HDAC activity in nuclear extracts from vector control, early and late passage ERK1-GFP cells. Increased relative fluorescence units (RFU) is an index for increased HDAC activity, which was measured using the Fluor de Lys – Green kit (Enzo Life Sciences, Plymouth Meeting, PA) according to manufacturer’s directions. HDAC activity was increased in late passage ERK1-GFP cells, relative to early passage ERK1-GFP cells and vector control. *Significantly different from vector control. Values represent the mean ± se (n = 3). Similar results were observed in three independent experiments.