NEURAL RECEPTOR TYROSINE KINASE

Abstract: Novel receptor tyrosine kinase protein and isoforms thereof which are expressed in migrating axons, and nucleic acid molecules encoding the novel protein isoforms and parts thereof are disclosed. The invention also relates to methods for identifying substances which are capable of binding to the receptor protein and methods for screening for agonists or antagonists of the binding of the protein and substance. Diagnostic and therapeutic methods using the protein and nucleic acid molecules are also described.

Description: Neura l receptor tyro si ne ki nase FIELD OF THE INVENTION The invention relates to a novel receptor tyrosine kinase protein and isoforms and parts thereof, nucleic acid molecules encoding the novel protein and fragments thereof, and uses of the protein and nucleic acid molecules. BACKGROUND OF THE INVENTION Embryonic development of multicellular organisms is a highly ordered process that requires coordination of individual cells. Every cell must decipher the numerous signals it receives and then properly execute commands in order to achieve the correct position and differentiated state in the animal. The exquisite controls over cell growth, determination, migration and adhesion are mediated by molecules located on the plasma membrane surface. A class of membrane associated molecules known to regulate cellular interactions are receptor tyrosine kinase proteins. The evolutionary conservation of genes encoding receptor tyrosine kinase proteins and their targets has emphasized the importance of these proteins in intracellular communication, and has also provided model systems for genetic analysis of tyrosine kinase signalling pathways. Such studies have shown that some tyrosine kinases function to specify a particular cell fate, such as the sevenless (sev) receptor in Drosophila R7 photoreceptor cells and the Let-23 receptor in nematode vulval cells (reviewed by Greenwald and Rubin, Cell 68:271-281, 1992). The binding of sev with its ligand, boss, results in cell clustering suggesting a role in cell-cell adhesion for these molecules (Kramer et al, Nature 352:207-212, 1991). The receptor tyrosine kinase encoded by torso functions in pattern formation by specifying the terminal poles of Drosophila embryos (Sprenger et al, Nature 338:478-483, 1989). Genetic analysis has recently provided insight into the functions of a small number of receptor tyrosine kinases in mouse development, including the α-platelet-derived growth factor receptor, the colony stimulating factor-1 receptor, and c-Kit/W (Pawson and Bernstein, Trends in Genetics 6:350-356, 1990). A growing number of closely related transmembrane receptor tyrosine kinase proteins containing cell adhesion-like domains on their extracelluar surface have recently been identified. Collectively, this group of proteins defines the Eph/Elk Eck subfamily, which is made up of at least fifteen related but unique gene sequences in higher vertebrates (Hirai et al, Science 238:1717-1720, 1987; Letwin et al, Oncogene 3:621-627, 1988; Lindberg et al., Mol. Cell Biol 10:6316-6324, 1990; Lhotak et al, Mol. Cell. Biol 11:2496-2502, 1991; Chan and Watt, Oncogene 6:1057-1061, 1991; Lai and Lemke, Neuron 6:691-704, 1991; Pasquale, Cell Regulation 2:523-534, 1991; Sajjadi et al, New Biologist 3:769-778, 1991; Wicks et al., PNAS 89:1611-1615, 1992; Gilardi-Hebenstreit et al., Oncogene 7:2499-2506, 1992; Bohme et al, Oncogene 8:2857-2862, 1993; Sajjadi and Pasquale, Oncogene 8:1807-1813, 1993). Eph family members encode a structurally related cysteine rich extracelluar domain containing a single immunoglobulin (Ig)-like loop near the N-terminus and two nbronectin L (FN El) repeats adjacent to the plasma membrane. Examples of Eph family members include Cek5 (Pasquale, Cell Regulation 2:523-534, 1991) and Erk; (Chan and Watt, Oncogene 6:1057-1061 1991). Another Eph family member, Sek, has been shown to be segmentally expressed in specific rhombomeres of the mouse hindbrain (Nieto et al, Development 116:1137-1150, 1992). The presence of cell adhesion-like domains in this family of tyrosine kinases suggests that these proteins function in cell-cell interactions. The other major families of proteins implicated in cell adhesion include the cadherins, selectins, integrins, and those of the immunoglobulin superfamily (reviewed by Hynes, R.O. and Landers, A.D., Cell 68, 303-322, 1992). The extracelluar regions of cell adhesion molecules frequently contain peptide repeats, such as FN HI motifs, epidermal growth factor (EGF) repeats, or Ig loops that may direct protein-protein interactions at the cell surface. A number of cell adhesion molecules in both vertebrates (Dodd, J. and Jessell, T.M., Science, 242, 692-699, 1988; Jessell, T.M., Neuron, 1, 3-13, 1988; Furley et al, Cell 61, 157-170, 1990; Burns et al, Neuron, 7, 209-220, 1991) and invertebrates (Bastiani et al., Cell 48:745-755, 1987; Elkins et al., Cell 60:565-575, 1990; Grenningloh et al, Cold Spring Harb. Symp. Quant. Biol. 55, 327-340, 1991; Nose et al, Cell 70:553-567, 1992) have been implicated in axonal growth cone guidance and pathway/target recognition. Other aspects of neuronal morphogenesis involving cell-cell interactions may also require the activities of cell adhesion molecules (Edelman and Thiery, In The Cell in Contact: Adhesions and Junctions as Morphogenetic Determinants, Wiley, New York, 1985; Hatta et al, Dev. Biol 120:215-227, 1987; Takeichi, Development 102:639-655, 1988; Takeichi, Annu. Rev. Biochem. 