Neurtrophin-3 (NTF3, also known as NT-3) is the main ligand for the neuronal receptor tyrosine kinase NTRK3 (TRKC) (Lamballe et al. 1991). NTRK3 is not activated by other member of the neurotrophin family (NGF, BDNF and NTF4) (Lamballe et al. 1991, Ip et al. 1993). Binding to the NTF3 homodimer induces dimerization of the NTRK3 (TRKC) receptor, so that the stoichiometry of the ligand:receptor complex is 2:2 (Philo et al. 1994). In the study by Philo et al., a recombinant human NTF3 was used with a recombinant extracellular region of NTRK3, but the origin of NTRK3 was not specified.