The article studies whether 4-1BB and 4-1BB ligand interaction plays a role in osteoclastogenesis, using 4-1BB-deficient mice. 4-1BB is an inducible T-cell constimulatory molecule and a member of the tumor necrosis factor (TNF) receptor family. Enhanced osteoclastogenesis was observed in bone marrow-derived macrophage cells from 4-1BB-deficient mice than in those from wildtype mice.