CDw137 is a 39 kD TNF-receptor superfamily member also known as 4-1BB, TNFRSF9, Ly-63, and ILA (Induced by Lymphocyte Activation). CDw137 is expressed as monomers, dimers, and tetramers on activated T and B cells. Interaction of CDw137 with its ligand, 4-1BBL (CDw137L), is important in T-B cell costimulation. The 17B5 antibody has been reported to block 4-1BBL costimulated T cell proliferation.