H2AX phosphorylation. HL60 cells (A) and MOLT 4 cells (B) were treated for 8 hours with vorinostat (Vor) 2.5 μM or IDA 20 nM or the combination. H2AX phosphorylation was analyzed using immunohistochemistry in paraffin-embedded cell suspensions. Numbers at the top of each image represent the percentage of cells staining for γH2AX. Images were acquired using a Zeiss Axiovert S100 inverted microscope equipped with a 40×/0.65 objective lens (Zeiss, Thornwood, NY) and a Hamamatsu cooled CCD camera (Hamamatsu, Hamamatsu City, Japan) and were digitally stored with WebSlide Browser (Bacus Laboratory, Lombard, IL). Images were obtained using 100× magnification.