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Background:Histones are basic nuclear proteins that assemble into nucleosomes, the fundamental repeating units of eukaryotic chromatin. Multiple post‑translational modifications can co‑exist on the same histone H3 N‑terminal tail to generate combinatorial epigenetic codes. H3T6pK9me3 represents the dual modification of phosphorylated Threonine 6 and trimethyl‑Lysine 9 on histone H3. H3K9me3, catalyzed by Suv39h‑family methyltransferases, is a hallmark mark of constitutive heterochromatin responsible for transcriptional silencing. H3T6 phosphorylation is mainly mediated by PKC‑family kinases and accumulates during mitosis. This combinatorial mark is enriched on heterochromatin regions in mitotic cells. In interphase, H3K9me3 recruits HP1 family proteins to establish compact, transcriptionally‑silenced heterochromatin. Upon mitotic entry, T6 phosphorylation occurs on H3K9me3‑containing histones within heterochromatin domains. H3T6 phosphorylation interferes with the binding of modification‑specific reader proteins, participates in mitotic heterochromatin compaction, and coordinates chromosome structural rearrangement. The underlying repressive H3K9me3 mark remains intact during mitosis, safeguarding epigenetic memory for heterochromatin re‑establishment after cell division. Impaired deposition or turnover of H3T6pK9me3 disrupts heterochromatin dynamics, mitotic chromosome behavior and epigenetic inheritance, and correlates with genomic instability.
