Previous studies have identified three primary cell types that are permissive for EWS-FLI1 expression and thus represent prime candidates for the elusive tumor cell of origin: (i) mesenchymal stem cells (MSCs) [17–19], (ii) neural crest stem cells [20], and (iii) embryonic osteochondrogenic progenitor cells [21]. Here, EWSR1 is linked to neoplasm.