Based on the previous studies reporting that BDNF and the 5-HT system are involved in various brain functions including anxiety, depression, aggression, learning and memory, and that dysregulations of these molecules are closely related to neuropsychiatric diseases [19–23], we hypothesized that these molecules are vulnerable to gravity changes and most likely mediate the effects of gravity changes on various types of behavior. The gene discussed is BDNF; the disease is major depressive disorder.