This class of small-molecule inhibitors has thus become a new class of anti-tumor drugs, but some clinical trials exhibit numerous side effects of pan-HDAC inhibitors, such as hematologic toxicity, diarrhea, weight loss, taste disturbances, electrolyte changes, disordered clotting, fatigue, and cardiac arrhythmias [66]. The gene discussed is HDAC9; the disease is neoplasm.