Cre expression in MEFs induces accumulation in G2/M and aneuploidy. Cell cycle analysis of MEFs infected with viruses encoding Cre-ERT, Cre(R173K)-ERT, Cre (lane 3), and R26cre-ERT MEFs, either untreated or treated with 1 μM OHT, for 3 days. Cells were stained with propidium iodide for DNA content and analyzed by FACS. The numbers indicate the percentage of nonpolyploid cells in G0/G1, S, and G2/M phases of the cell cycle. In addition, a region marker is set to determine the percentage of polyploid cells in the total population.