Friend leukemia integration 1 transcription factor (FLI-1) is a protein encoded by the proto-ocogene FLI-1.  The FLI-1 protein is best known for its critical role in the pathogenesis of ES/pPNET. FLI-1 is normally expressed in endothelial cells and in hematopoietic cells, including T-lymphocytes. The immunohistochemical detection of FLI-1 protein has been shown in studies to be valuable in the discrimination of Ewing sarcoma/peripheral primitive neuroectodermal tumor (ES/pPNET) from most of its potential mimics. ES/pPNET is a rare primary tumor of the bone/soft tissue that resembles other undifferentiated tumors.^1-3