The Trk family of nerve growth factor receptors includes Trk A, Trk B and Trk C. The prototype member of this gene family, Trk A encodes a 140-kDa cell surface receptor (gp140) expressed in neurons of the sensory spinal and cranial ganglia of neurocrest origin. Nerve growth factor (NGF) stimulates tyrosine phosphorylation of Trk A in neural cell lines and embryonic dorsal root ganglia. By comparison, BDNF and to a lesser extent, neurotrophin-3 (NT-3), but not NGF, can induce tyrosine phosphorylation of Trk B gp 145. Trk C, the third member of the Trk receptor family, encodes a 140-kDa protein, Trk C gp 140, that is preferentially expressed in brain tissue and primarily functions as a receptor for NT-3. An additional component of the Trk receptor complex, NGFR p 175 , binds to neurotrophic factor with low affinity but is required for efficient signaling.