Fig. 2. Loss of expression of CDKIs does not result in escape from requirements for MAPK activation. MEFs derived from p21^Waf1/Cip1, p27^Kip1 or p16^Ink4-knockout mice were plated at a density of 2.5x10⁴/ml. The following day the cells were placed in medium containing 0.5% (v/v) FCS for 24-48 hours. The cells were then treated with U0126 (20 µM final concentration) or vehicle control (DMSO) 60 minutes and then stimulated with 10% (v/v) FCS for 16-20 hours followed by immunofluorescence.