The TRK Family of Tyrosine Kinase Receptor consists of 3 members: TrkA, TrkB and TrkC. The three TRK family proteins have different ligand specificities. They connect to different neurotrophins, including NGF, BDNF, NT-3NT-4/5. TRKA binds NGF, TRKB binds BDNF and NT-3, TRKC binds NT-4/5. At the protein sequence level, human and rat TRKB have greater than 90% sequence identity and the proteins exbihit cross-species activity. TRKB is primarily expressed in the nervous system and it also expression in a wide variety of tissues with low levels.