-!- This enzyme phosphorylates and activates its downstream protein kinase, EC 2.7.12.2, but requires MAPKKKK for activation. -!- Some members of this family can be activated by p21-activated kinases (PAK/STE20) or Ras. -!- While c-Raf and c-Mos activate the classical MAPK/ERK pathway, MEKK1 and MEKK2 preferentially activate the c-Jun N-terminal protein kinase(JNK)/stress-activated protein kinase (SAPK) pathway. -!- Mitogen-activated protein kinase (MAPK) signal transduction pathways are among the most widespread mechanisms of cellular regulation. -!- Mammalian MAPK pathways can be recruited by a wide variety of stimuli including hormones (e.g. insulin and growth hormone), mitogens (e.g. epidermal growth factor and platelet-derived growth factor), vasoactive peptides (e.g. angiotensin-II and endothelin), inflammatory cytokines of the tumor necrosis factor (TNF) family and environmental stresses such as osmotic shock, ionizing radiation and ischemeic injury. -!- Formerly EC 2.7.1.37.