PARP1 and neoplasm: One possible theory for the mechanism of action of PARP inhibitors in tumours with HRR deficiencies is synthetic lethality due to the simultaneous blockade of the pathways involved in base excision repair and HRR DNA repair.43,44 An additional proposed mechanism of action is PARP trapping, whereby PARP inhibitors act as DNA poisons by trapping PARP on damaged DNA, resulting in cytotoxic PARP–DNA complexes.