SP7 and osteosarcoma: To further define the importance of Fgfr3+ endosteal stromal cells as a potential cell-of-origin of osteosarcoma-like lesions, we performed comparative p53-deficiency-induced bone tumorigenic analyses with additional bone-related cre/creER lines, including Osx-creER (targeting osteoblast precursors), Gli1-creER (targeting growth plate chondrocytes and metaphyseal mesenchymal progenitors), Pthrp-creER (targeting resting-zone chondrocytes) and Lepr-cre (targeting bone marrow stromal cells).