Role of RasGRP1 in tipifarnib sensitivity. (A) Twenty-four hours after transfection with cDNA encoding H-Ras, cells were treated with the indicated tipifarnib concentration for 72 hours in the presence of 5μM Q-VD-OPh and prepared for immunoblotting. (B) Forty-eight hours after transfection with control siRNA (lane 1) or RasGRP1 siRNA (lane 2), whole cell lysates were prepared and subjected to immunoblotting with antibody to RasGRP1 or, as a control, poly(ADP-ribose) polymerase (PARP). (C) Beginning 24 hours after transfection with control siRNA or RasGRP1 siRNA, cells were treated for 48 hours with tipifarnib, stained with propidium iodide, and subjected to flow cytometry. Error bars, mean ± SD of 3 independent experiments.