RUNX2 and cleidocranial dysplasia 1: Mice homozygous for a Runx2 null allele exhibit neonatal lethality and skeletal abnormalities particularly in clavicle and cranial bones, recapitulating the underdeveloped or absence of clavicles seen in cleidocranial dysplasia (OMIM: 119600) that has been associated with mutations in RUNX2.