Extracellular signal-regulated kinases （ERKs） or classical MAP kinases are widely expressed protein kinaseintracellular signalingmolecules which are involved in functions including the regulation of meiosis， mitosis， and postmitotic functions in differentiated cells. Many different stimuli， including growth factors， cytokines， virusinfection， ligands for heterotrimeric G protein-coupled receptors， transforming agents， and carcinogens， activate the ERK pathway.
The term， "extracellular signal-regulated kinases"， is sometimes used as a synonym for mitogen-activated protein kinase（MAPK）， but has more recently been adopted for a specific subset of the mammalianMAPK family. In the MAPK/ERK pathway， Rasactivates c-Raf， followed by MEKand then MAPK1/2 （below）. Ras is typically activated by growth hormones through receptor tyrosine kinasesand GRB2/SOS， but may also receive other signals. ERKs are known to activate many transcription factors and some downstream protein kinases. Disruption of the ERK pathway is common in cancers， especially Ras， c-Raf and receptors such as HER2.Mitogen-activated protein kinase 3 （MAPK3） is also known as "extracellular signal-regulated kinase 1" （ERK1）. Transgenic gene knockoutmice lacking MAPK3 are viable and it is thought that MAPK1 can fulfill most MAPK3 functions in most cells. The main exception is in T cells. Mice lacking MAPK3 have reduced T cell development past the CD4+CD8+ stage.
