Abstract: The present invention relates to a newly identified family of protein serine/threonine kinases which phosphorylate microtubule-associated protein 2 (MAP2). It is based, in part, on the cloning and characterization of novel MAP2 kinases designated extracellular signal-regulated kinase 1, 2, and 3 (ERK1, ERK2, ERK3) which are expressed in the central nervous system, and on the identification of another ERK family member, ERK4, with antisera. The present invention provides for recombinant nucleic acid molecules and proteins representing members of the MAP2 kinase family, and also for microorganisms, transgenic animals, and cell lines comprising recombinant MAP2 kinase molecules. In additional embodiments of the invention, the present invention provides for methods for assaying cellular factor activity, including, but not limited to, nerve growth factor activity, in which the activation of MAP2 kinase serves as an indicator of cellular factor activity.