CD99 and Ewing sarcoma: Resulting tumors were confirmed to be Ewing sarcoma with positive IHC staining for CD99, a membrane protein expressed uniformly in Ewing tumors, and nuclear staining of NK homeobox 2 (NKX2.2), a transcription factor upregulated by EWSR1::FLI1 fusion oncoprotein (Fig. 2B).