CHEK1 and Ewing sarcoma: To identify therapeutically actionable DDR pathway vulnerabilities, we screened six ARMS cell lines, eight Ewing sarcoma cell lines and five embryonal rhabdomyosarcoma (ERMS) cell lines compared to five primary untransformed myoblasts derived from healthy human donors for their sensitivity to small molecule inhibitors of DDR kinases ATR (AZD6738, BAY 189534428,29), ATM (KU60019), CHK1/2 (AZD7762) and WEE1 (AZD1775) (Fig. 1a–f and Supplementary Fig. 1a–p).