EWSR1 and Ewing sarcoma: Furthermore EWSR1-FLI1 expression in mesenchymal cells resulted in development of ES tumors [65, 66] and EWSR1-FLI1 introduction into a population of cells enriched for osteochondrogenic progenitors derived from the embryonic superficial zone of murine long bones revealed a subpopulation of precursor cells that further enhanced EWSR1-ETS–dependent tumor induction [67].