Its importance has recently been highlighted by the development of a monoclonal antibody against RANKL (denosumab, Amgen Inc., Thousand Oaks, CA, USA) approved in the clinical practice for the treatment of postmenopausal osteoporosis and cancer-related osteolysis [12] and under evaluation in a Phase 4 clinical trial for rheumatoid arthritis (clinical trial identifier: NCT01770106). Here, TNFSF11 is linked to rheumatoid arthritis.