The clinical picture of depression varies from one major depressive episode to another in any given patient. This suggests that major depression, despite its various symptom profiles, may have a common underlying cause. If so, the clinically evident symptom profiles may result from differing patterns of neurotransmitter abnormalities in various brain regions.17 Consonant with such hypotheses, a host of deficiencies — in serotonin, norepinephrine, dopamine, γ-aminobutyric acid (GABA), and peptide neurotransmitters or trophic factors such as brain-derived neurotrophic factor, somatostatin, and thyroid-related hormones — have been proposed as contributing to depression.18 Furthermore, overactivity in still other neurotransmitter systems involving acetylcholine, corticotropin-releasing factor, and substance P are thought to be implicated in depression.18 Although no specific abnormalities in genes that control neurotransmitter or hormonal synthesis or release have been identified with certainty, both major depressive disorder and bipolar disorder are clearly heritable. How a genetic predisposition interacts with adverse early-life experience to alter brain development and lead to major depression remains unclear.