Nerve growth factor (NGF) is a small secreted protein which induces the differentiation and survival of particular target neurons. It is perhaps the prototypical growth factor; in that it is one of the first to be described. NGF is critical for the survival and maintenance of sympathetic and sensory neurons. NGF is released from the target cells, binds to and activates its high affinity receptor (TrkA), and is internalized into the responsive neuron. There is some data that shows that NGF can be transported from the axon tip to soma. One of the most important molecules in the nervous system, nerve growth factor and its other neurotrophin family members control the development of the nervous system in the embryo and the maintenance of nervous tissue and neural transmission in the adult.