59:237-252 1990; Takeichi, Science 251:1451-1455, 1991; Edelman, Biochemistry 27:3533-3543, 1988; Grumet, Curr. Opin. Neurobiol. 1:370-376, 1991; Hynes and Lander, Cell 68:303-322, 1992). For example, ectopic N-cadherin expression during gastrulation stage Xenopus embryos has been shown to interfere with segregation of the neural tube from the ectoderm (Detrick et al, Neuron 4:493-506, 1990; Fujimori et al, Development 110:97-104, 1990). Although many different types of cell adhesion molecules have been identified, little is known about how these adhesive interactions are regulated and how they function in cell signalling pathways during normal development. A critical stage in the development of the nervous system is the projection of axons to their targets. Navigational decisions are made at the growth cones of the migrating axons. As axons grow their growth cones extend and retract filopodia and lamellipodia processes which are implicated in the navigational decisions and pathfinding abilities of migrating axons. Like peripheral nervous system axons, the growth cones of neurons associated with the central nervous system follow stereotyped pathways and apparently can selectively chose from a number of possible routes (reviewed by Goodman and Shatz, Cell 72:77-98, 1993). Early pathways in the vertebrate embryonic brain are thought to be arranged as a set of longitudinal tracts connected by commissures. However, the molecular mechanisms that underly growth cone navigation and axon pathfinding in development are poorly understood (Hynes, R.O. and Lander, A.D., 1992, Cell 68:303). Evidence indicates that the development of the endolymphatic duct is under the control of neuronal induction (Van De Water and Represa, Van De Water, T.R. and Represa, J. (1991). Ann. NY Acad. 630:116-128, 1991). The endolymphatic duct pinches off from the otic vesicle and elongates to form a tube that apparently functions in regulating the endolymph fluid pressure in the membranous labyrinth of the internal ear (Guild, Amer. }. Anat. 39:57-81, 1927; Rugh, The Mouse: Its reproduction and Development. Minneapolis: Burgess, 1968; Sher, 1971; Hendriks and Toerien, 1973; Theiler, 1989; Kaufman, In Postimplantation Mammalian Embryos: a Practical Approach (ed. A.J. Copp and D.L. Cockroft) pp. 81-91. New York: Oxford University Press, 1990). The developmental function of tyrosine kinases during axonogenesis has been studied in Drosophila. A function in axonal pathfinding is evident for the Drosophila abl tyrosine kinase when abl mutations are combined with mutations in other genes including the neural cell adhesion molecule, fasciclin I (fas I, Elkins et al, Cell 60:565-575, 1990) or disabled (dab, Gertler et al, Cell 58:103-113, 1989). These studies have shown that the abl tyrosine kinase is specifically localized to the axonal compartment of the embryonic Central Nervous System (CNS) (Gertler et al, Cell 58:103-113, 1989). Moreover, genetic analysis has indicated that subcellular localization to axons is essential for abl function during development (Henkemeyer et al, Cell 63:949-960, 1990) and that mutations in second-site modifier genes including fas I and dab can reveal a role for abl in axonogenesis (Elkins et al, Cell 60:565-575, 1990; Gertler et al, Cell 58:103-113 1989). The requirement for tyrosine phosphorylation in axonal outgrowth and adhesion in Drosophila is strengthened by the identification in CNS axons of three transmembrane tyrosine phosphatases containing FN m motifs (Tian et al, Cell 67:675-685, 1991; Yang et al, Cell 67:661-673, 1991). SUMMARY OF THE INVENTION The present inventors have identified and characterized a receptor tyrosine kinase protein that plays an important role in cell-cell interactions and axonogenesis in the development of the nervous system. In particular, the present inventors have cloned a novel murine gene, designated neural kinase (Nuk). The gene encodes a new member of the Eph subfamily of receptor tyrosine kinases, designated Nuk protein. The murine Nuk locus was mapped to the distal end of mouse chromosome 4 near the ahd-1 mutation. The biological function of Nuk protein was investigated using antibodies having anti-Nuk protein specificity. A detailed immunohistochemical analysis of its subcellular localization in whole-mount mouse embryos indicated that during early embryogenesis Nuk protein is confined to the developing nervous system, where it marks segments along the axis of the neural tube in the hindbrain (rhombomeres r2, r3, and r5) and specific morphological bulges of the midbrain and forebrain. Nuk protein was also found to be concentrated at sites of cell-cell contact, often involving migrating neuronal cells or their extensions. Most notably, high levels of Nuk protein were found within initial axon outgrowths and associated nerve fibers, including most if not all peripheral nervous system (PNS) axons. The axonal localization of Nuk protein was also found to be transient and was not detected after migrations have ceased. The subcellular localization of Nuk protein, as well as the presence of fibronectin type El and immunoglobulin-like adhesive domains on the extracelluar region, indicates that this receptor tyrosine kinase functions to regulate specific cell-cell interactions during early development of the nervous system and that it has a role during the early pathfinding and/or fasciculation stages of axonogenesis in animals. The present invention therefore provides a purified and isolated nucleic acid molecule containing a sequence encoding a receptor tyrosine kinase protein which is expressed in migrating