Pretreatment with olaparib sensitizes ATM null cells to cytotoxic agents. (A) Effect of 1μM olaparib on killing by 12.5μM bendamustine, 0.1μM fludarabine, 0.1μM 4HC, 5mM VPA, and 1 Gy IR in ATM mutant Granta-519 cells. Synergistic effects (see supplemental Table 2) for the given doses are shown below the graph (+++ synergism; ++ moderate synergism; + slight synergism; ± nearly additive). (B) Western blot analysis of Granta-519 cells showing an increased cleavage of PARP1, caspase 7, and caspase 3 after combined olaparib plus 4HC, VPA, or IR treatment compared with treatment with 4HC, VPA, or IR alone. Actin shows equal loading.