NGF was the first neurotrophin to be discovered, more than 50 years ago. Three other proteins have been added to the list: BDNF, NT-3 and NT-4 (NT-4/5). These neurotrophins bind two groups of receptors. The p75^NTR 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.