The MAP kinases mediate a wide range of cellular functions via a variety of signal transduction pathways in eukaryotic cells. The most studied mammalian MAP kinase pathway is the Ras-Raf-MEK-ERK pathway. The binding of GTP to Ras protein initiates a phosphorylation cascade through Raf-1 and MEK1/2 (MAPK kinase), which results in stimulation of the MAP kinases, ERK1/2. Upon stimulation, ERKs are known to phosphorylate a variety of cytosolic substrates and are also translocated into the nucleus where they initiate the transcription of immediate early genes. The Ras-Raf-MEK-ERK pathway is stimulated by various growth factors and extracellular stimuli and plays important roles in cell survival, differentiation and proliferation. The observation that MEK1/2 inhibitors potentiate the antitumor activity of various cytotoxic agents, including ara-C, cisplatin and paclitaxel, suggests a possible role for MEK1/2 inhibitors in the treatment of human malignancies.Synonyms: Dual specificity mitogen-activated protein kinase kinase 1, ERK activator kinase 1, MAP kinase kinase 1, MAPK/ERK kinase 1, MAPKK 1, MEK1, PRKMK1