(2Z,3Z)-U0126 is a highly selective inhibitor of both MEK1 and MEK2, a type of MAPK/ERK kinase. It was found to functionally antagonize AP-1 transcriptional activity via noncompetitive inhibition of the dual specificity kinase MEK with IC50 of 72 nM for MEK1 and 58 nM for MEK2. U0126 inhibited anchorage-independent growth of Ki-ras-transformed rat fibroblasts by simultaneously blocking both extracellular signal-regulated kinase and mammalian target of rapamycin pathways.