6a However, MCS differs from EWS as the only bone tumor with a component of developed cartilage, which stains positive for the S‐100 protein3, 8. Recent studies have identified a novel fusion gene HEY1‐NCOA2 which has both high sensitivity and specificity for MCS10. Detection of this fusion gene may play a role in future cytogenetic diagnosis of obscure presentations of MCS.1 The gene discussed is HEY1; the disease is bone neoplasm.