axons, or an oligonucleotide fragment of the sequence which is unique to the receptor tyrosine kinase protein. In a preferred embodiment of the invention, the purified and isolated nucleic acid molecule comprises (a) a nucleic acid sequence encoding a protein having the amino acid sequence as shown in SEQ ID NO:2 and Figure 2; (b) nucleic acid sequences complementary to (a); (c) nucleic acid sequences which are at least 97% identical to (a); or, (d) a fragment of (a) or (b) that is at least 15 bases and which will hybridize to (a) or (b) under stringent hybridization conditions. Most preferably, the purified and isolated nucleic acid molecule comprises (a) a nucleic acid sequence as shown in SEQ ID NO:l and Figure 1, wherein T can also be U; (b) nucleic acid sequences complementary to (a); (c) nucleic acid sequences which are at least 85% identical to (a); or, (d) a fragment of (a) or (b) that is at least 15 bases and which will hybridize to (a) or (b) under stringent hybridization conditions. It is contemplated that a nucleic acid molecule of the invention may be prepared having deletion and insertion mutations. For example, the extracelluar domain or parts thereof, such as the FN HI and Ig domains; the transmembrane region or parts thereof; the tyrosine kinase domain or parts thereof, such as the ATP binding site and; the carboxy terminal tail may be deleted. In a preferred embodiment, the deletions are in a portion of the nucleic acid molecule of the invention encoding the extracelluar domain of Nuk protein, most preferably the portion comprising codons 29 to 50 in SEQ ID NO:l. In another preferred embodiment, the deletions are in a portion of the nucleic acid sequence of the invention encoding the kinase domain of Nuk protein, most preferably the portion comprising the ATP-binding site amino acid number 623-707. The invention further contemplates a purified and isolated double stranded nucleic acid molecule containing a nucleic acid molecule of the invention or a fragment thereof, hydrogen bonded to a complementary nucleic acid base sequence. The nucleic acid molecules of the invention, or fragments thereof may be inserted into an appropriate expression vector, i.e. a vector which contains the necessary elements for the transcription and translation of the inserted protein-coding sequence. Accordingly, recombinant DNA molecules adapted for transformation of a host cell may be constructed which comprise a nucleic acid molecule of the invention and one or more transcription and translation elements operatively linked to the nucleic acid molecule. In an embodiment of the invention, a recombinant molecule is provided which contains a nucleic acid molecule of the invention having a deletion or insertion mutation. Such a recombinant molecule may further comprise a reporter gene. The recombinant molecule can be used to prepare transformed host cells expressing the protein or part thereof encoded by a nucleic acid molecule of the invention. Therefore, the invention further provides host cells containing a recombinant molecule of the invention. The invention also contemplates transgenic non-human mammals whose germ cells and somatic cells contain a recombinant molecule of the invention. The invention further provides a method for preparing a novel receptor tyrosine kinase protein or isoforms or parts thereof utilizing the purified and isolated nucleic acid molecules of the invention. The invention further broadly contemplates a purified and isolated receptor tyrosine kinase protein which is expressed in migrating axons, or an isoform or a part of the protein. In a preferred embodiment, a purified receptor tyrosine kinase protein is provided which has the amino acid sequence as shown in SEQ ID NO:2 or Figure 2, or a sequence having between 97 and 100 percent identity thereto. The receptor tyrosine kinase protein of the invention may also be phosphorylated. Conjugates of Nuk protein of the invention, or parts thereof, with other molecules, such as proteins or polypeptides, may be prepared. This may be accomplished, for example, by the synthesis of N-terminal or C-terminal fusion proteins. In a preferred embodiment a fusion protein is provided comprising a part of the protein of the invention, preferably the extracelluar domain, most preferably having the amino acid sequence as shown in SEQ ID NO: 2 from amino acid number 26 to 548 or amino acids 600 to 618; or the carboxy terminal, most preferably having the amino acid sequence as shown in SEQ ID NO:2 from amino acid number 601 to 994; or sequences having at least 97% identity thereto. The invention also permits the construction of nucleotide probes which are unique to the nucleic acid molecules of the invention and accordingly to the novel receptor tyrosine kinase protein of the invention or a part of the protein. Thus, the invention also relates to a probe comprising a nucleotide sequence coding for a protein which displays the properties of the novel receptor tyrosine kinase of the invention or a part which is unique to the protein. The probe may be labelled, for example, with a detectable substance and it may be used to select from a mixture of nucleotide sequences a nucleotide sequence coding for a protein which displays the properties of the novel receptor tyrosine kinase protein of the invention, or a part thereof. The invention still further provides a method for identifying a substance which is capable of binding to the novel receptor tyrosine kinase protein of the invention, or an isoform or part of the protein, comprising reacting the novel