Positive staining for CD56 and NKX2.2 and negative results for markers such as CK19, Pan-CK, Desmin, and S100 effectively ruled out other small round-cell tumors, including lymphomas, rhabdomyosarcoma, and neuroblastoma. Immunohistochemistry remains essential for screening: diffuse membranous CD99 expression is seen in most ES cases but is not entirely specific, while NKX2-2 is a useful adjunct that increases specificity when used together with CD99. The gene discussed is CD99; the disease is Ewing sarcoma.