Fig. 1. Rb null but not wild-type MEFs enter the cell cycle in the presence of the Mek inhibitor U0126. Primary MEFs were plated at a 2500 cells/13 mm coverslip. The following day cells were placed in medium containing 0.5% (v/v) fetal calf serum (FCS) for 16-20 hours to render the cells quiescent. For cell cycle re-entry MEFs were stimulated with medium containing 10% FCS (v/v) for 16-20 hours. Cells were treated with U0126 at a final concentration of 20 µM, or vehicle control (DMSO) or LY294002 at a final concentration of 20 µM, or vehicle control (ethanol; ETOH) for 60 minutes prior to the addition of serum. (A) Immunofluorescent images of cells treated with U0126 stained with antibody against BrdU and Texas-Red-X-phalloidin. (B) Histogram of percentage BrdU-positive cells that have been treated with U0126, LY294002 or a combination.