A, WT and IKKβ−/− MEFs transduced with LZRS or IKKβ as shown (parentheses) were either untreated (shaded) or treated overnight with TNF (10 ng/ml; unshaded) in the presence of zVADfmk (25 μM). 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 PI-positive MEFs is shown in panel B. C, WT and NEMO-deficient MEFs were either untreated (shaded) or treated overnight with TNF (10 ng/ml; solid line) in the absence (Control) or presence of zVADfmk (25 μM). PI uptake was determined as described for panel A, and the percentage of PI-positive cells is shown in D.