EWSR1 and neoplasm: Moreover, ectopic EWSR1-FLI1 expression in human mesenchymal stem cells (hMSCs), defined as the putative cells of origin for ES, does not result in cell transformation or in tumor formation in immunocompromised mice, although hMSCs are initially permissive to EWSR1-FLI1 ectopic expression [15, 16].