Panel A: Expression of ERK1-GFP chimera was confirmed by Western blot analysis (bottom). Phosphorylation of both ERK1-GFP and endogenous ERKs is increase by growth factor stimulation (FBS), relative to quiescent controls (top). Panel B: Epifluorescent microscopy confirmed ERK1-GFP nuclear translocation following stimulation with FBS, relative to unstimulated ERK1-GFP cells. Blocking nuclear export with leptomycin B (30 min treatment) resulted in retention of ERK1-GFP in the nucleus as expected. Similar results were observed in two independent experiments. Methods for retroviral expression of ERK1-GFP can be found in .