Histone methylation adds methyl groups to selected residues of histones. The position and number of methyl groups affect which binding partners recognize the mark. H3K4 methylation, H3K9 methylation and H3K27 trimethylation therefore cannot be treated as one interchangeable signal. Their distributions can be associated with different transcriptional states, and direct causation requires more than correlation between a mark and gene expression. [ref]
Regulation of heterochromatic silencing and histone H3 lysine-9 methylation by RNAi by Thomas A. Volpe and others. 2002, RNA interference and H3K9-linked silencing
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