APOE and Alzheimer disease: Similarly, young APOE ε4 carriers (mean age = 21–30), although had no difference in cognition and GM volume compared to their age-mated controls, showed increase in default mode network (involving medial temporal, medial prefrontal, and retrosplenial cortical areas) coactivation [46], suggesting that the function of these areas subject to the disease process in AD is modulated by APOE ε4 allele at very early stage.