GSK-J1 is a potent and selective inhibitor of jumonji H3K27 histone demethylases JMJD3 and UTX (IC₅₀ = 60 nM, human JMJD3). This is the first known inhibitor selective for the H3K27me3-specific JMJ subfamily which binds to the active catalytic site of the enzyme. The COOH group confers cell impermeability and as such is useful as a standard in in vitro assays. A cell permeable ethyl ester analog is also available (Cat. Nos. 2259 & 2762).