Figure 1: Histone Modifications at different classes of cis-regulatory elements. Active promoters tend to be marked by H3K4me3 in the absence of H3K27me3. Inactive promoters tend to be marked by H3K27me3, with the additional presence of H3K4me3 termed the “bivalent” mark, and are thought to represent promoters “poised” for activation after removal of the repressive H3K27me3 mark. Enhancers tend to be marked differently, with active enhancers marked by H3K4me1, H3K27Ac, and Cyclic AMP-responsive element binding protein (CBP)/p300 binding. Poised enhancers tend to be marked by H3K4me1 alone, CBP/p300, and in human ESCs H3K27me3. The combination of histone marks and their implications for gene expression remain an open question. Insulators are defined by the binding of CCCTCbinding factor (CTCF). Transcriptional start sites (TSS) are indicated with arrows.