Alterations in topo IIα and IIβ levels in logarithmically growing HL-60 cells as a function of DNA content, ie, the cell cycle position. (A) DNA histogram showing the cell cycle distribution of logarithmically growing HL-60 cells. (B and C) Two-dimensional flow cytometric analyses of DNA content and topo IIα and IIβ signals, respectively. Isotype-matched negative controls are depicted as contour maps. (D and E) Histograms of topo IIα and IIβ contents, respectively. Isotype-matched control fluorescence curves are the most proximal to the Y-axis.