Recent findings have identified protein phosphatase methylesterase-1 (PME-1) as a protector of sustained ERK pathway activity in malignant gliomas. PME-1 is a protein methylesterase that functions in the regulation of protein phosphatase 2A (PP2A) by reversible methylation. Biochemical elucidation of PME-1 would thus greatly benefit from the development of potent and selective chemical inhibitors. The probe compound ML136 (CID-44607965), containing a sulfonyl acrylonitrile core, represents the first potent, selective inhibitor of PME-1. Moreover, the probe does not appear to exhibit cytotoxicity. Thus, ML136 should serve as a useful tool for in vitro and in situ research assays in which it is desirable to specifically block PME-1 activity.