receptor tyrosine kinase protein of the invention, or part of the protein, with at least one substance which potentially can bind with the receptor tyrosine kinase protein, isoform or part of the protein, under conditions which permit the formation of substance-receptor kinase protein complexes, and assaying for substance-receptor kinase protein complexes, for free substance, for non-complexed receptor kinase proteins, or for activation of the receptor tyrosine kinase proteins. In an embodiment of the method, ligands are identified which are capable of binding to and activating the novel receptor tyrosine kinase protein of the invention. The extracellular ligands which bind to and activate the novel receptor tyrosine kinase protein of the invention may be identified by assaying for protein tyrosine kinase activity. Still further, the invention provides a method for assaying a medium for the presence of an agonist or antagonist of the interaction of a receptor tyrosine kinase protein of the invention and a substance which binds to the receptor tyrosine kinase protein, preferably a ligand. In an embodiment, the method comprises providing a known concentration of a receptor tyrosine kinase protein of the invention, isoforms thereof, or part of the protein, preferably the extracelluar domain of the protein, incubating the receptor tyrosine kinase protein with a substance which is capable of binding to the receptor tyrosine kinase protein, isoforms thereof, or part of the protein, and a suspected agonist or antagonist substance under conditions which permit the formation of substance-receptor protein complexes, and assaying for substance-receptor protein complexes, for free substance, for non-complexed proteins, or for activation of the receptor tyrosine kinase protein. The methods of the invention permit the identification of potential stimulators or inhibitors of axonal migration and nerve cell interactions in development and regeneration, which will be useful in the treatment of nerve disorders and nerve damage. The invention also contemplates a method for identifying a substance which is capable of binding to an activated receptor tyrosine kinase protein of the invention, or an isoform or part of the activated protein, comprising reacting the activated receptor tyrosine kinase protein of the invention, or part of the protein, with at least one substance which potentially can bind with the receptor tyrosine kinase protein, isoform or part of the protein, under conditions which permit the formation of substance-receptor kinase protein complexes, and assaying for substance-receptor kinase protein complexes, for free substance, for non-complexed receptor kinase proteins, or for phosphorylation of the substance. In an embodiment of the method, intracellular ligands such as Src homology region 2 (SH2)-containing proteins which are capable of binding to a phosphorylated receptor tyrosine kinase protein of the invention, or intracellullar ligands which may be phosphorylated by the novel receptor tyrosine kinase of the invention may be identified. The invention further contemplates antibodies having specificity against an epitope of the receptor tyrosine kinase protein of the invention or part of the protein which is unique to the receptor tyrosine kinase protein. Antibodies may be labelled with a detectable substance and they may be used to detect the novel receptor tyrosine kinase of the invention in tissues and cells. The antibodies may accordingly be used to monitor axonal migration and nerve cell interactions. Substances which affect axonal migration may be identified using the methods of the invention by comparing the pattern and level of expression of the novel receptor tyrosine kinase protein of the invention in tissues and cells in the presence and in the absence of the substance. Thus the invention provides a method for screening for substances having pharmaceutical utility in the treatment and diagnosis of nerve disorders and nerve damage. Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples while indicating preferred embodiments of the invention are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. DESCRIPTION OF THE DRAWINGS The invention will be better understood with reference to the drawings in which: Figure 1 shows the nucleotide sequence encoding the novel receptor tyrosine kinase protein of the invention as shown in SEQ ID NO: 1; Figure 2 shows the amino acid sequence of the novel receptor tyrosine kinase protein of the invention as shown in SEQ ID NO:2 and a schematic diagram of the regions of the novel receptor tyrosine kinase protein of the invention; Figure 3A shows immunoprecipitation of the novel receptor tyrosine kinase protein of the invention; Figure 3B shows Western Blot analysis of immunoprecipitates of the novel receptor tyrosine kinase protein of the invention; Figure 4A shows localization of mRNA of the novel receptor tyrosine kinase protein of the invention in whole-mount sections of 7.5 day old mouse embryo; Figure 4B shows localization of the novel receptor tyrosine kinase protein of the invention in whole-mount sections of 8 day old mouse embryo; Figure 4C shows localization of the novel receptor tyrosine kinase protein of the invention in whole-mount sections of 8.75 day old mouse embryo; Figure 4D shows localization of the novel receptor tyrosine kinase protein of the invention in whole-mount sections of 9.5 day old mouse embryo; Figure 4E shows localization of the novel receptor tyrosine kinase protein of the invention in whole-mount sections of 9.5 day old mouse