Imatinib induces classical apoptosis, while zVAD + imatinib induces atypical cell death. Diff-Quick–stained cytocentrifuge specimens of BV173 cells (A) and K562 cells (B). BV173 cells and K562 cells were treated for 12 hours and 48 hours, respectively, with DMSO, zVAD, imatinib, and zVAD + imatinib. After treatment with imatinib, BV173 cells and K562 cells exhibited cell shrinkage, nuclear condensation, and nuclear fragmentation, which are characteristic of apoptosis. In contrast, after treatment of zVAD + imatinib, BV173 cells and K562 cells exhibited cell shrinkage, nonfragmented nuclei, and marked cellular pyknosis. Results are representative of 5 individual experiments. Arrows indicate apoptotic cells; arrowheads, atypical cells. Original magnification, × 1000.