Examples of genes that implicate a neurobiological etiology of MDD are the dopamine receptor D2 (DRD2) gene, which is related to emotion processing, and the CUGBP Elav-Like Family Member 4 (CELF4) gene, which is associated with regulating synaptic activity for excitatory neurons (Table 1) (23). The gene discussed is DRD2; the disease is major depressive disorder.