U0126 is a highly selective inhibitor of both MEK1 (IC50 of 72nM) and MEK2 (IC50 of 58 nM). U0126 was found to functionally antagonize AP-1 transcriptional activity via noncompetitive inhibition. U0126 inhibited anchorage-independent growth of Ki-ras-transformed rat fibroblasts by simultaneously blocking both extracellular signal-related kinase (ERK) and mammalian target of rapamycin (mTOR)-p70(S6K) pathways. The effects of U0126 on the growth of eight human breast cancer cell lines have shown that U0126 selectively represses anchorage-independent growth of MDA-MB231 and HBC4 cells, two lines with constitutively activated ERK. Upon treatment with U0126, cells deprived of anchorage entered apoptosis.