Activating mutations in the gene encoding the Wnt co-receptor LRP5 which makes it resistant to extracellular Wnt inhibitors, like Dickopff-related 1 (Dkk-1), leads to a high bone mass phenotype [73–77], whereas loss-of-function mutations in the same co-receptor leads to a juvenile form of osteoporosis, associated with decreased de novo bone formation, called osteoporosis-pseudoglioma syndrome [78]. This evidence concerns the gene DKK1 and osteoporosis.