The purpose of this study is: to find out the structural or functional effects of selective serotonin reuptake inhibitors (SSRI) in major depressive disorder (MDD); find special abnormalities in depression secondary to other disease, e.g., autoimmune disease like systemic lupus erythematosus (SLE). find the relationship between the efficacy of antidepressant and the change of neuroimaging in MDD to find possible predispose to MDD to explore the DNA methylation status in depression;