ERK1 is a protein serine/threonine kinase that is a member of the extracellular signal-regulated kinases (ERKs), which are activated in response to numerous growth factors and cytokines. Activation of ERK1 requires both tyrosine and threonine phosphorylation that is mediated by MEK. ERK1 is ubiquitously distributed in tissues with the highest expression in heart, brain and spinal cord. In vitro studies indicate that ERK1 phosphorylate both nuclear and cytoplasmic proteins. Activated ERK1 translocates into the nucleus where it phosphorylates various transcription factors (e.g., Elk-1, c-Myc, c-Jun, c-Fos, and C/EBP β). The consensus primary sequence for substrate phosphorylation by ERK1 has been identified as -Pro-Leu-Ser/Thr-Pro-. ERK1