EWSR1 and synovial sarcoma: Once the respective fusion has occurred, oligo-mutated cancer cells show a strong oncogene addiction toward their disease-defining fusion oncogene (e.g., EWSR1-FLI1 in EwS [3], PAX3/7-FOXO1 in alveolar rhabdomyosarcoma (ARMS) [8, 9] and SS18-SSX1/2/4 in synovial sarcoma (SS) [10–12]) in terms of tumor progression and metastasis.