Although the brain has, for decades, been considered an insulin-insensitive organ, insulin is now clearly seen to play an essential role in a plethora of brain physiological processes (e.g., regulation of glucose homeostasis; energy metabolism; neuronal differentiation, maturation, and proliferation) and in the regulation of psychic processes (e.g., anxiety, depression) and cognitive functions (e.g., execution, attention, learning, memory) [11,16,76]. The gene discussed is INS; the disease is major depressive disorder.