Catalytic domain of the dual-specificity Protein Kinase, Mitogen-Activated Protein (MAP)/Extracellular signal-Regulated Kinase (ERK) KinasePKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine (ST) or tyrosine residues on protein substrates. MEK1 and MEK2 are MAPK kinases (MAPKKs or MKKs), and are dual-specificity PKs that phosphorylate and activate the downstream targets, ERK1 and ERK2, on specific threonine and tyrosine residues. The ERK cascade starts with extracellular signals including growth factors, hormones, and neurotransmitters, which act through receptors and ion channels to initiate intracellular signaling that leads to the activation at the MAPKKK (Raf-1 or MOS) level, which leads to the transmission of signals to MEK1/2, and finally to ERK1/2. The ERK cascade plays an important role in cell proliferation, differentiation, oncogenic transformation, and cell cycle control, as well as in apoptosis and cell survival under certain conditions. This cascade has also been implicated in synaptic plasticity, migration, morphological determination, and stress response immunological reactions. Gain-of-function mutations in genes encoding ERK cascade proteins, including MEK1/2, cause cardiofaciocutaneous (CFC) syndrome, a condition leading to multiple congenital anomalies and mental retardation in patients. The MEK subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.