SOST and bone disorder: This rare Mendelian skeletal disease is caused by mutations in SOST, which encodes the protein sclerostin, an osteocyte‐specific secreted WNT inhibitor.33 Sclerostin is a tonic inhibitor of bone formation;34 therefore, a mechanism through which bone anabolic signals may trigger new osteoblast activity is by reducing SOST expression in osteocytes.