HDAC9 and Alzheimer disease: Histone acetylation, which is regulated by the activities of histone deacetylases (HDAC) and histone acetyltransferases (HAT), has been primarily implicated in AD-like phenotypes in Drosophila. HDAC6 is a unique member of the HDAC family that acts mainly on cytoplasmic non-histone substrates [164] and increased in a postmortem study of human AD brain [165].