MEK1 is a dual specificity protein kinase, which is known as a mitogen-activated protein (MAP) kinase kinase 1 (MAPKK 1). MAP kinases are recognized as extracellular signal-regulated kinases (ERKs) that act as an integration position for multiple biochemical signals. MEK1 is located upstream of MAP kinases and catalyzes phosphorylation of threonine and tyrosine residues on MAP kinases ERK1 and ERK2. MEK1 is activated by phosphorylation of serine 218 and 222 residues by Raf1. As a key player of the MAP kinase signal transduction pathway, MEK1 is involved in many cellular processes such as proliferation, differentiation, transcription regulation and development.