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Nucleosomal asymmetry shapes histone mark binding and promotes poising at bivalent domains
Bivalent promoters are thought to poise developmental genes for expression. Bryan et al. show that asymmetric bivalent nucleosomes recruit repressive H3K27me3 but not activating H3K4me3 readers, thereby promoting poising. Bivalency further attracts specific factors, including the acetylase KAT6B, which is required for proper activation of bivalent genes during differentiation.