RUNX2 and osteosarcoma: The proliferation of Runx2−/− calvarial cells was greater than that of wild-type calvarial cells; Runx2 induces G1 cell-cycle arrest through the induction of p27KIP1 in osteosarcoma cells; Runx2 expression is up-regulated in the cessation of cell proliferation and down-regulated to minimal levels during the early S phase and mitosis in MC3T3-E1 preosteoblastic cells; Runx2 suppresses the proliferation of cells with osteogenic potential and osteosarcoma cells, and the introduction of siRNA against Runx2 into human mesenchymal stem cells increases proliferation3–7.