The few studies that, have found that several HDAC inhibitors exert a protective effect in AD, improving dendritic spine density, and facilitating learning and memory formation in different mouse models of the disease (Fischer et al., 2007; Francis et al., 2009; Ricobaraza et al., 2009, 2012; Zhang and Schluesener, 2013; Rumbaugh et al., 2015), although the precise mechanisms by which the HDAC inhibitors work remain to be determined. Here, HDAC9 is linked to Alzheimer disease.