RB1 and spindle cell neoplasm: al’s data demonstrating p53−/− and Rb−/− MSCs derived from mouse bone marrow, when subcutaneously injected into SCID mice, form malignant spindle cell tumors consistent with leiomyosarcomas, however, when the MSCs are first placed in osteogenic differentiation media and then injected into immunodeficient mice they form osteosarcoma [28].