text: Histone deacetylase (HDACs) There are a total of four classes that categorize Histone Deacetylases (HDACs). Class I includes HDACs 1, 2, 3, and 8. Class II is divided into two subgroups, Class IIA and Class IIB. Class IIA includes HDACs 4, 5, 7, and 9 while Class IIB includes HDACs 6 and 10. Class III contains the Sirtuins and Class IV contains only HDAC11. Classes of HDAC proteins are divided and grouped together based on the comparison to the sequence homologies of Rpd3, Hos1 and Hos2 for Class I HDACs, HDA1 and Hos3 for the Class II HDACs and the sirtuins for Class III HDACs.
  Class I HDACs
  HDAC1 & HDAC2
  HDAC1 & HDAC2 are in the first class of HDACs are most closely related to one another. By analyzing the overall sequences of both HDACs, their similarity was found to be approximately 82% homologous. These enzymes have been found to be inactive when isolated which led to the conclusion that they must be incorporated with cofactors in order to activate their deacetylase abilities. There are three major protein complexes that HDAC 1 & 2 may incorporate themselves into. These complexes include Sin3 (named after its characteristic protein mSin3A), Nucleosome Remodelling and Deacetylating complex (NuRD), and Co-REST. The Sin3 complex and the NuRD complex both contain HDACs 1 and 2, the Rb-associated protein 48 (RbAp48) and RbAp46 which make up the core of each complex. Other complexes may be needed though in order to initiate the maximum amount of available activity possible. HDACs 1 and 2 can also bind directly to DNA binding proteins such as Yin and Yang 1 (YY1), Rb binding protein 1 and Sp1. HDACs 1 and 2 have been found to express regulatory roles in key cell cycle genes including p21.