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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.

A chromatin complex that catalyses H3K27 methylation, with locus-specific recruitment and developmental functions.

Its catalytic activity deposits H3K27 methylation, including the trimethylated state associated with many repressed developmental genes. Recruitment and recognition of modified chromatin can stabilize a regulatory state while retaining the possibility of later reversal. ^[ref]