The methylation of histones, proteins that make up the bulk of chromatin in eukaryotes, plays a critical role in the epigenetic regulation of gene expression. Although the enzymes that put this mark onto chromatin are well known, the class of enzymes that take it off again, the Jumonji C (JmjC) family of demethylases, are a more recent discovery (see the Perspective by Rivenbark and Strahl). Although several JmjC lysine demethylases are known, no JmjC protein has been identified that can remove methyl groups from arginine residues in histones. Chang et al . now report the discovery of an enzyme, JMJD6, that demethylates histone H3 at arginine 2 and histone H4 at arginine 3, marks that are likely a critical part of the "histone code" that modulates chromatin function. Di- and trimethylation of histone H3 on lysine 27 (H3K27me2-3) are exclusively repressing signals and are implicated in X chromosome inactivation, imprinting, stem cell maintenance, circadian rhythms, and cancer. The enzyme that places the marks has been known, and now Lee et al . have identified the human enzyme, UTX (ubiquitously transcribed mouse X chromosome gene), a JmjC-domain-containing protein (similar to other demethylase enzymes), responsible for removing the H3K27me2-3 marks and promoting the activation of gene expression. B. Chang, Y. Chen, Y. Zhao, R. K. Bruick, JMJD6 is a histone arginine demethylase. Science 318 , 444-447 (2007). [Abstract] [Full Text] M. G. Lee, R. Villa, P. Trojer, J. Norman, K.-P. Yan, D. Reinberg, L. Di Croce, R. Shiekhattar, Demethylation of H3K27 regulates polycomb recruitment and H2A ubiquitination. Science 318 , 447-450 (2007). [Abstract] [Full Text] A. G. Rivenbark, B. D. Strahl, Unlocking cell fate. Science 318 , 403-404 (2007). [Summary] [Full Text]