The origin of bone marrow osteoblasts is not totally understood. Recent findings demonstrated that bone marrow osteoblasts could be derived from a subpopulation of hypertrophic Col2+/Col10+ chondrocytes which migrate from the growth plate into the bone marrow cavity underneath the growth plate and dedifferentiate into mesenchymal progenitor cells and then differentiate into mature osteoblasts.1 This process is called chondrocyte-osteoblast transdifferentiation. This type of osteoblast participates in bone formation and is involved in maintaining bone remodeling, especially in the epiphyseal and diaphyseal regions of long bone. Several growth factors, such as Ihh, PTH, and Wnt signaling molecules have been demonstrated to play a critical role in the regulation of chondrocyte-osteoblast transdifferentiation2; however, the role of Runx2, the key transcription factor controlling skeletal development,3 in chondrocyte-osteoblast transdifferentiation has not been fully defined.