A phase II study of single agent, Olaparib failed to demonstrate clinical activity [35]; however, strong evidence that PARP inhibitors were also able to selectively potentiate the activity of DNA-damaging cytotoxic agents such as irinotecan and temozolomide in Ewing sarcoma animal models led to several phase I and II combination clinical trials [36–38]. Here, PARP1 is linked to Ewing sarcoma.