The extracellular signal-regulated kinase 1 and 2 (Erk1 and Erk2) are closely related mitogen activated protein (MAP) kinases that are activated throμgh the extracellular stimulation by many growth factors, mitogens and differentiation promoting agents via a protein kinase cascade. Erk1/2 are activated approximately 1000 fold by phosphorylation of neighboring threonine and tyrosine residues by Mek1 and Mek2. Both sites must be phosphorylated for maximum activity. Erk1/2 kinases are ubiquitously distributed in the nervous system, as well as a broad range of cells and tissues. These two kinases represent proximal kinases in the classical MAP kinase cascade pathway which links growth and differentiation signals at the cell surface (throμgh tyrosine kinase) with transcription in the nucleus.