{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n Histone H4 plays a central role in establishing a chromatin “code” via specific post‐translational modifications that govern higher‐order chromatin structure, gene expression, DNA repair, and centromere function. For example, acetylation at lysine 16 on histone H4 (H4K16Ac) has been shown to disrupt compact chromatin fiber formation and to antagonize ATP‐dependent remodeling, thereby increasing chromatin accessibility for transcription and other DNA‐mediated processes."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "3"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n In parallel, methylation events on histone H4, such as dimethylation at lysine 20 (H4K20me2), serve as docking sites for pivotal DNA repair factors like 53BP1; these marks help to direct repair pathway choice at double‐strand breaks."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "4", "end_ref": "6"}]}, {"type": "t", "text": " Moreover, removal of repressive H4K20 monomethylation by specific demethylases (e.g. PHF8) underpins proper cell cycle progression."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n The acetylation pattern of H4 also provides a combinatorial binding surface for bromodomain‐containing effectors. Proteins such as Brd4 and Brdt recognize di‐ or tetra–acetylated H4 tails, thereby linking histone modification patterns to recruitment of complexes that stimulate transcriptional elongation and mediate chromatin remodeling in critical processes like circadian gene regulation and spermatogenesis."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "8"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n In addition, the assembly of centromeric chromatin requires the formation of a uniquely rigid CENP-A/H4 heterotetramer; this specialized nucleosomal structure is fundamental for centromere identity and accurate chromosome segregation."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "10"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n Collectively, these studies illustrate that the diverse modifications on histone H4—collectively conceptualized here as the “H4 code” (or “H4C4”)—act as an integrated epigenetic platform. The balance between acetylation and methylation on H4 fine-tunes nucleosomal behavior, regulating access to the genome and thereby influencing transcription, DNA repair, cell cycle progression, and maintenance of genomic stability. Disruption of this balance has wide-ranging biological consequences, including impacts on aging and tumorigenesis."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "12"}]}, {"type": "t", "text": "\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Michael Shogren-Knaak, Haruhiko Ishii, Jian-Min Sun, et al. 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