Cleidocranial dysplasia (CCD) is an autosomal-dominant heritable skeletal disease caused by heterozygous mutations in the RUNX2 gene. We studied a Chinese family that included three affected individuals with CCD phenotypes; the clinical features of patients with CCD include delayed closure of fontanelles, frontal bossing, dysplasia of clavicles, late tooth eruption, and other skeletal anomalies. X-ray analysis showed aplasia of the clavicles. The RUNX2 gene was studied by PCR and direct sequencing of the entire coding region and the exon-intron boundaries of the gene. A novel missense mutation (c.1259C-->T[p.T420I]) in RUNX2 gene exon 7 was identified; it was found in the affected individuals in this Chinese family, but was not present in an unaffected family member or in 100 unrelated normal controls. This is the first report that gives evidence that the T420I mutation of RUNX2 is associated with CCD, expanding the spectrum of RUNX2 mutations causing CCD.