The signaling system that involves receptor activation of nuclear factor-κB (RANK) by its ligand (RANKL), which is regulated by the soluble decoy receptor osteoprotegerin (OPG), plays a role in the RANKL/RANK/OPG pathway of gene control of fetal development of bones and osteoporosis. The gene discussed is TNFRSF11B; the disease is osteoporosis.