EWSR1 and rhabdomyosarcoma: The presence of ctDNA in plasma has been proposed as a prognostic biomarker in pediatric solid tumors (reviewed in ref. 7), and targeted detection of EWSR1 and FOXO1 fusions in Ewing sarcoma and alveolar rhabdomyosarcoma (ARMS) has been demonstrated in small cohorts of patients29,30.