In this context, it has been shown that overexpression of EWS-FLI1 in murine MSCs leads to their transformation and the formation of sarcoma once implanted in vivo, with characteristics (CD99 expression) and morphology similar to those of ES (Torchia et al., 2003; Castillero-Trejo et al., 2005; Riggi et al., 2005). This evidence concerns the gene EWSR1 and sarcoma.