Core component of nucleosome. Nucleosomes wrap and compact DNA into chromatin, limiting DNA accessibility to the cellular machineries which require DNA as a template. Histones thereby play a central role in transcription regulation, DNA repair, DNA replication and chromosomal stability. DNA accessibility is regulated via a complex set of post-translational modifications of histones, also called histone code, and nucleosome remodeling. The nucleosome is a histone octamer containing two molecules each of H2A, H2B, H3 and H4 assembled in one H3-H4 heterotetramer and two H2A-H2B heterodimers. The octamer wraps approximately 147 bp of DNA. Mono-, di- and trimethylated to form H3K4me1/2/3. H3K4me activates gene expression by regulating transcription elongation and plays a role in telomere length maintenance. H3K4me enrichment correlates with transcription levels, and occurs in a 5' to 3' gradient with H3K4me3 enrichment at the 5'-end of genes, shifting to H3K4me2 and then H3K4me1 (By similarity). Acetylation of histone H3 leads to transcriptional activation. Belongs to the histone H3 family. To ensure consistency between histone entries, we follow the 'Brno' nomenclature for histone modifications, with positions referring to those used in the literature for the 'closest' model organism. Due to slight variations in histone sequences between organisms and to the presence of initiator methionine in UniProtKB/Swiss-Prot sequences, the actual positions of modified amino acids in the sequence generally differ. In this entry the following conventions are used: H3K4me1/2/3 = mono-, di- and trimethylated Lys-5; H3K9ac = acetylated Lys-10; H3K9me1 = monomethylated Lys-10; H3K14ac = acetylated Lys-15; H3K14me2 = dimethylated Lys-15; H3K18ac = acetylated Lys-19; H3K18me1 = monomethylated Lys-19; H3K23ac = acetylated Lys-24; H3K23me1 = monomethylated Lys-24; H3K56ac = acetylated Lys-56; H3K64ac = acetylated Lys-64.