U0126 is a chemically synthesized organic compound that was initially recognized as a cellular AP-1 antagonist, and found to be a very selective and highly potent inhibitor of Mitogen-Activated Protein Kinase (MAPK) cascade by inhibiting its immediate upstream activators, Mitogen Activated Protein Kinase Kinase 1and 2 (also known as MEK1 and MEK2, IC₅₀: 70 and 60 nM respectively). U0126 inhibits both active and inactive MEK1,2, unlike PD098059 which only inhibits activation of inactive MEK (1,2). Blockade of MEK activation would prevent downstream phosphorylation of a number of factors including p62^TCF (Elk-1), an upstream inducer of c-Fos and c-Jun, components of the AP-1 complex.^1-4

Inhibition of MEK/ERK pathway by U0126 also prevents all effects of oncogenic H-Ras and K-Ras, inhibits part of the effects triggered by growth factors and blocks the production of inflammatory cytokines and matrix metalloproteinases.^{5- 11}