Phosphorylation of specific tyrosine and threonine residues in the activation loop of this enzyme by EC 220.127.116.11, mitogen-activated protein kinase kinase (MAPKK) is necessary for enzyme activation. Once activated, the enzyme phosphorylates target substrates on serine or threonine residues followed by a proline . A distinguishing feature of all MAPKs is the conserved sequence Thr-Xaa-Tyr (TXY). 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 tumour necrosis factor (TNF) family and environmental stresses such as osmotic shock, ionizing radiation and ischaemic injury.