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High/low bone density in rare human diseases have been linked to mutations in components of WNT signaling.Mutations in the receptors LRP4,5 or 6 affect their affinity for an endogenous WNT inhibitor, sclerostin.Other mutations affect the expression of sclerostin itself.Antibody inhibition or deletion of sclerostin increases markedly bone formation along bone surfaces and decreases bone resorption.Clinical trials show a rapid but temporary increase in bone formation and bone mass with a decreased fracture rate.Osteoporosis is a widespread chronic disease characterized by low bone density, altered microstructure and bone fragility, leading to low impact fractures in affected individuals. The discovery of a few mutations that cause extremely rare human diseases has identified the WNT signaling pathway as a candidate for therapeutic intervention aimed at increasing bone mass and strength. In particular, inhibition of sclerostin, a WNT antagonist secreted by osteocytes, has proven in clinical trials to be a very efficient osteo-anabolic approach. One year of monthly administration of antibodies to sclerostin rapidly decreases bone resorption and increases bone formation and bone density at all sites, decreasing markedly fracture risk in treated patients. Their effect is however limited in time and cardiovascular adverse events have been reported in one clinical trial.