MEK is a mitogen-activated protein kinase protein kinase that phosphorylates and activates ERK in the RAS-MAPK signalling pathway. Its dual-specificity chemistry can modify both a threonine and a tyrosine in the activation region of its substrate. MEK therefore connects one protein kinase tier to the next. When testing developmental requirements with an inhibitor, unchanged fate is informative only if inhibition of the downstream pathway has been established. [ref]
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