HDAC9 and cancer: Abnormal gene expression is induced by an imbalance between acetylation and deacetylation, which leads to cell proliferation, DNA replication, mitosis, and carcinogenesis.HDAC inhibitors are gaining popularity as new anti-cancer drugs due to their ability to kill cancer cells by autophagy, apoptosis, cellular necrosis, cell cycle arrest, production of reactive oxygen species (ROS), reduction of tumour angiogenesis, and immunomodulatory effects.