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Unlike other members of the MAPK family, ERK5 contains a large C-terminal domain with transcriptional activation capability in addition to an N-terminal canonical kinase domain. Genetic deletion of ERK5 is embryonic lethal, and tissue-restricted deletions have profound effects on erythroid development, cardiac function, and neurogenesis. In addition, depletion of ERK5 is antiinflammatory and antitumorigenic. Small molecule inhibition of ERK5 has been shown to have promising activity in cell and animal models of inflammation and oncology. Here we report the synthesis and biological characterization of potent, selective ERK5 inhibitors. In contrast to both genetic depletion/deletion of ERK5 and inhibition with previously reported compounds, inhibition of the kinase with the most selective of the new inhibitors had no antiinflammatory or antiproliferative activity. The source of efficacy in previously reported ERK5 …