Mitochondrial caspase signaling works in imatinib-treated cells, while caspases are completely inhibited in zVAD + imatinib–treated cells. BV173 cells were treated with DMSO (control), imatinib (hatched bars), and zVAD + imatinib (black bars) for 1, 2, 3, 6, 12, 24, and 72 hours, followed by caspase activity assay (“Materials and methods”). In imatinib-treated cells, significant activation of caspase-9 and caspase-3 preceded that of caspase-8. In zVAD + imatinib–treated cells, all these caspases were inhibited below the control level throughout the experiments. Results are expressed as fold increase compared with control. Data represent means ± SEM of 4 individual experiments. ^* indicates P is less than .05.