SSX1 and synovial sarcoma: However, entity-defining fusion oncogenes acting as the main oncogenic driver mutations are frequently found in pediatric bone and soft-tissue sarcomas such as Ewing sarcoma (EWSR1-FLI1), alveolar rhabdomyosarcoma (PAX3/7-FOXO1), and synovial sarcoma (SS18-SSX1/2/4).