BDNF and major depressive disorder: In this study, we explore the evolutionary history of the BDNF gene in Mammalia. The BDNF gene is implicated in a number of human diseases including a variety of brain disorders such as neurodevelopmental disorders (e.g., autism spectrum disorder), neuropsychiatric disorders (e.g., depression, PTSD, and schizophrenia), and some neurodegenerative disorders [1].