Topo IIα level decreases dramatically and a new G2/M cell population expressing almost no topo IIα emerges during the ATRA-induced differentiation. Logarithmically growing HL-60 cells are treated with 1 μmol/L of ATRA, before the treatment (A, E, I, M, and Q), for 2 days (B, F, J, N, and R), for 4 days (C, G, K, O, and S), and for 6 days (D, H, L, P, and T). DNA histograms with insets showing the percentage of cells in each phase of the cell cycle (A through D). Two-dimensional flow cytometric analyses of DNA contents and topo IIα and IIβ signals (E through H and I through L, respectively). Histograms of topo IIα and IIβ contents (M through P and Q through T, respectively).