{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n Histone H4 is a core component of the nucleosome whose tail serves as a versatile platform for multiple post‐translational modifications that regulate chromatin architecture and function. In its various modified states, H4 modulates higher order chromatin compaction, gene transcription, DNA repair, centromere identity, and cell cycle progression. For example, acetylation of H4 at lysine 16 (H4K16Ac) directly interferes with the formation of compact chromatin fibers and reduces the recruitment or activity of remodeling enzymes, thereby increasing chromatin accessibility and impacting transcription and DNA repair processes"}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": ", and."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": " In addition, specific methylation marks on H4—such as dimethylation at lysine 20 (H4K20me2)—provide recognition sites for DNA damage response factors like 53BP1, which is recruited to double-strand breaks to promote repair fidelity"}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": "and."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "5"}]}, {"type": "t", "text": " Other modifications, including arginine methylation and deacetylation mediated by sirtuins (e.g. SIRT1 and SIRT2), further fine‐tune the balance between chromatin decondensation and condensation, influencing processes ranging from centromeric nucleosome assembly to cell cycle‐dependent chromatin restructuring"}, {"type": "fg", "children": [{"type": "fg_f", "ref": "6"}]}, {"type": "t", "text": ", and."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "8"}]}, {"type": "t", "text": " Moreover, the combinatorial “readout” of H4 acetylation by bromodomain‐containing proteins such as BRD4 plays a critical role in sustaining transcriptional programs and establishing epigenetic memory, as evidenced during processes like neuronal plasticity and stress responses"}, {"type": "fg", "children": [{"type": "fg_f", "ref": "9"}]}, {"type": "t", "text": "and."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "10"}]}, {"type": "t", "text": " In stem cell biology and aging, alterations in H4 modification patterns have been linked to changes in metabolic state and polarity, thereby influencing stem cell renewal and organismal homeostasis"}, {"type": "fg", "children": [{"type": "fg_f", "ref": "11"}]}, {"type": "t", "text": "and."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "12"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Collectively, the H4 “code”—comprising acetylation, methylation, and other modifications—acts as an integrative signal that modulates chromatin dynamics and orchestrates diverse cellular responses. The ability of these modifications to serve as recruitment platforms for effector proteins, to alter nucleosome interactions, and to influence higher order chromatin organization underscores their central role in maintaining genomic integrity and in regulating gene expression during development, stress responses, and disease.\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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