The human APOE gene has three polymorphic alleles, namely APOE2, APOE3, and APOE4. Although various genetic backgrounds and life experiences may cause differences in adaptability and response capability of individual brains to stressful events [3], ApoE4 has been documented to be an age-dependent and a gene-dose-effect risk factor for late-onset familial and sporadic Alzheimer’s disease (AD) [4–6] and psychiatric disorders, such as depression [7]. Here, APOE is linked to major depressive disorder.