The disruption of p53/Rb in mesenchymal progenitors (Prx1-cre), osteoblast precursors (Osx-Cre), and osteoblast committed cells (Col1a1-Cre and OCN-Cre) leading to osteogenic sarcoma confirmed that cells with mesenchymal origin and osteogenic lineage were responsible for osteogenic tumor formation (4, 10–12). The gene discussed is COL1A1; the disease is sarcoma.