Neurotrophin-3 (NT-3) and Neurotrophin-4 (NT-4) are members of the NGF family of neurotrophic factors that are required for the differentiation and survival of specific neuronal subpopulations. The neurotrophin family is comprised of at least four proteins including NGF, BDNF, NT-3, and NT-4/5. These secreted cytokines are synthesized as prepropeptides that are proteolytically processed to generate active propeptides and mature protein homodimers. Mature NT-4 also forms heterodimers with BDNF or NT-3. Neurotrophins induce dimerization and activation of Trk family receptors and promote the development and survival of selected peripheral and CNS neurons. All neurotrophins have six conserved disulfide-linked cysteine residues.