The molecular mechanisms underlying the dental defects seen in CCD are also poorly understood; however, Runx2 is widely expressed in odontogenic mesenchyme and interestingly, tooth development arrests at the bud stage in Runx2 mutant mice.32,33,34,35 Investigating the supernumerary phenotype in humans has been compounded to some extent by the arrested tooth development seen in Runx2 mutant mice. The gene discussed is RUNX2; the disease is cleidocranial dysplasia 1.