RUNX2 and cleidocranial dysplasia 1: Mice that are deficient in RUNX2 have complete absence of bone [5, 13, 14], while those with haploinsufficiency of this transcription factor mimic some of the human CCD phenotypes, while others, such as supernumerary teeth, some skeletal abnormalities, and osteoporosis, have not been clearly demonstrated in mice [13].