Histone acetyltransferases serve many biological roles inside the cell. Chromatin is a combination of proteins and DNA found in the nucleus, and it undergoes many structural changes as different cellular events such as DNA replication, DNA repair, and transcription occur. Chromatin in the cell can be found in two states: condensed and uncondensed. The latter, known as euchromatin, is transcriptionally active, whereas the former, known as heterochromatin, is transcriptionally inactive. Histones comprise the protein portion of chromatin. There are five different histone proteins: H1, H2A, H2B, H3, and H4. A core histone is formed when two of each histone subtype, excluding H1, form a quaternary complex. This octameric complex, in association with the 147 base pairs of DNA coiled around it, forms the nucleosome. Histone H1 locks the nucleosome complex together, and it is the last protein to bind in the complex.