Acetylation of the tail of histone is known to cause chromatin to adopt a more ÔopenÕ 3D conformation, allowing trans factors grearter access to DNA. Histone acetyltransferases (HATs), complexes interact with sequence-specific activator proteins and target specific genes. In addition to histones, HATs acetylate non-histone proteins, implicating them in a wide variety of regulartory roles for these enzymes. By comparison, histone deacetylation promotes a more ÔclosedÕ chromatin conformation and in general leads to repression of gene activity. Mammalian histone deacetylases are divided into three classes on the basis of their similarity to various the yeast deacetylases. Class I (HDACs 1, 2, 3 and 8) related to the yeast Rpd3-like proteins, class II (HDACs 4, 5, 6, 7, 9 and 10) related to yeast Hda1-like proteins and class III related to the yeast protein Sir2. Inhibitors of HDAC have enormous potential as cancer therapeutic agents.