The compaction of chromatin into a conformation that is refractory to transcription but that can be converted to euchromatin and allow transcription in specific contexts. These can be temporal (e.g., developmental states or specific cell-cycle stages), spatial (e.g., nuclear localization changes from the center to the periphery or vice versa due to exogenous factors/signals), or parental/heritable (e.g., monoallelic gene expression). In metazoa, this involves the methylation of histone H3K27. [PMID:17936700]
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C5: Ontology GO: Gene Ontology GO:BP: GO Biological Process