BDNF and NT-4 belong to the neurotrophin family which also includes NGF and NT-3. These neurotrophins bind two groups of receptors. The p75NTR receptor is common to all four neurotrophins and is a member of the tumor necrosis factor receptor family. The tropomyosin-related kinase (Trk) receptors are receptor tyrosine kinases (RTKs) and three receptors form this family: TrkA, TrkB, and TrkC. TrkA is activated by NGF binding, TrkB by that of BDNF and NT-4, while TrkC is stimulated by the binding of NT-3.