HDAC9 and Parkinson disease: Mounting evidence highlights a pathological imbalance between histone acetylation-mediated histone acetyl transferease (HAT) and deacetylation by histone deacetylase (HDAC) in PD models, through which gene expression is dysregulated and downstream function is subsequently altered in DAergic neurons (Song et al., 2010; Labbé et al., 2016).