A 2-year-old boy with classical CCD showed a heterozygous deletion (c.593_601delCCTTGACCA, p.Thr198_Thr200del) in the Runt domain; the 3D modeling assessment demonstrated that this mutation abolished heterodimerization of the RUNX2 protein with its partner subunit: the polyomavirus enhancer-binding protein 2b (PEBP2b); in this report, the transactivation study showed that the p.Thr198_Thr200del and p.Leu199Phe mutants had significantly lower transcription activities (44% and 63% of the wild type, respectively) [21]. The gene discussed is RUNX2; the disease is cleidocranial dysplasia 1.