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H3K27 methylation is addition of one or more methyl groups to the side chain of lysine 27 in histone H3. Mono-, di- and trimethylated states are different chemical states; a measurement of one is not automatically a measurement of the others. H3K27 trimethylation is particularly associated with Polycomb-mediated gene silencing. ^[ref]

A mechanistic test distinguishes correlation from catalysis. A purified complex can be supplied with histones and a methyl donor to test transfer, while changing a catalytic region tests whether that activity depends on an intact enzyme. In a cell, the distribution of the mark also depends on recruitment, removal and replacement of histones, not only on total enzyme abundance.

H3K27 trimethylation adds three methyl groups to lysine 27 of histone H3, conventionally H3K27me3. It is associated with Polycomb-linked repression of developmental genes in many eukaryotes. Deposition and removal help regulate the persistence of a chromatin state. A broad marked domain describes a molecular state; demonstrating that the mark is required for repression at a particular gene needs a perturbation that separates it from associated factors. ^[ref]

An enzyme that removes particular histone methyl marks.

Different demethylases act on different residues and methylation states, so removal of H3K4 methylation and removal of H3K27 trimethylation are distinct activities. Regulated removal makes a chromatin mark reversible without requiring replacement of the underlying DNA sequence. ^[ref]

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]