HDAC9 and Parkinson disease: In previous studies, HDAC inhibitors (HDACi) such as sodium butyrate or trichostatin A (TSA) were reported to rescue memory deficits in both aged and gene-mutant mice by elevating the level of hippocampal histone acetylation, and these compounds also showed therapeutic potential for depression and some neurodegenerative disorders such as Huntington’s disease (HD), Parkinson's disease (PD), and Alzheimer’s disease (AD) [6, 8–13].