HDAC9 and Alzheimer disease: These observations may be associated with overexpression of histone deacetylases (HDACs), particularly nuclear HDAC2 and HDAC3 (class I) and cytosolic HDAC6 (class II) (Ding et al., 2008; Gräff et al., 2012; Zhu et al., 2017), although decreased HDAC levels were also reported in AD brains (Pascoal et al., 2022).