A, WT, IKKα−/−, or IKKβ−/− MEFs were either untreated (shaded) or treated overnight with TNF (10 ng/ml; unshaded) in the absence (Control) or presence of zVADfmk (25 μM). FADD DN-transduced IKKα−/− and IKKβ−/− MEFs were also either untreated or treated overnight with TNF. Cells were harvested, incubated in PBS containing PI, and then analyzed by FACS. Membrane-permeable cells that took up PI were determined in region M1, and the percentage of PI-positive cells is shown in B.