We have now discovered that FLI-1-EWS is frequently expressed in Ewing sarcoma and have obtained data indicating that endogenous FLI-1-EWS is required for Ewing sarcoma growth and that FLI-1-EWS cooperates with EWS-FLI-1 in human MSCs through abrogation of the growth arrest induced by EWS-FLI-1. Here, EWSR1 is linked to Ewing sarcoma.