C.Y. Zhang1, S.G. Zheng1*,

Cleidocranial dysplasia (CCD) is an inherited autosomal-dominant skeletal disease caused by heterozygous mutations in the osteoblast-specific transcription factor, RUNX2. We performed muta-tion analysis of RUNX2 on four unrelated Chinese individuals with CCD. Three novel distinct muta-tions were detected in the coding region of RUNX2: two missense and one frameshift. These mutations were exclusively clustered within the Runt domain. One missense mutation converts threonine to iso-leucine at codon 200 (T200I). The other one sub-stitutes leucine for arginine at codon 225 (R225L), which affects many family members. The frame-shift mutation (214fs) in exon3 leads to the intro-duction of a translational stop codon at codon 221, resulting in a truncated RUNX2 protein. The reporter gene assays revealed that all the mutants exhibited significantly reduced transactivation activities on the osteocalcin promoter. Our results provide new genetic evidence that mutations involved in RUNX2 contribute to CCD. Abbreviations: AML3, gene encoding acute myeloid leukemia protein 3; bp, base pair; CBFA1, gene encoding core-binding factor 1; CBFβ, gene encoding core-binding factor β; CCD, cleidocranial dysplasia; NLS, nuclear local-ization signal; OSE2, osteoblast-specific cis-acting element 2; PEBP2A, gene encoding polyoma enhancer binding protein 2A; PST, proline/serine/ threonine-rich domain; Q/A, glutamine-alanine repeat domain; Runt, Runt Homology Domain; RUNX2, the mammalian runt-related genes 2; RUNX2, Runt-related protein 2