Osteoclast differentiation requires the interaction of RANK (receptor activator of NF kappaB) and RANK ligand, conserved member of the TNF (tumor necrosis factor) receptor and TNF families, respectively. We tested the ability of a peptide mimic (WP9QY) of a critical contact site on the TNF receptor to inhibit RANK ligand induced osteoclastogenesis and discuss whether it could work as both TNF-alpha and RANKL antagonists, and the possibility of the drug development using the techniques in the structural biology.