CD137 is a member of the tumor necrosis factor (TNF) receptor family. CD137 is expressed by activated T cells, dendritic cells, NK cells, granulocytes and cells of blood vessel walls at sites of inflammation. CD137 has costimulatory activity for activated T cells. Crosslinking of CD137 enhances T cell proliferation, IL-2 secretion survival and cytolytic activity. CD137 was shown to enhance immune activity to eliminate tumors in mice.