embryo in greater detail than in Figure 4D; Figure 4F shows localization of the novel receptor tyrosine kinase protein of the invention in paraffin serial transverse sections of 9.5 day old mouse embryo; Figure 4G shows localization of the novel receptor tyrosine kinase protein of the invention in paraffin serial transverse sections of 9.5 day old mouse embryo; Figure 4H shows localization of the novel receptor tyrosine kinase protein of the invention in paraffin sagittal section along the midline of 10.5 day old mouse embryo; Figure 41 shows localization of the novel receptor tyrosine kinase protein of the invention in paraffin adjacent transverse sections of 10.5 day old mouse embryo immunoreacted with anti-Nuk protein antibodies; Figure 4J shows localization of the novel receptor tyrosine kinase protein of the invention in paraffin adjacent transverse sections of 10.5 day old mouse embryo immunoreacted with a trpE-Nuk peptide; Figure 5A shows an adjacent transverse section of an 11.5 day embryo at the level of the caudal/posterior spinal cord immunoreacted with anti-Nuk antibodies; Figures 5B shows an adjacent transverse section of an 11 day embryo at the level of the caudal/posterior spinal cord immunoreacted with anti-Nuk antibodies preincubated with a trpE-Nuk peptide; Figures 5C shows transmission electron microscopy immunolocalization of Nuk protein in ventral midbrain cells of a 9.5 day embryo; Figure 5D shows transmission electron microscopy (EM) immunolocalization of Nuk protein in ventral midbrain cells of a 9.5 day embryo; Figure 6A is a photomicrograph of the head of an 11.5 day anti-Nuk whole-mount immunostained embryo with one of the pair of oculomotor nerve fibers in focus showing strong labelling for Nuk protein (filled arrow); Figure 6B shows a clearer view of Nuk protein staining in the oculomotor axons fibers obtained when the whole-mount staged embryo was filleted down the midline to minimize tissue thickness; Figure 6C shows a paraffin section of a 10.5 to 11 day mouse embryo immunostained with anti-Nuk; Figure 6D shows a paraffin section of a 10.5 to 11 day mouse embryo immunostained with anti-Neurofilament antibodies. Figure 6E is a frontal section immunostained with anti-Nuk antibodies which label the ventral midbrain and oculomotor axon fibers as they exit the neural tube (filled arrow) and extend (open arrow) towards their target tissue, the pre-optic muscle mass; Figure 6F shows a sagittal section immunostained with anti-Nuk antibodies which label the oculomoter axon fascicule as it enters the pre-optic muscle mass (open arrow); Figure 7A is a 10.5 day whole-mount embryo immunostained with anti-Nuk antibodies which label the trigeminal nerve V and facial nerve VII; Figures 7B shows transverse sections of a 10.5 day embryo showing Nuk-positive trigeminal nerve V axon fascicules labelled with the anti-Nuk antibodies; Figures 7C shows transverse sections of a 10.5 day embryo showing Nuk-positive trigeminal nerve V axon fascicules labelled with the anti-Nuk antibodies; Figures 7D is an 11.5 day anti-Nuk whole-mount showing localization of Nuk protein in the vagus nerve X associated fibers as they pathfind to their target visceral organs (curved open arrows); Figures 7E is an 11.5 day anti-Nuk whole-mount showing localization of Nuk protein in the vagus nerve X associated fibers as they pathfind to their target visceral organs (curved open arrows); Figure 7F is a whole-mount 10 day embryo showing Nuk protein concentrated within the earliest spinal nerve fibers exiting the neural tube (arrows); Figure 7G shows a slightly later stage embryo from that shown in Figure 7F; Figures 7H shows a transverse section bisecting the rostral spinal cord of an 11 day embryo demonstrating that the darkly stained Nuk-positive fibers shown in Figures 7F and 7G are ventral motor axons (open arrows). Figures 71 shows a transverse section bisecting the rostral spinal cord of an 11 day embryo demonstrating that the darkly stained Nuk-positive fibers shown in Figures 7F and 7G are ventral motor axons (open arrows). Figures 7J shows both sides of an 11 day whole-mount embryo demonstrating that Nuk protein is localized within the spinal motor nerves as they elongate to the plexus regions (open arrows); Figures 7K shows both sides of an 11 day whole-mount embryo demonstrating that Nuk protein is localized within the spinal motor nerves as they elongate to the plexus regions; Figure 7L shows a close-up of a 12 day whole-mount embryo immunostained with anti-Nuk antibodies which label the nerve fibers of the parasympathetic chain ganglion; Figure 8A shows a whole-mount 10.5 day embryo showing Nuk protein localization (arrow) at the dorsal region of the otic vesicle (ov) surrounding the budding endolymphatic duct; Figure 8B shows a whole-mount 11.5 day embryo showing elongation of the endolymphatic duct; Figure 8C shows a whole-mount 11.5 day embryo at a slightly different focal plane from Figure 8B showing that the vestibulocochlear sensory fibers connecting to the developing ear stain positive for Nuk protein (arrow); Figure 8D shows a transverse section of an 11.5 day embryo showing high levels of Nuk protein localized to the basement membrane of the endolymphatic duct cells where they contact the surrounding mesenchymal cells (open arrow; Figure 9 shows a recombinant DNA molecule of the invention having a null mutation obtained by deletion of exon 2, corresponding to codons 29 to 50 as shown in SEQ ID NO: 1; Figure 10 shows a recombinant DNA molecule of the invention encoding the N u fi mutation