As HDAC inhibitors are powerful drugs, which itself could potentially induce the side-effects (45), understanding which individual HDACs or HATs involved in the aging-induced hypoacetylation of key histones would greatly improve the potential for adjunct HDAC inhibitors, as more selective HDAC inhibitors may lead to better tolerability of this adjunct treatment approach in elderly and AD patients. This evidence concerns the gene HDAC9 and Alzheimer disease.