CD99 and Ewing sarcoma: F: female; M: male; CT: chemotherapy; RT: radiotherapy; VDC/IE: vincristine, doxorubicin, cyclophosphamide/ifosfamide, etoposide; ES: Ewing sarcoma; NGS: next-generation sequencing; CK: cytokeratin; CD: cluster of differentiation (e.g., CD99, CD45); NKX2.2: homeobox protein NKX2.2; FLI-1: Friend leukemia integration 1 transcription factor