HDAC9 and Alzheimer disease: Two HDAC inhibitors, the pan-HDAC inhibitor sodium valproate (VPA), and the novel HDAC6-specific inhibitor WT161, which is currently used for antitumor therapy [30, 31], were chosen to observe the effects of both on the expression levels of Aβ in cells and mouse brains, and on the improvement of behaviour and memory in AD mice, aiming to explore the underlying molecular mechanisms.