MEK can be divided into MEK1 and MEK2 with molecular weight of 44kD and 45kD. After Raf is activated, its C-terminal catalytic region can bind to MEK and phosphorylate two Ser in the ⅷ subregion of its catalytic region, thus activating MEK. MEK is one of the rare dual-specific kinases that activate ERK by phosphorylation at regulatory sites Tyr and Thr. However, it is not clear how MEK has both Tyr and Thr bispecific phosphorylation activity, but MEK has important physiological significance for Tyr/Thr bispecific phosphorylation of ERK, because ERK signaling pathway plays a pivotal role in cell signal transduction network, and any wrong activation will have a profound impact on cell life activities. This bi-specific recognition and activation mechanism greatly improves the accuracy of signal transduction and prevents the wrong activation of ERK.