HDAC9 and cancer: Given that histone acetylation level alters the structure of chromatin and the accessibility of genomic DNA by endo/exogenous biological molecules, impacting many intrinsic biological processes and pathological processes, in particular cancer cell growth and apoptosis [43], many small molecular HDAC inhibitors (HDACi) have been synthesized and used to explore the crucial roles of histone acetylation in genomic stability and transcription processes [121,122].