PARP1 and Ewing sarcoma: While single-agent treatment with olaparib, a PARP1 inhibitor, effectively inhibited Ewing’s sarcoma cell growth in vitro, monotherapy with olaparib failed to suppress Ewing’s sarcoma xenograft tumor growth or provide survival advantage in vivo [50,52] and showed no clinical benefit in a phase II trial with continuous high-dose treatment as a single agent [50].