Brain-derived neurotrophic factor, also known as BDNF, is the second member of NGF family of neurotrophic factors (neurotrophins), which includes nerve growth factor (NGF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5). BDNF shares approximately 55% amino acid identity with other family members. The amino acid sequence of mature BDNF is identical in all mammals examined. All neurotrophins are noncovalently linked homodimers and initially produced as proneurotrophins. These proneurotrophins have distinct biological activity different from their mature counterparts. Mature BDNF promotes the survival and differentiation of selected neuronal populations of the peripheral and central nervous systems during the development. BDNF participates in axonal growth, pathfinding, and in the modulation of dendritic growth and morphology. BDNF is also a major regulator of the synaptic transmission and plasticity at adult synapses in many regions of the central nervous system (CNS). High levels of expression of BDNF have been detected in the hippocampus, cerebellum, fetal eye, and placenta. BDNF binds with high affinity and specifically activates the TrkB tyrosine kinase receptor.