CD137, also known as TNFRSF9 or 4-1BB, is an inducible T cell surface receptor which belongs to the tumor necrosis factor receptor superfamily. CD137 is a transmembrane protein expressed on the surface of activated T-cells. In addition, activation-dependent expression of CD137 has also been found in B lymphocytes, monocytes, and diverse nonlymphoid cell types. CD137 provides a co-stimulatory signal that enhances the survival, and differentiation of cells, and has a crucial role in the development of CD8 cytotoxic T cells and anti-tumor immunity. Soluble forms of CD137 (sCD137) are generated by differential splicing and are released by activated lymphocytes. Elevated serum sCD137 level has been associated with some diseases, including rheumatoid arthritis, multiple sclerosis, chronic lymphocytic leukemia, and acute coronary syndrome.