ATO-induced apoptosis in 4 human myeloma cell lines. U266, MM.1s, 8226/S, and KMS11 were treated for 24, 48, and 72 hours with 0, 0.5, 1, 2, and 4 μM ATO. Cell viability was determined by flow cytometry following annexin-V–FITC/PI staining. Percentage (%) of control viability was plotted versus ATO concentration. Graphs are presented for each time point. The data are presented as the means (± SD) of 3 independent experiments.