APOE and Alzheimer disease: Conversely, APOE4 carriers who may be less susceptible to developing AD neuropathology (e.g., those without a family history of AD) could be underrepresented in the NACC data, since they may not be aware of carrying APOE4. Further research on characterizing APOE4 and other factors associated with abnormal cognitive ageing, and developing needed preventive measures are warranted.