HDAC9 and cancer: HDACs are categorized into 4 different classes: Class I (HDAC 1–3 and 8), Class IIA (HDAC 4–7 and 9) and IIB (HDAC 6 and 10), Class III (SIRT1–7), and Class IV (HDAC 11) based on their function and sequence homology to yeast proteins.37 HDACs play a critical role in cancer progression by inhibiting acetylation at promoters of genes involved in apoptosis, cell cycle, DNA damage, and angiogenesis.38 However, the mechanisms involved in regulating tumorigenesis by different classes of HDAC is not well understood.