Members of the Trk family (TrkA, B, and C) of receptor tyrosine kinases act as receptors for the neurotrophin family: BDNF, NGF, NT-3, and NT-4. Trks exhibit selectivity for specific neurotrophins. TrkA, TrkB and TrkC preferentially bind NGF, NT-4 and BDNF, and NT-3, respectively. All Trk family proteins share a common structural organization consisting of an extracellular region with two cysteine-rich domains, a cluster of three leucine-rich motifs, and two immunoglobulin-like domains, and an intracellular region that contains the tyrosine kinase domain. They are best known for their regulation of neuronal survival and axon outgrowth.