The in situ expression levels and subcellular localization of molecules involved in signal transduction using specific antibodies can be useful for prognosis and diagnosis of human diseases such as cancer. In addition, it has the potential to be helpful in monitoring biologic response to targeted therapies. The increasing availability of such antibodies makes these studies feasible. However, compared to typical immunohistochemical stains in which stabile molecules such as cytokeratins are targeted, additional -validation may be required for signal transduction immunohistochemistry.