EWSR1 and Ewing sarcoma: Ewing sarcoma (ES) is a malignant small round blue cell tumor most frequently arising in the long bones of adolescents and young adults, characterized by a translocation involving the Ewing sarcoma RNA-binding protein 1 (<i>EWSR1</i>) gene, most commonly the <i>EWSR1</i>/friend leukemia integration 1 (<i>FLI1</i>) fusion.