Major depressive disorder (MDD) is one of the most challenging psychiatric disorders to study. It is a polygenic disorder that has a lifetime incidence of 10-15%, with women affected twice as often as men, for unknown reasons. The high lifetime prevalence of this non-infectious but debilitating disorder necessitates progress in this field so that better therapeutic drugs can be developed to more effectively treat patients. Genome-wide association studies on major depression have yet to produce significant findings. This is likely because the effects of individual loci are small and/or because...