Mitogen-activated protein kinase (MAPKs) are a widely conserved family of serine/threonine kinase involved in many cellular programs such as cell proliferation, differentiation, motility, and death. MAPK3 (ERK1) signaling pathway can be activated in response to a diverse range of extracellular stimuli including mitogens, growth factors, and cytokines and is on important target in the diagnosis and treatment of cancer. Upon stimulation, a sequential three-part protein kinase cascade is initiated, consisting of a MAP kinase kinase kinase (MAPKKK or MAP3K), a MAP kinase kinase (MAPKK or MAP2K), and a MAP kinase (MAPK). Multiple p44/42 MAP3Ks have been identified, including members of the Raf family as well as Mos and Tpl2/Cot. MEK1 and MEK2 are the primary MAPKKs in this pathway.Synonyms: ERK-1, ERT2, Extracellular signal-regulated kinase 1, Insulin-stimulated MAP2 kinase, MAP kinase 3, MAP kinase isoform p44, MAPK 3, Microtubule-associated protein 2 kinase, Mitogen-activated protein kinase 1, Mitogen-activated protein kinase 3, PRKM3, p44-ERK1, p44-MAPK