MEK1 and MEK2 are homologous dual specificity kinases that share ERK as their only known catalytic substrate,15 making MEK an appealing target for cancer drug development. Although many kinase inhibitors target the ATP-binding region of an enzyme, MEK contains a hydrophobic allosteric pocket adjacent to the ATP-binding site that is not shared with other kinases.36 This offers the opportunity to design highly selective inhibitors of MEK that do not simply target the conserved ATP region of the kinase.