Larger studies of localized Ewing sarcoma and osteosarcoma demonstrated that the abundance of ctDNA molecules present at the time of diagnosis correlated with outcome (19, 22), and higher ctDNA concentrations of EWS during or after treatment were associated with known clinical risk factors such as larger initial tumor volume, pelvic or osseous localization, metastatic disease, higher age at diagnosis, and poor histological response to chemotherapy (22). This evidence concerns the gene EWSR1 and osteosarcoma.