Sclerosteosis and Van Buchem disease are two closely related bone disorders characterized by progressive bone thickening due to increased bone formation. Sclerosteosis is associated with mutations in the SOST gene and Van Buchem disease with a 52 kb deletion downstream of the SOST gene that probably affects transcription of the gene. Expression of the gene product sclerostin in bone is restricted to osteocytes and it is a negative regulator of bone formation. It inhibits BMP-stimulated bone formation, but cannot antagonize all BMP responses. The exclusive bone phenotype of good quality of patients with sclerosteosis and Van Buchem disease and the specific localization of sclerostin make it an attractive target for the development of bone forming therapeutics.