HDAC9 and leukemia: HDACs can be classified into four major groups based on sequence homology: class I (HDAC 1–3 and HDAC8), class II (HDAC 4–7 and HDAC 9-10), class III HDAC (sirtuin 1–7), and class IV only (HDAC11).828 HDACs promote leukemia development by mediating abnormal gene silencing in malignant tumors.