EWSR1 and osteosarcoma: Such mutations occur in up to 37.5% of patients with metastatic Ewing sarcoma.254–256 PTPL1 (encoded by Ptpn13) is a direct transcriptional target of EWS-FLI1 that modulates EWS tumorigenesis.257 RPTPζ is also involved in the pathology of osteosarcoma.