DYSPLASIA CLEIDOCRANIAL Best known for its dental and clavicular abnormalities, dysplasia is a disorder of bone caused cleidocranial by a defect in the CBFA1 gene (also known as the RUNX2 gene) of chromosome 6p21. This gene nor- differentiation and appropri mally guides osteoblastic ate bone formation. This condition initially was thought to involve only membranous bones (e.g., clavicles, skull, flat bones), but it is now known to affect endochondral disorder ossification and to represent a generalized of skeletal structures. Recent evidence suggests that the CBFA1 gene additionally plays an important role in odontogenesis via participation in odontoblast differentiation, enamel organ formation, and dental lamina proliferation. Disruption of these functions might explain the distinct dental anomalies found in patients with this disorder. The disease has an esti. mated prevalence of 1:1,000,000 and shows an autosomal dominant inheritance pattern, but as many as 40% of cases appear to represent spontaneous mutations. This condition formerly was known as cleidocranial dysostosis.