First, the development of therapeutic RANKL (receptor activator of NFκB-ligand) antibodies (denosumab) for the treatment of osteoporosis, for which the insights came from very rare osteolytic bone diseases (familial expansile osteolysis—OMIM 174810) affecting the RANK/RANKL axis of osteoclast development (107). Here, TNFSF11 is linked to osteoporosis.