Treatment with each of four most potent HDAC inhibitors (IC50 0.089–0.26 μM) decreased the viability of the human breast cancer MDA-MB-231 cells (IC50 2.98–6.14 μM) as well as chronic myelogenous leukemia K562 cells (IC50 6.75–12.9 μM). The gene discussed is HDAC9; the disease is breast carcinoma.