FLI1 and osteosarcoma: To address this, mice expressing the Ews-Fli1 transgene were then crossed to mice harboring Prx-Cre-directed deletion of p53. The Prx-Cre;Ews-Fli1;p53−/− mice rapidly developed poorly differentiated sarcomas (median age of 21 weeks); while Prx-Cre mediated deletion of p53 alone resulted in the development of osteosarcomas (median age of 50 weeks), demonstrating the cooperative interactions between Ews-Fli1 and p53 in sarcomas.