The histone H3 lysine 27 (H3K27) demethylase JMJD3 plays important roles in the transcriptional regulation of cell differentiation, development, the inflammatory response, and cancer. GSK-J1 is a potent inhibitor of the H3K27 histone demethylases JMJD3 and UTX (ICs = 18 and 56 µM, respectively as measured by mass spectrometry; IC = 60 nM in JMJD3 antibody-based assays). It is inactive against a panel of additional JMJ family demethylases, including several variants of JMJD2 and JMJD1 and, at concentrations up to 30 μM, has no effect on more than 100 different kinases or other unrelated proteins, including other chromatin-modifying enzymes such as histone deacetylases. Since the highly polar carboxylate group of GSK-J1 restricts its cellular permeability, a prodrug ethyl ester, GSK-J4 has also been developed. See the Structural Genomics Consortium (SGC) website for more information.