JIB-04 (E-isomer) is a potent, cell-permeable inhibitor of Jumonji histone demethylases in vitro and in vivo. (In vitro IC₅₀ values are 230, 340, 435, 445, 855 and 1100 nM for JARID1A, JMJD2E, JMJD2B, JMJD2A, JMJD3 and JMJD2C respectively). JIB-04 is not a competitive inhibitor of α-ketoglutarate ; it selectively inhibits viability of several human cancer cell lines with little toxicity towards normal cells, and also diminishes tumor growth in xenograft mouse models.