Induction of apoptosis. The percentage of apoptotic cells was determined using annexin V flow cytometry assays. Cell lines were treated with increasing doses of VPA (0.25-3 mM) or vorinostat (0.075-1 μM) in combination with idarubicin at 0.5 nM in MOLT4 or 1.5 nM in HL60 cells daily for 3 days. Numbers on the y-axis represent percentage of apoptotic cells. (A) MOLT4 cells. (B) HL60 cells. IDA indicates idarubicin; VPA, valproic acid; and Vor, vorinostat (suberoylanilide hydroxamic acid). Error bars indicate standard deviation.