FLI1 and Ewing sarcoma: In the latest version of the WHO classification of soft tissue and bone tumors, Ewing sarcoma (ES) is defined as a malignant small blue round cell tumor that occurs in bone and soft tissue, with common genetic characteristics, including fusions of FET (EWSR1/FUS/TAF15, etc.)gene family members with transcription factor ETS (FLI1/ERG, etc.)family members.