HDAC9 and Alzheimer disease: Altered HDAC levels have been detected in neurological degenerative diseases, such as Alzheimer disease, bipolar disorder, schizophrenia, major depressive disorder, stroke, epilepsy, and multiple sclerosis.137 Because of their role in epigenetic regulation, HDACs have also been linked to cancer.135 Class 1 and 2 HDAC activities have been particularly linked to disease and have been best studied.