||Neurotrophins play very important roles in the development and maintenance of the vertebrate nervous system. In mammals, there are four members in the family: NGF, BDNF, NT-3, and NT-4/S. Different members of the neurotrophin family activate different receptors that belong to a class of receptor tyrosine kinases called "Trks". NGF is the specific ligand of TrkA while BDNF activates TrkB. To elucidate which regions of the two neurotrophins determine the receptor specificities, chimeric neurotrophins were constructed using BDNF as the backbone with various regions being substituted by the corresponding regions of NGF. The activity of the chimeras on the Trk receptors was assayed in fibroblasts over-expressing the receptors. The results suggested that individual substitution involving several variable regions of BDNF did not lead to significant loss in TrkB nor gain in TrkA activation. Moreover, important determinants of TrkB activation might be located near the carboxyl terminus of BDNF while critical elements for TrkA activation might be located within the first thirty-three amino acids in the amino-terminal half of the mature NGF molecule. In addition to the well-studied neurotrophins described above, a new member of the family, called neurotrophin-6 (NT-6), was recently cloned from the aquarium fish Xiphophorus macdatus. NT-6 contained an insertion of twenty-two amino acids responsible for heparin binding, a feature that is distinct from all other neurotrophins. In order to clone the NT-6 counterpart in higher vertebrates, PCR using degenerate oligonucleotides was performed, but no NT-6 counterpart was identified thus far. However, PCR using carp genomic DNA as template resulted in a partial sequence that contained structural feature characteristics of all neurotrophins but only shared 56% and 60% arnino acid identity to Xiphophoms NGF and NT-6, respectively. Moreover, there was an insertion of fifteen amino acids at the position corresponding to the insertion in Xiphophorus NT-6.