In contrast to the more differentiated osteosarcomas that formed without EWS-FLI1 in mice with conditional mutation of p53 induced by Prx1-cre, the tumors that arose with EWS-FLI1 were undifferentiated sarcomas, similar to Ewing's sarcoma. This evidence concerns the gene EWSR1 and undifferentiated (embryonal) sarcoma.