1BBL, is a TNF ligand superfamily member (TNFSF9) expressed on the surfaces of activated antigen presenting cells and T and B lymphoma cell lines. 4-1BB transduces a T cell costimulatory signal to promote survival, proliferation, cytokine production, and effector functions. On dendritic cells, 4-1BB ligation enhances cytokine production and upregulates expression of B7-1 and B7-2 costimulatory molecules.