Reinitiation of the cell cycle progression in quiescent cells. Blocking the ER function inhibits the insulin-induced reinitiation of the cell cycle progression. MCF-7 cells were placed in serum- and phenol red-free medium containing or not 10 nM ICI 182780 for 48 h. Subsequently the cells were stimulated by addition of insulin (Ins; 1 μM) or E2 (1 μM). The cells were harvested for analysis of their DNA contents by flow cytometry at t = 0, 12, 16, 20, 24 and 28 h as shown. The data (means of 2 to 4 experiments) are plotted as percent of cells in the G1 phase and S phase. S.e.m. are shown (unless smaller than the size of the symbol).