HDAC9 and cancer: Butyrate acts as an HDAC inhibitor (HADCi), thus preventing the deacylation of histones (Figure 2) and increasing the expression of repressed genes.43 This process is crucial in activating extrinsic and intrinsic apoptotic pathways, reactive oxygen species (ROS), and cell cycle arrest in cancer cells.44–46 The inhibition of HDAC also impacts several other diseases, such as brain trauma, dementia, and autoimmune encephalitis.47,48 By inhibiting HDAC, chromatin is exposed to aryl hydrocarbon receptor (AhR)-ligand complexes and binding sites in the promoter of AhR target genes.