Hyperostosis corticalis generalisata (Van Buchem disease; MIM 239100) and sclerosteosis (MIM 269500) are autosomal recessive sclerosing bone dysplasias with similar clinical and radiological findings.  Both disorders are characterized by a progressive osteosclerosis involving the skull, jaw, clavicles, ribs and long bones.  Overgrowth of the cranial and facial bones may lead to facial palsy, optic atrophy, proptosis, deafness, massive overgrowth of the mandible, and increased intracranial pressure. Patients with sclerosteosis have a more severe phenotype, which includes syndactyly, radial deviation of the terminal phalanges, nail dysplasia and gigantism. Sudden death due to impaction of the medulla oblongata in the occipital foramen has also been reported. Sclerosteosis is caused by mutations in the SOST gene encoding sclerostin, a protein that decreases osteoblast activity and inhibits bone formation. Hyperostosis corticalis generalisata (Van Buchem disease) is caused by a 52 kb deletion approximately 35 kb downstream of the SOST gene and possibly by mutations within the SOST gene.  The 52 kb deletion contains regulatory elements essential for proper expression of SOST. A phenotypically similar but less severe autosomal dominant disorder, known as Van Buchem disease type 2 (MIM 607636), is caused by mutations in the LDL receptor-related protein 5 gene (LRP5). Mutations in SOST also cause craniodiaphyseal dysplasia(CDD; MIM 122860).