APOE and Alzheimer disease: Our hypotheses are that 1) in individuals with a heightened risk for AD, PA will improve AD-related cognitive performance relative to controls; 2) the effects on cognitive performance will be moderated by APOE4 carrier status; and 3) changes in neural and blood biomarker will be observed in response to PA, will differ as a function of APOE4 carrier status, and will contribute to cognitive changes.