in the ATP binding region of the kinase domain of Nuk protein, and a lac Z reporter gene; Figure 11A shows expression of the Nuk 2 mutation in a mouse embryo at the six somite stage (8.25 days development) in the brain and developing heart; Figure 11B shows expression of the N k 2 mutation in a mouse embryo at the 14 somite stage (8.75 days development) in the hindbrain rhombomeres, the midbrain, diencephalon and in the heart (B) Figure 12A shows Nuk 2 expression in a 10.5 day old mouse embryo in the ventral midbrain, dienchephalon and retinal cells; Figure 12B shows Nuk 2 expression in a 10.5 day old mouse embryo in the brain and spinal cord; Figure 13 shows an immunoblot illustrating that Nuk protein autophosphorylation is induced by Elk-ligand stimulation; Figure 14 is an immunoblot showing expression of Nuk protein in cell lines; Figure 15 is an immunoblot showing binding of phosphorylated/non-phosphorylated Nuk protein to SH2-containing GST- fusion proteins; Figure 16 is an immunoblot showing binding of phosphorylated/non-phosphorylated Nuk protein to SH2-containing GST- fusion proteins; Figure 17 is an immunoblot showing binding of Nuk Protein to SH2- containing GST-fusion proteins in the presence of a competing phosphorylated peptide; Figure 18 shows immunoblots illustrating the phosphorylation of proteins after ELK-ligand stimulation of COS cells. DETAILED DESCRIPTION OF THE INVENTION I. Characterization of Nucleic Acid Molecules and Proteins of the Invention As hereinbefore mentioned, the present inventors have identified and sequenced a nucleic acid molecule encoding a novel receptor tyrosine RECTIFIED. SHEET (RULE..91) ISA/EP kinase protein with a unique expression pattern as described herein. The receptor tyrosine kinase protein of the invention is also referred to herein as Neural Kinase (Nuk) protein. The Nuk coding region was cloned using a λgtlO expression library constructed from mouse embryo mRNA. The library was probed with a partial n Ql Nuk cDNA insert. Additional 5'Nuk coding sequences were obtained by rapid amplification of cDNA ends (RACE). Translation of combined RACE and cDNA clones revealed a single open reading frame of 994 codons. The Nuk locus mapped to the distal end of mouse chromosome 4 near the QM mutation. The Nuk protein belongs to the EphlElklEck family, of which many members are expressed in the developing nervous system. The protein encoded by the deduced amino acid sequence of Nuk has all the hallmarks of an Eph family member, including a number of conserved residues of the Eph family, for example the 20 cysteine residues whose position is conserved in the extracelluar domain of Eph family members (bold type, Figure 2), an immunoglobulin-like domain near the amino terminus (Ig-like), and two fibronectin type HI repeats (FN HI); between Nuk residues 330-420 and 444-534. The Ig-like domain of Nuk protein contains specific residues (Cys 70 , Trp 80 , Cys 115 ) known to be conserved in the Ig superfamily (Williams and Barclay, Ann. Rev. Immunol 6:381-405, 1988). When compared to other known members of the Eph family, Nuk protein was found to be most highly related to the full length amino acid sequence of chicken Cek5 (96% identity). In accordance with an embodiment of the invention a purified and isolated nucleic acid molecule is provided containing a sequence encoding a protein having the amino acid sequence as shown in SEQ ID NO:2 and Figure 2. Most preferably, the purified and isolated nucleic acid molecule of the invention contains a nucleic acid sequence as shown in SEQ ID NO:l and Figure 1. Fragments of the nucleic acid molecules are contemplated by the present invention. In a preferred embodiment, the fragments include fragments of the nucleotide sequence as shown in SEQ. ID. NO. 1 and in Figure 1 that have at least 15 bases to 18 bases, preferably at least 15 bases, and which are capable of hybridizing to the nucleotide sequence as shown in SEQ ID NO. 1 and Figure 1 under stringent hybridization conditions as described herein. These fragments may encode, for example, the extracellular domain (amino acids 26 to 548, SEQ ID NO:2) or the carboxy tail (amino acids 601 to 994, SEQ ID NO:2). It will also be appreciated that a double stranded nucleotide sequence comprising a nucleic acid molecule of the invention or a fragment thereof, hydrogen bonded to a complementary nucleotide base sequence, and an RNA made by transcription of this double stranded nucleotide sequence are contemplated by the present invention. Further, it will be appreciated that the invention includes nucleic acid or amino acid sequences which have substantial sequence identity with the nucleic acid and amino acid sequences shown in SEQ ID NOS:l and 2 and in Figures 1 and 2, and fragments thereof. The term "sequences having substantial sequence identity" means those nucleic acid and amino acid sequences which have slight or inconsequential sequence variations from the sequences disclosed in Figures 1 and 2 and SEQ ID NOS: 1 and 2, i.e. the homologous sequences function in substantially the same manner to produce substantially the same polypeptides as the actual sequences. The variations may be attributable to local mutations or structural modifications. Nucleic acid sequences having substantial identity include nucleic acid sequences which encode proteins having at least 97% sequence identity with the amino acid sequences as shown in SEQ. ID. NO:2 and in Figure 2; nucleic acid sequences having at least 85%, preferably at least 90%, most preferably at least 95% identity with the nucleic acid