Effect of Bcl-2/Bcl-X_L inhibitor on ATO-induced apoptosis. MM.1s and KMS11 cell lines were treated with 2 μM ATO and indicated concentrations of ABT-737 or the less active enantiomer (-)ABT for 24 hours. Viability was measured by annexin-V–FITC/PI staining. Percentage (%) of apoptosis was plotted versus the concentration of ABT-737. The data are presented as the means (± SD) of 5 independent experiments. t test was used to compare differences between expected additive effect and actual cotreatment results, with confidence intervals of 95%. ND indicates no difference; *P < .05; **P < .01; ***P < .001.