Ewing sarcoma: A malignant tumor that arises in a primitive nerve cell within bone or soft tissue and affects children and adolescents, especially between ages 10 and 20. Ewing sarcoma usually appears in the large bones of the arms and legs and the flat bones of the pelvis, spine, and ribs. Treatments include chemotherapy, surgery, and radiation therapy. The primitive nerve cell from which Ewing sarcoma arises also gives rise to a number of tumors, known as the Ewing family of tumors, which include Ewing sarcoma of bone, extraosseus (nonbone) Ewing sarcoma, primitive neuroectodermal tumor (PNET), and Askin tumor (PNET of the chest wall). Most Ewing family tumors have a translocation between chromosomes 11 and 22 that results in the fusion of the EWS gene on chromosome 22 with the transcription factor gene FLI1 on chromosome 11, leading to the production of a chimeric (fusion) protein. The remaining tumors in the Ewing family engage the EWS gene in other translocations that lead to formation of chimeric proteins. In all cases the chimeric protein is oncogenic; that is, it is responsible for the malignancy.