Two closely related mitogen-activated protein (MAP) kinases, extracellular signal-regulated protein kinase (ERK)1 and ERK2, are known to be involved in the regulation of cell proliferation. These ubiquitous protein-serine/threonine kinases are well known as key players in signaling pathways downstream of growth-factor receptor-tyrosine kinases, cytokine receptors and G-protein-coupled receptors ; they often indirectly mediate the actions of members of the Ras family of small GTPases. Gain-of-function mutations have been implicated in more than 30% of human tumors, but chronic activation of Ras by mutated mitogen receptors occurs in even higher frequency than this . Most previously published work has inferred that ERK1 and ERK2 are commonly regulated and that they target the same substrates. In this issue of the Journal of Biology, however, Riccardo Brambilla and colleagues  provide compelling evidence that the two ERK proteins in fact counteract each other in the regulation of the cell-proliferation effects of Ras in mouse fibroblasts.