CD99 and primitive neuroectodermal tumor: FLI-1 is still a highly specific marker to distinguish EWS/PNET from all types of malignancies. In addition, in poorly differentiated SS, alveolar rhabdomyosarcoma, mesenchymal chondrosarcoma, and other tumors with positive CD99 expression, FLI-1 is negative (28, 30, 31).