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Background:Histones are basic nuclear proteins forming the nucleosome, the fundamental repeating unit of eukaryotic chromatin. Lysine‑20 on histone H4 (H4K20) undergoes mono‑, di‑ and trimethylation, among which H4K20me2 (dimethyl‑Lys20) is the most abundant H4K20 methylation mark in mammalian cells. H4K20me2 is sequentially generated: SET8/KMT5A first catalyzes H4K20me1, followed by Suv4‑20h1/h2 (KMT5B/C)‑mediated conversion to dimethylated state. H4K20me2 plays central roles in genome stability maintenance. It serves as the primary docking mark for 53BP1 via its tandem Tudor domain, driving non‑homologous end‑joining (NHEJ)‑dependent DNA double‑strand break repair pathway. This modification is also recognized by ORC1 BAH domain, participating in DNA replication origin licensing and cell‑cycle progression. H4K20me2 broadly distributes across chromatin, correlates with repressive chromatin environment and participates in heterochromatin homeostasis. Dysregulation of H4K20me2 is linked to DNA‑repair defects, replication stress, developmental syndromes and tumor progression.
