Induction of histone H3 and H4 acetylation. HL60 cells were treated with (A) vorinostat (Vor) at increasing concentrations or (B) VPA. The effect on the sequence of IDA with vorinostat using concentrations of 1 μM of vorinostat and IDA at 1.5 nM administered concomitantly, or with IDA preceding vorinostat (IDA, A; vorinostat, A-B) or the reverse (IDA, B; vorinostat, A), were studied (C).