The beneficial effect of HDAC inhibitors in learning and AD can be interpreted as a proof of the involvement of histone acetylation in these processes (Vogel-Ciernia and Wood, 2012; Gräff and Tsai, 2013a) but, instead, it is just proving the involvement of the inhibited enzyme per se, and not necessarily of the acetylation of histones since other non-histones substrates can be also acetylated and deacetylated (Martinez-Redondo and Vaquero, 2013; Li et al., 2014a). Here, HDAC9 is linked to Alzheimer disease.