H3K27me3 - tri-methylation of lysine 9 of H3. Repressive mark. (IHEC reference epigenome). This repressive mark is a result of histone methyltransferase activity of PRC2 (Polycomb Repressive Complex 2). When found in promotors it represses gene expression and in combination with H3K4me3 forms a so-called bivalent chromatin state. Genes marked by bi-valent states are poised for transcription; often they are expressed at a very low level and become active when loose H3K27me3 during cellular differentiation. H3K27me3 appears as a broad mark forming enriched clusters of 10K and more. H3K4me3 can form extreme domains of ~100Kb (e.g. in the Hox genes regions). H3K27me3 is anticorrelated with gene expression.
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