Figure 2: Schematic representation of the classical Class I, II, and IV mammalian HDACs. HDAC enzymes have been separated into 4 classes based on their homology to yeast proteins. Class I enzymes are structurally similar to the yeast, reduced potassium dependency-3 (Rpd3) and consisting of HDAC1, 2, 3, and 8. Class II HDACs are related to the yeast, histone deacetylase-1 (Hda 1). This class is subdivided into two classes—Class IIa (HDAC4, 5, 7, 9) and Class IIb (HDAC6, 10). The HDACs have a conserved deacetylase (DAC) domain. The number of amino acid residues of the longest isoform of each HDAC is shown on the right.