sequence as shown in SEQ. ID. NO.:l and in Figure 1; and fragments thereof having at least 15 to 18, preferably at least 15 bases which will hybridize to these sequences under stringent hybridization conditions. Stringent hybridization conditions are those which are stringent enough to provide specificity, reduce the number of mismatches and yet are sufficiently flexible to allow formation of stable hybrids at an acceptable rate. Such conditions are known to those skilled in the art and are described, for example, in Sambrook, et al, (1989, Molecular Cloning, A Laboratory Manual, Cold Spring Harbor). By way of example only, stringent hybridization with short nucleotides may be carried out at 5-10° below the T m using high concentrations of probe such as 0.01-1. Opmole/ml. The invention further provides amino acid sequences which have substantial identity with the amino acid sequence shown in SEQ ID NO:2 and in Figure 2. Substantially identical sequences include sequences having at least 97% sequence identity. The invention still further provides peptides which are unique to the receptor tyrosine kinase protein of the invention. Preferably, the peptides have at least 10 to 20 amino acids. The sequence of the nucleic acid molecule of the invention or a fragment thereof, may be inverted relative to its normal presentation for transcription to produce antisense nucleic acid molecules. The antisense nucleic acid molecules may be constructed using chemical synthesis and enzymatic ligation reactions using procedures known in the art. The antisense nucleic acid molecules may be used in gene therapy to treat inherited disorders of the nervous system. A number of unique restriction sequences for restriction enzymes are incorporated in the nucleic acid sequence identified in SEQ ID NO: 1 and in Figure 1 and these provide access to nucleic acid sequences which code for polypeptides unique to the receptor tyrosine kinase protein of the invention. Nucleic acid sequences unique to the receptor tyrosine kinase protein of the invention or isoforms or parts thereof, can also be constructed by chemical synthesis and enzymatic ligation reactions carried out by procedures known in the art. The invention contemplates isoforms of the receptor tyrosine kinase protein of the invention. An isoform contains the same number and kinds of amino acids as the protein of the invention, but the isoform has a different molecular structure. The isoforms contemplated by the present invention are those having the same functional properties as the novel receptor tyrosine kinase protein of the invention as described herein. The present invention also includes conjugates of the receptor tyrosine kinase protein of the invention, or parts thereof. For example, the receptor tyrosine kinase protein or portions thereof may be conjugated with a selected protein or marker protein to produce fusion proteins. The present invention also includes a receptor tyrosine kinase protein of the invention or part thereof, preferably the catalytic domain, which is enzymatically active. The catalytically active form of the protein or part thereof is also referred to herein as an "activated receptor tyrosine kinase protein or part thereof. 1L Expression Pattern of the Receptor Tyrosine Kinase Protein of the Invention The expression of Nuk protein has been localized during early embryogenesis. The restricted expression of Nuk imposes constraints on the cellular range of activity of the putative Nuk ligand, and indicates that the Nuk locus plays unique and important roles in the determination, migration and pathfinding of axons, in axogenesis and fasciculation, in neural tube formation, and in the regulation of specific cell-cell interactions during early development of the nervous system. As many features of regeneration in the peripheral nervous system recapitulate development, for example, following injury and wallerian degeneration, axons regrow and migrate to reinervate their targets, the Nuk locus also plays an important role in axonal migration during regeneration following injury to the peripheral nervous system. The present inventors have shown that, in the early stages of embryonic development, Nuk protein is confined to the developing nervous system, where it marks segments along the axis of the neural tube in the hindbrain and specific morphological bulges of the midbrain and forebrain. Nuk is expressed in a rhombomere-specific pattern early during hindbrain segmentation. The restriction of Nuk protein to specific anterior-posterior and dorsal- ventral compartments during early development of the rostral neural tube indicates this receptor tyrosine kinase protein functions in the patterning of specific brain structures. Nuk protein was found to be expressed in the developing nervous system and, in particular, is highly expressed very early in the retinal ganglion cells and in the group of cells that form the optic chiasm just prior to axonogenesis of the retinal cells. These observations indicate that Nuk protein participates in early development of the visual system components and in pathfinding of retinal axons. The present inventors have also detected Nuk in cells of the ventral midbrain and in the endolymphatic duct of the developing ear. The present inventors have also localized Nuk protein to specific locations within the cells of the developing nervous system and have shown that Nuk protein is associated with the plasma membrane of migrating neural cells. In particular, Nuk protein is concentrated at sites of cell-cell contact, of migrating