CD137 exists on the cell surface as a monomer with a molecular mass of 30 kDa and as a dimer of 55 kDa. Human and mouse CD137 share 60% amino acid identity. CD137 (4-1BB), a member of the tumour necrosis factor receptor superfamily, is a type I transmembrane glycoprotein expressed on the cell surface of activated splenic T cells and thymocytes. The functions of CD137 in T lymphocytes include regulating activation, proliferation and apoptosis. CD137 and CD28 are costimulatory molecules of T cell activation. Costimulatory molecules are important in initiating anti-tumor immune responses. CD137 plays an important role in regulating T-cell-dependent immune responses. Expression of CD137 correlates negatively with lymphocyte proliferation and positively with the degree of activation-induced cell death caused by mitogen overstimulation. In monocytes, CD137 induces activation, promotes adherence and prolongs survival.