HDAC9 and liver cancer: Among HDAC types, substantial expression of class I HDACs is commonly observed in most cancer types, including HDACs 1 and 2 in breast cancer (52, 53); HDAC 1 in lung cancer (54); HDACs 2 and 3 in colorectal cancer (55); and HDACs 1, 2, and 3 in liver cancer (56, 57).