Additionally, clinical trials of HDAC inhibitors have revealed numerous adverse reactions in patients, including thrombocytopenia-induced bleeding, neutropenia-induced susceptibility to infections, anemia due to hemoglobin reduction, arrhythmias, myocardial hypertrophy, and neurotoxicity, which also pose major limitations in the development of HDAC inhibitors (262). This evidence concerns the gene HDAC9 and Arrhythmia.