ERK1 ablation in mouse embryo fibroblasts results in enhancement of ERK2 activity and facilitates cell proliferation. (a) Wild-type (+) and ERK1-deficient (-) mouse embryonic fibroblasts (MEFs) were serum starved for 24 h and then stimulated with 20% serum for the indicated times. Western blotting was performed with both anti-ERK and anti-phospho-ERK antibodies. (b) Bands from (a) were quantified and the fold increase in phospho-ERK2 levels over total ERK2 levels calculated. (c) RNA from cells stimulated as in (a) was subjected to an RNase protection assay and probed for either c-fos or zif-268. A histone H4 probe was used as internal standard for normalization. (d) Wild-type (+) and ERK1-deficient (-) MEFs were seeded in triplicate in the presence of either 10% or 2.5% serum and cells were counted after the indicated times. Data are the mean ± standard error of the mean (SEM) of three independent experiments.