The modification of histone proteins is a major event in epigenetics. It includes the introduction and removal of methyl, acetyl, phosphate, and other groups. The acetylation of lysine side chains of histones loosens the compact chromatin structure, making the genome accessible for the transcription machinery. The removal of histone acetyl residues is catalyzed by histone deacetylases and results in the silencing of the respective genes. Several isoforms of the HDAC family have been discovered so far that are divided into four classes. Class I enzymes (HDAC1, 2, 3, and 8) are mainly responsible for histone deacetylation, while other classes and isoforms have different substrate scopes, thus influencing various cellular functions. Class II is further subdivided into subclasses IIa (HDAC4, and . Class III enzymes, also called sirtuins (Sirt1-7), are not zincdependent like classes I, II, and IV, but use NAD + as a cofactor. Class IV consists only of one isoform (HDAC11).