CD137 (4-1BB), or tumor necrosis factor receptor superfamily member 9 (TNFRSF9), is a promising target for enhancing antitumor immune responses without the autoimmune side effects associated with immunotherapy approaches (1). CD137 signaling plays a significant role in multiple cells and regulates the activity of many immune cells. It can activate CD8+ T cells, induce cytokine release, and increase Cytotoxic T lymphocyte (CTL) activity. The selective expression of CD137 on cells of the immune system and oncogenic cells in several types of cancers including breast, melanoma and lymphoma leads CD137 to be an attractive target for cancer immunotherapy. Anti-CD137 or anti-CD137L (the ligand of CD137) targeted immunotherapy has been extensively studied, seeking to enhance anticancer immune responses (2). Specific antibodies against CD137 are currently in clinical trials aiming to activate and enhance anti-cancer immune responses as well as suppress oncogenic cells (3).