RUNX2 and cleidocranial dysplasia 1: Variants in the runt-related transcription factor 2 gene (RUNX2, MIM 600211) account for 60% of the CCD cases.[6,7]RUNX2 is located on chromosome 6p21 and has 8 coding exons.[6] RUNX2 is a key regulator of osteoblast differentiation and bone development.[7] Homozygous Runx2 knockout mice (Runx2−/−) exhibited a lack of osteoblasts, failed in both intramembranous and endochondral ossification, and showed early lethality after birth.