The present invention relates to the use of a HER2 protein or a nucleic acid coding therefor as a target for the modulation of the mitogen-activated protein (MAP) kinase pathway. Further, the use of a PYK2 protein and a nucleic acid coding therefor as a target for the modulation of the MAP kinase pathway is described. By inhibiting HER3 kinase activity, the phosphorylation of PYK2 and thus the stimulation of the MAP kinase pathway is inhibited. The present invention is preferably suitable for applications, particularly diagnostic or medical applications, wherein an inhibition of the MAP kinase pathway is desired. Thus, the invention relates to novel methods for diagnosing, treating or preventing MAP kinase associated disorders such as tumors.