Neurotrophic factors (NTFs) are molecules which act to promote the differentiation of neurons and to maintain their phenotype. The discovery by Hamburger and Levi- Montalcini in 1949 that the number of neurons in dorsal root ganglia was related to the size of the target field suggested that there exists a mechanism of neuronal support by which factors produced by target organs are able to support neurons projecting to them. The isolation of nerve growth factor (NGF) proved that such a factor could indeed promote the growth of a specific type of neuron (Levi- Montalcini and Hamburger 1953), and the importance of NGF in neuronal development was indicated in experiments showing that specific antibodies against NGF could interfere with the normal development of dorsal root ganglion neurons (Johnson et al. 1980).