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. Acetylation is generally linked to gene activation. Can be acetylated to form H3K9ac, H3K14ac, H3K18ac and H3K23ac. H3K9ac could compete with H3K9me and prevent gene silencing. H3K9ac is restricted to euchromatin (By similarity). Methylated to form mainly H3K4me, H3K9me, H3K18me, H3K23me, H3K27me and H3K36me. H3K4me1/2/3, H3K9me3, H3K27me3 and H3K36me1/2/3 are typical marks for euchromatin, whereas heterochromatic chromocenters are enriched in H3K9me1/2 and H3K27me1/2. H2BK143ub1 is probably prerequisite for H3K4me (By similarity). Can be phosphorylated to form H3S10ph, H3T11ph and H3S28ph. 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: H3K4me = methylated Lys-5; H3K9ac = acetylated Lys-10; H3K9me = methylated Lys-10; H3S10ph = phosphorylated Ser-11; H3T11ph = phosphorylated Thr-12; H3K14ac = acetylated Lys-15; H3K18ac = acetylated Lys-19; H3K18me = methylated Lys-19; H3K23ac = acetylated Lys-24; H3K23me = methylated Lys-24; H3K27me = methylated Lys-28; H3S28ph = phosphorylated Ser-29; H3K36me = methylated Lys-37.