HDAC9 and Alzheimer disease: HDAC inhibitors are known to inhibit GSK3β, a kinase that participates in the pathogenic hyperphosphorylation of tau during AD pathogenesis.66,69–71 Additionally, a selective inhibitor of HDAC3 has also been shown to decrease tau hyperphosphorylation by directly inhibiting HDAC3-dependent mechanisms that mediate tau phosphorylation.72