Scheme of the three-tiered modules of the mitogen-activated protein kinases (MAPK) in eukaryotic cells. MAPK signaling cascades are organized hierarchically into three-tiered modules (a) (b) and (c). MAPKs are phosphorylated and activated by MAPK-kinases (MAPKKs), which in turn are phosphorylated and activated by MAPKK-kinases (MAPKKKs). MAPKKKs are in turn activated by interaction with the family of small GTPases and/or other protein kinases, connecting the MAPK module to cell surface receptors or external stimuli. Phosphoproteomics strategies allow identification of several phosphorylated proteins at a given time and space, thus it is possible to study the dynamic processes of signaling networks via phosphoproteomic MS-based tools (GPCR: G-protein coupled receptor).