CD137, a member of the tumor necrosis factor receptor superfamily, represents a promising target for enhancing antitumor immune responses (1). The functions of CD137 in T lymphocytes include regulating activation, proliferation and apoptosis (2). CD137 helps regulate the activation of many immune cells, including CD4(+) T cells, CD8(+) T cells, dendritic cells, and natural killer cells. Recent studies indicate that the antitumor efficacy of therapeutic tumor-targeting antibodies can be augmented by the addition of agonistic antibodies targeting CD137. As ligation of CD137 provides a costimulatory signal in multiple immune cell subsets, CD137 antibody has potential to improve cancer treatment, and has been implicated in breast cancer, melanoma and lymphoma (3-5). Therefore, CD137 agonists represent a promising immunotherapeutic approach to treating cancers.