TNFRSF11B and osteoporosis: Recent studies showed the biological functions of the RANKL–RANK–OPG system in development, immunity, and disease and its potential as a target for therapeutic agents in the treatment of osteoporosis and cancer (Baud'huin et al., 2013; Danks and Takayanagi, 2013; Walsh and Choi, 2014; Sigl and Penninger, 2014).