Runx2 [also known as: core binding factor ␣ 1 (CBFA1); acute myeloid leukemia transcription factor 3 (AML3); polyoma enhancer-binding protein 2␣ (PEPB2␣)] is a member of the runt-domain gene family of DNA binding proteins (Runx1, Runx2, and Runx3), which control the expression of numerous genes involved in cell growth, proliferation, and determination of cell lineage (1). Aberrant expression and activity of Runx proteins are implicated in leukemia, metastatic breast cancer, and defects in skeletal development and bone remodeling (2-4). Runx2 is expressed during early embryonic development in mesenchymal and cartilage condensations of the developing bone anlagen, and its transcription, activity, and stability are tightly regulated (5-7). Considered the master regulator of osteoblast differentiation, Runx2 also contributes to chondrocyte maturation (8,9) and, through these cell-types, controls intramembranous and endochondral ossification culminating in the formation of the mineralized skeleton (5, 6, 10). A gain-of-function mutation of Runx2, resulting from duplication of exons 3 to 5, is linked to dysplastic long bone formation, enlarged clavicles, and thickening of the cranial vault (11). Conversely, the rare autosomal dominant disorder, cleidocranial dysplasia, has been traced to multiple loss-of-function mutations within the Cbfa1 gene, and is characterized by defective formation or absence of the clavicle, enlarged fontanelles, and dental abnormalities (9, 12, 13). Mice nullizygous for Runx2 develop a cartilaginous skeletal framework that fails to undergo mineralization caused by the absence of osteoblasts, and these mice die at birth because of respiratory failure (5, 10).