TNFRSF11B and Osteopenia: The significance of the RANK-RANKL–communication is portrayed in several monogenic conditions with abnormal bone mass resulting from defective RANK-RANKL-OPG–axis: osteoclast-poor osteopetrosis with excessive bone formation due to mutated RANKL, juvenile Paget's disease with osteopenia and progressive skeletal deformity from mutated OPG, and familial expansile osteolysis (FEO) with osteolytic lesions and increased bone remodeling from mutated RANK (33–35).