Cited from the paper "Targeting Polycomb systems to regulate gene expression: modifications to a complex story"
Polycomb group proteins usually belong to one of two multi-subunit protein complexes: Polycomb repressive complex 1 (PRC1), which adds a ubiquityl moiety to histone H2A at Lys119 (H2AK119ub1); and PRC2, which catalyses the addition of one to three methyl groups to histone H3 at Lys27, leading to H3K27me1, H3K27me2 and H3K27me3.
PRC1 and PRC2 usually co-occupy target sites in the genome, at which their combined activities create Polycomb chromatin domains consisting of the Polycomb group proteins themselves, H2AK119ub1, and H3K27me3.
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