We have screened an adult rat cerebeliar cDNA library in search of novel protein tyrosine-kinase (PTK) cDNAs. A cDNA for a putative PTK, trkB, was cloned, and its sequence indicates that it is likely to be derived from a gene for a ligand-regulated receptor closely related to the human irk oncogene. Northern (RNA) analysis showed that the trkB gene is expressed predominantly in the brain and that trkB expresses multiple mRNAs, ranging from 0.7 to 9 kb. Hybridization of cerebral mRNAs with a variety of probes indicates that there are mRNAs encoding truncated trkB receptors. Two additional types of cDNA were isolated, and their sequences are predicted to encode two distinct C-terminally truncated receptors which have the complete extracellular region and transmembrane domain, but which differ in their short cytoplasmic tails.

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Expression of neurotrophins and neurotroph-