understanding of the activity and liabilities of individual PARP inhibitors is therefore important to aid clinical decisions and benefit cancer patients as well as to guide the design of future PARP inhibitors. Several differences between individual PARP inhibitors have already been reported at the cellular and clinical levels. When used at micromolar concentrations, differences in DNA strand break repair, phosphorylation of several proteins, cell cycle arrest, and anti-proliferative activities have been described between olaparib, rucaparib and veliparib in cancer cell lines 24,25. Moreover, the different capacity of PARP inhibitors to trap PARP at the DNA damage site is widely accepted as important for the action of PARP inhibitors. Yet, the exact molecular mechanism of this is not completely understood 26. Differences in cancer cell sensitivity and synthetic lethality also emerge from recent large-scale profiling experiments, in some instances leading to the prediction of distinct predictive genomic biomarkers (Supplementary Table 1) 27,28. Overall, it seems important to investigate further the differences between PARP inhibitors and to explore the potential impact on clinical use. The selectivity and polypharmacology of PARP inhibitors within the PARP-family was recently characterised in vitro using an enzymatic inhibition assay 29. Of the four approved PARP inhibitors, niraparib was shown to be more selective for PARP1 and PARP2 compared to olaparib, rucaparib and talazoparib which show broader pan-PARP activity (Fig. 1b) 29. However, this differential intra-family PARP selectivity is insufficient to explain all the differences observed between clinical PARP inhibitors. In 2014, we reported for the first time that the different polypharmacology patterns between PARP inhibitors extended beyond the PARP enzyme family 30. We demonstrated that rucaparib inhibited at least nine kinases with micromolar affinity whereas veliparib inhibited only two kinases and olaparib did not exhibit activity against any of the 16 kinases tested 30. From a high-throughput screen for RPS6KB1 kinase inhibitors, we identified a series of carboxamidobenzimidazoles that were confirmed to bind RPS6KB1 by orthogonal methods including X-ray crystallography 31. The carboxamidobenzimidazoles are known inhibitors of PARP 32 and the existence of a crystal structure of a carboxamidobenzimidazole bound to RPS6KB1 kinase 31 prompted our speculation that all PARP inhibitors could have an intrinsic capacity to inhibit kinases. This capacity could result from the ability of their shared benzamide pharmacophore to interact with the highly-conserved kinase hinge region (Fig. 1a) 30,31. Accordingly, depending on its individual molecular size and decoration, each PARP inhibitor could have a unique off-target kinase profile that may remain as yet unexplored and would be important to characterise 4,30. More recently, an unbiased, large scale, mass spectrometry-based chemical proteomics approach uncovered new, low-potency affinities of the PARP inhibitor ...