The TNF (tumor necrosis factor) ligand family promotes tumor growth and progression. Torrey et al. developed an antibody that locks TNFR2, a TNF receptor found on immunosuppressive regulatory T cells and some tumor cells, in an inactive state (see the Focus by Chen and Oppenheim). The antibody inhibited the proliferation of regulatory T cells while promoting the proliferation of effector T cells isolated from metastatic sites in ovarian cancer patients. The antibody had less of an effect on T cells from normal donors. Thus, this antibody may be more specific and less toxic than current antibodies against TNFRs.