Histones, the most abundant proteins in eukariote nucleus, are protein octamers integrated by two tetramers, containing two copies of an H4/H3 dimmer and two copies of the H2A/H2B dimer, respectively. Thanks to multiple hydrogen bounds and salt bridges histones bound to double helix of DNA, forming a complex know as chromatin. The histone-octamer/DNA complex comprises the first level of chromatin organisation. 

For many years it was believed that histones only served as structural components, but during the last 20 years intensive research developed around the key role of histone in gene regulation. Subtle covalent modifications on the N-terminal tails of the histone are the main responsables behind the DNA epigenetic control. This illustration displays the main known modifications occurring in histone tails.