It has been shown that the presence in MCS of undifferentiated small round cells, positive for CD99 immunohistochemistry in a membranous pattern, and also positive for NK2 homeobox 2 (NKX2.2) in a nuclear pattern, may be a pitfall as Ewing sarcoma (EWS), another relatively frequent primary bone tumor, is also positive for these markers. The gene discussed is EWSR1; the disease is bone neoplasm.