Induction of tetraploidy and apoptosis in MM cells. (A) Cell cycle analysis (propidium iodide staining) was performed in OPM-2 and MM1.S MM cell lines after 48-hour exposure to vehicle alone (control), ZK (500 nM), or VX-680 (100 nM). Induction of tetraploidy is shown. (B) Similarly treated OPM-2 and MM1.S cells were assayed at 72 hours for apoptosis by flow cytometric analysis of activated caspase 3 expression. Results are mean percent apoptosis ± SD of 3 separate experiments (control cells treated with vehicle alone showed a mean of 16% positive staining with the caspase 3 antibody).