The reason for failure of permanent tooth eruption in patients with cleidocranial dysplasis is not understood well. Microscopic studies of unerupted permanent teeth have shown that these teeth lack secondary cementum. However, a recent histomorphometric study demonstrated no statistically significant difference in the percentage of root surface covered by cementum between teeth extracted from a patient with cleidocranial dysplasia and teeth from control patients. Some investigators alternatively have proposed that insufficient alveolar bone resorption is the reason for impaired tooth eruption; this theory is based on observations of decreased osteoclasts in the alveolar bone heterozygous CBFA 1 knockout mice.