Mitogen-activated protein kinases (MAPKs) are a widely conserved family of serine/threonine protein kinases involved in many cellular programs such as cell proliferation, cell differentiation, cell movement and cell death. MAPK signaling cascades are organized hierarchically into three-tiered modules. MAPKs are phosphorylated and activated by MAPKkinases (MAPKKs), which in turn are phosphorylated and activated by MAPKK-kinases (MAPKKKs). The MAPKKKs are in turn activated by interaction with a family of small GTPases and/or other protein kinases connecting the MAPK module to the cell surface receptor or external stimuli. The Phospho-Erk1/2 Pathway Sampler Kit provides a fast and economical means to evaluate the activation status of the entire Erk1/2 pathway. The kit includes enough primary and secondary antibodies to perform four Western mini-blot experiments, as well as a specific inhibitor of MEK1/2 (U0126).