Data show increased HDAC3 protein levels and decreased acetyl‐H3 in AD human cortex, and increased HDAC2‐3 in MCI peripheral human cells, HT22 mouse hippocampal cells exposed to Aβ1–42 oligomers (AβO) and APP/PS1 mouse hippocampus. The gene discussed is HDAC3; the disease is Alzheimer disease.