YH004 is an anti-CD137 agonist monoclonal antibody with properties that stimulate the immune system and exhibit anti-tumor activities. It activates CD137, present on various leukocyte subgroups, including activated T lymphocytes and natural killer cells. YH004 enhances CD137-mediated signaling, leading to proliferation of cytotoxic T lymphocytes (CTL), cytokine production, and promotion of CTL-mediated anti-tumor responses. Additionally, YH004 induces natural killer cell-mediated tumor cell lysis and inhibits the immunosuppressive activity of regulatory T cells.