We only included those cases of ES in our study that have unequivocal IHC profile, i.e., all tumors were positive with CD99 and FLI1, some show focal synaptophysin immunoreactivity, and all were negative with CKAE1/AE3, desmin, myogenin, LCA, and TdT immunostains. This evidence concerns the gene CD99 and Ewing sarcoma.