The role of the RANKL/RANK system in bone and other tissues. RANKL is produced by a variety of cell types and its expression is regulated by many physiologic and pathologic factors. Preclinical studies in mice and studies of human tissues have revealed functions for RANKL/RANK signaling in normal and pathologic states. OPG can bind to RANKL and prevent its interaction with RANK to inhibit osteoclast formation, but its effects on other cellular functions of RANKL have yet to be determined. OPG, osteoprotegerin; RANKL, receptor activator of nuclear factor-κB ligand.