4-1BB (also known as CD137 or TNFRSF9), a co-stimulatory receptor belonging to the tumor necrosis factor receptor superfamily. 4-1BB is expressed on activated T cells (both CD4+ and CD8+), natural killer (NK) cells, and some dendritic cells. Upon engagement by its natural ligand or an agonistic antibody, it delivers a powerful signal that enhances T-cell proliferation, survival, cytokine production (e.g., IFN-γ), and cytotoxic function, while protecting against activation-induced cell death. This makes it a highly attractive target for reinvigorating anti-tumor immunity, particularly for reversing the dysfunctional state of exhausted T cells that express 4-1BB.