Neurotrophins function to regulate naturally occurring cell death of neurons during development. The prototype neurotrophin is nerve growth factor (NGF), originally discovered in the 1950s as a soluble peptide promoting the survival of, and neurite outgrowth from, sympathetic ganglia. More recently, three additional structurally homologous neurotrophic factors have been identified. These include brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4), also designated NT-5. These various neurotrophins stimulate the in vitro survival of distinct but partially overlapping populations of neurons. The Trk A receptor is the preferential receptor for NGF, but also binds NT-3 and NT-4. The Trk B receptor binds equally well to both BDNF and NT-4 and to a lesser extent NT-3, while the Trk C receptor only binds NT-3. BDNF promotes the survival of neuronal populations that are all located either in the central nervous system or directly connected to it. Belongs to the NGF-beta family.