sentences:  However, this pathway is stimulated by the upstream binding of growth factors (GF), either from within the cell itself or from neighboring cells.Stimulation by growth factors activates any of a number of receptor tyrosine kinase (RTK) proteins.These receptor tyrosine kinases in turn phosphorylate and activate many other proteins, including Fos/Jun/Myc and phosphatidylinositol 3 kinase (PI-3-K).