HDAC9 and Alzheimer disease: HDAC inhibition of ΔFos in APP mutant AD mice via 4-phenylbutyric acid (Class I HDAC 4-PBA) or MS-275 (inhibitor of HDAC1-3) has now been shown to reverse the suppression of c-Fos and thus increases cognition performance in AD mice as observed with object location memory tasks and hippocampus-dependent spatial memory tasks (Corbett et al., 2017).