PARP1 and neoplasm: Importantly, these radiosensitizing effects are only observed in proliferating cells, in which unresolved SSBs are converted into DSBs during replication, thus enhancing the cytotoxic effects of radiation.86 In the context of GBM, therefore, PARP inhibition is predicted to enhance the capacity of RT to ablate rapidly proliferating tumor cells while having minimal effect on cells in the surrounding normal brain, which are almost exclusively nonreplicating.