Histone acetylation is generally considered a mark involved in activating gene expression by making chromatin structures less compact. In the April 1, 2010, issue of Genes & Development, Xhemalce and Kouzarides (pp. 647-652) demonstrate that the acetylation of histone H3 at Lys 4 (H3K4) plays a role in the formation of repressive heterochromatin in Schizosaccharomyces pombe. H3K4 acetylation mediates a switch of chromodomain proteins associated with methylated H3K9 during heterochromatin assembly.