A potent cell-permeable inhibitor of ERK1 and ERK2 (Ki = 310 and 140 nM, respectively); much less effective against p38α (IC50 = 10 µM) and does not inhibit a range of other serine/threonine or tyrosine kinases; blocks ERK-mediated signaling from TGF-β to AP-1 (IC50 = 3.1 μM).