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-J4 is a cell-permeable prodrug which is modified by intracellular esterases to give GSK-J1, an inhibitor of JMJD3. GSK-J5 is a pyridine regio-isomer of GSK-J4. Like GSK-J4, this isomer is cell-permeable and hydrolyzed to a free base. However, the free base is a weak inhibitor of JMJD3 (IC50 > 100 μM), making it an ideal inactive control molecule for elucidating the functional role of JMJD3 inhibition.