To probe the role of ATM and HRR status further, and assess PARP as a therapeutic target in CLL, we analysed the effect the highly potent PARP inhibitor, talazoparib (cell-free IC50 = 0.57 nM, [27] on the proliferation of CLL cells using the CD40L-stimulated co-culture model. This evidence concerns the gene CD40LG and B-cell chronic lymphocytic leukemia.