HDAC9 and cognitive disorder: Inhibition of specific HDAC enzymes, and the resulting increase in lysine acetylation is therapeutic for cognitive disorders like Alzheimer’s disease (AD) and enhances cognitive performance in rodent models (Swank and Sweatt, 2001; Korzus et al., 2004; Levenson and Sweatt, 2005; Guan et al., 2009; Xu et al., 2011; Yang et al., 2017).