PARP1 and neoplasm: In order to show the dependence of the tumor cells on specific repair processes and to exploit DNA repair deficiencies, we treated the MB cells isolated from the Nestin-Cre p53−/−Xrcc4−/− mouse tumors with inhibitors of HR (B02 or Ri1, RAD51 inhibitors) and/or Alt-EJ (olaparib, PARP inhibitor) or induced DNA damage using topotecan (a topoisomerase 1 inhibitor), or applied a combination of all three.