{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n Histone H4 modifications are central to regulating chromatin structure, gene expression, and genome stability. For example, acetylation at lysine 16 on histone H4 (H4K16Ac) impedes the formation of compact 30‐nm-like fibers and limits inter‐nucleosomal interactions, thereby facilitating a more open chromatin state that is accessible to remodeling enzymes and factors critical for transcription and DNA repair."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Dynamic regulation of H4K16Ac is achieved through the opposing activities of histone acetyltransferases and deacetylases. In particular, SIRT2 selectively deacetylates H4K16Ac during the G2/M transition, a modification essential for the chromatin condensation needed during mitosis."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": " Moreover, signaling pathways such as those initiated by physiologic levels of vitamin D can induce H4 acetylation at gene promoters (for example, of the MKP-1 gene), thereby dampening inflammatory responses in monocytes/macrophages."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Besides acetylation, methylation of histone H4—most notably on lysine 20 (H4K20)—plays an essential role in genome maintenance. The dimethylated form (H4K20me2) is specifically recognized by repair proteins such as 53BP1 via tandem Tudor domains, a recognition event that is crucial for the recruitment of repair factors to double-strand breaks."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": " In addition, demethylation of H4K20me1 by enzymes like PHF8 alleviates repressive chromatin marks to promote appropriate cell cycle progression."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "6"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Chromatin “readers” further interpret combinatorial H4 modifications. Bromodomain proteins, such as Brd4, specifically bind to di- and tetraacetylated H4 tails, which is critical for their role in recruiting transcriptional elongation complexes and propagating active chromatin states."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": " Moreover, in the context of centromere identity, H4 forms a tight, rigid complex with the centromere-specific histone variant CENP-A—structurally optimizing centromeric nucleosomes to ensure accurate kinetochore assembly and faithful chromosome segregation."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "9", "end_ref": "11"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n In the DNA damage response, coordinated H4 modifications are also key. For instance, the acetyltransferase TIP60 is activated upon binding to specific histone marks and acetylates histone H4, thereby facilitating the recruitment and assembly of repair complexes at double-strand breaks."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "12"}]}, {"type": "t", "text": " Similarly, the MOF-containing MSL complex acetylates H4K16, linking metabolic signals to chromatin remodeling necessary for normal developmental processes and repair."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "13"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Collectively, these studies demonstrate that post‐translational modifications of histone H4 not only govern higher order chromatin structure but also orchestrate critical processes such as transcriptional control, DNA repair, and centromere function. The diverse and dynamic “histone code” written on H4 thereby functions as a master regulator of cellular homeostasis and genome integrity."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "14", "end_ref": "26"}]}, {"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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