EWSR1 and Ewing sarcoma: Ewing Sarcomas are derived from mesenchymal progenitor cells (MPCs) [100], and forced expression of EWS-FLI1 in MPCs was shown to be stably maintained without growth arrest or apoptosis whilst inducing an gene expression profile similar to that of a Ewing Sarcoma, all in the presence of a functional, wild-type p53 [101–103].