Cells were resuspended in PBS and fixed with ethanol overnight at 220uC. Cells were then resuspended in PBS and treated with 100 mg/ml of ribonuclease A (bovine pancreas; Sigma), 0.1% Triton-X100, and 40 mg/ml propidium iodide (PI) for 30 min at 37uC. Cell cycles were detected with FACSCalibur, and analyzed by the ModFit LT program (Verity Software House, Topsham, ME, USA).