Mutation of the RB-1 and p53 tumor suppressors is associated with the development of human osteosarcoma. With the goal of generating a mouse model of this disease, we used conditional and transgenic mouse strains to inactivate Rb and/or p53 specifically in osteoblast precursors. The resulting Rb;p53 double mutant (DKO) animals are viable but develop early onset osteosarcomas with complete penetrance. These tumors display many of the characteristics of human osteosarcomas, including being highly metastatic. We established cell lines from the DKO osteosarcomas to further investigate their properties. These immortalized cell lines are highly proliferative and they retain their tumorigenic potential, as judged by their ability to form metastatic tumors in immunocompromised mice. Moreover, they can be induced to differentiate and, depending on the inductive signal, will adopt either the osteogenic or adipogenic fate. Consistent with this multipotency, a significant portion of these tumor cells express Sca-1, a marker that is typically associated with stem cells/uncommitted progenitors. By assaying sorted cells in transplant assays, we demonstrate that the tumorigenicity of the osteosarcoma cell lines correlates with the presence of the Sca-1 marker. Finally, we show that loss of Rb and p53 in Sca-1-positive mesenchymal stem/progenitor cells is sufficient to yield transformed cells that can initiate osteosarcoma formation in vivo.osx-cre ͉ Sca-1 ͉ hibernoma mouse model O steosarcomas account for Ϸ30% of malignant bone tumors and 3-4% of all childhood malignancies (1, 2). They arise primarily around the knee joint, lower femur and upper tibia, which are all regions of active bone growth and repair. These tumors are predominantly osteoblastic in nature, although there is a correlation between loss of differentiation and poor prognosis. The generation of new therapeutic treatments for osteosarcoma has improved the 5-year survival rate of affected individuals. However, like other mesenchymal neoplasms, osteosarcomas are predisposed to metastasize via the hematogenous route, and thus, pulmonary metastasis is a major cause of death. Analyses of both sporadic and hereditary tumors show that inactivation of the p53 and RB-1 tumor suppressors plays a key role in the development of this tumor type (1, 2). Li-Fraumeni patients, who often carry germ-line mutations in p53, are predisposed to a variety of tumors, 12% of which are bone sarcomas (3, 4). p53 mutations are also observed in 20-60% of sporadic osteosarcomas (5-7). Similarly, patients carrying germline mutations in RB-1 have an Ϸ500-fold higher incidence of osteosarcoma than the general population (8). Moreover, RB-1 mutations are detected in 70% of all adolescent osteosarcomas (9). Finally, human osteosarcomas can carry mutations in both p53 and RB-1 (10).Mouse models have provided considerable insight into the role of p53 in bone development and tumorigenesis. Experiments from three different settings suggest that p53 plays an important role in bone development by modul...