Based on the increased level of replication stress in Ewing sarcoma cells and the known sensitivity of Ewing sarcoma cells to ATR inhibitors, we tested a CHK1 inhibitor (LY2603618) as a single agent to evaluate the effect of inhibition of CHK1 on the viability of Ewing sarcoma cells [25]. Here, CHEK1 is linked to Ewing sarcoma.