Histone H3 is a histone protein. Histones are basic nuclear proteins that are responsible for the nucleosome structure of the chromosomal fiber in eukaryotes. They play a central role in transcription regulation, DNA repair, DNA replication, and chromosomal stability. Two molecules of each of the four core histones (H2A, H2B, H3, and H4) form an octamer, around which approximately 146 bp of DNA is wrapped in repeating units, called nucleosomes. Histone H3 is characterized by a main globular domain and a long N-terminal tail. It is involved with the structure of the nucleosomes of the 'beads on a string' structure. There are five main histone proteins involved in the structure of chromatin in eukaryotic cells. Histone H3 is the most extensively modified of the five histones. In the emerging field of epigenetics, it is thought that Histone H3 plays a key role, cause its sequence variants and variable modification states function in the dynamic and long term regulation of genes.