Caspase-8 is the dominant activated caspase in cells with mutated p53.

RPMI 8226 cells were cultured for 0, 16, 24, and 48 hours with or without 7.5 μM ATO followed by 1 hour of incubation with FITC-tagged caspase peptides specific for caspase-3, -8, and -9 as recommended by the manufacturer. FITC–caspase substrate peptides had 5% to 10% background staining (0 h). For comparison, apoptosis was determined in separate tubes by the FITC–annexin V. Fluorescence was quantitated by flow cytometry as described in “Materials and methods”; 10 000 cells were analyzed for each sample. Representative results from at least 3 different experiments are shown. For additional experimental details, see “Materials and methods.” Note the similar kinetics between apoptosis and caspase activity. Note also the preferential activation of caspase-8 over caspase-9 in RPMI 8226 cells.