HDAC9 and cancer: The imbalance between HAT and HDAC activities leads to abnormal changes in histone acetylation levels, which may disrupt gene transcription regulation and participate during the appearance and growth of tumors.90 Specifically, in human and mouse tumor samples, the levels of H4K16 acetylation (H4K16ac) and H4K20 trimethylation (H4K20me3) have been significantly reduced, and these changes have been confirmed as biomarkers for tumor progression.91,92 Alongside the variations in histone acetylation levels, the expression of enzymes related to histone acetylation is also altered in cancer.