The identification and cloning of histone deacetylase 9 (HDAC9) is disclosed, and in particular full length HCAC9 polypeptides and HDAC9 polypeptides which have deacetylase activity, and to nucleic acid molecules encoding these polypeptides. The uses of these polypeptides and nucleic acid molecules are disclosed, for example for screening for compounds that are capable of modulating HDAC9 biological activity.