neuronal cells or their extensions and high levels of Nuk protein are found within initial axon outgrowths and associated nerve fibers. The axonal localization of Nuk is transient and is not detected after the growth cones have reached their targets and migrations have ceased, indicating a role for this receptor tyrosine kinase protein during the early migration, pathfinding and fasciculation stages of axonogenesis. Nuk protein may function to transmit signals from the plasma membrane and may cooperate with other neuronal tyrosine kinases, such as Sek. The present inventors found that Nuk protein is localized to at least one CNS axon pathway. The subcellular localization of Nuk protein is similar to that observed for vimentin and the extracelluar matrix molecule laminin (Liesi, EMBO 4:1163-1170, 1985) and it coincides with pathways of neuronal cell migration along the radial glial fibers (Hatten, Trends Neurosci. 13:179-184, 1990). The specific subcellular localization of Nuk protein to the cell-cell contacts between the basement membrane of the endolymphatic duct cells and the surrounding mesenchyme /neural crest cells indicates Nuk protein functions to modulate this interaction. The concentration of Nuk protein at sites of cell-cell contact indicates that its ligand is a membrane-associated molecule. In addition, Nuk protein immunoreactivity was frequently observed on the membranes of both cells at the site of contact and was generally observed to localize to specific regions of the membrane (see Figures 5 and 8). Therefore, homophilic/heterophilic interactions between Eph receptors may play a role in their biological functions. III. Preparation of Nucleic Acid Molecules and Proteins of the Invention The nucleic acid molecules of the invention encoding the novel receptor tyrosine kinase protein, or fragments thereof, may be isolated and sequenced, for example, by synthesizing cDNAs from mouse embryo RNA and using rapid amplification of cDNA ends (RACE, Frohman, et al., Proc. Na+, Acad. Sd. USA, 85, 8998-9002, 1988) using oligonudeotides spedfic for the novel receptor tyrosine kinase protein, and analysing the sequences of the clones obtained following amplification. Oligonu eotides specific for the novel receptor tyrosine kinase protein may be identified by comparing the nucleic acid sequence of the nudeic acid molecules of the invention to known sequences, for example, sequences of the other members of the Eph subfamily. Nucleic acid molecules of the present invention encoding the novel receptor tyrosine kinase protein and oligonucleotide fragments thereof, may also be constructed by chemical synthesis and enzymatic ligation reactions using procedures known in the art. The novel tyrosine kinase receptor protein of the invention may be prepared using recombinant DNA methods. Accordingly, the nucleic add molecules of the present invention having a sequence which codes for the receptor tyrosine kinase protein of the invention, or a fragment thereof may be incorporated in a known manner into an appropriate expression vector which ensures good expression of the protein or part thereof. Possible expression vectors include but are not limited to cosmids, plasmids, or modified viruses, so long as the vector is compatible with the host cell used. The invention therefore contemplates a recombinant molecule of the invention containing a nucleic acid molecule of the invention, or a fragment thereof, and the necessary elements for the transcription and translation of the inserted protein-sequence. Suitable transcription and translation elements may be derived from a variety of sources, including bacterial, fungal, viral, mammalian, or insect genes. Selection of appropriate transcription and translation elements is dependent on the host cell chosen as discussed below, and may be readily accomplished by one of ordinary skill in the art. Examples of such elements include: a transcriptional promoter and enhancer or RNA polymerase binding sequence, a ribosomal binding sequence, including a translation initiation signal. Additionally, depending on the host cell chosen and the vector employed, other genetic elements, such as an origin of replication, additional DNA restriction sites, enhancers, and sequences conferring indudbility of transcription may be incorporated into the expression vector. It will also be appreciated that the necessary transcriptional and translation elements may be supplied by the native receptor tyrosine kinase protein and/or its flanking regions. The recombinant molecules of the invention may also contain a reporter gene which facilitates the selection of host cells transformed or transfected with a recombinant molecule of the invention. Examples of reporter genes are genes encoding a protein such as β-galactosidase, chloramphenicol acetyltransferase, firefly luciferase, or an immunoglobulin or portion thereof such as the Fc portion of an immunoglobulin preferably IgG. In a preferred embodiment, the reporter gene is lac Z. Transcription of the reporter gene is monitored by changes in the concentration of the reporter protein such as β-galactosidase, chloramphenicol acetyltransferase, or firefly luciferase. This makes it possible to visualize and assay for expression of recombinant molecules of the invention and in particular to determine the effect of a mutation on expression and phenotype. Recombinant molecules can be introduced into host cells via transformation, transfection, infection, electroporation etc. Methods for transforming transfecting, etc. host cells to express foreign DNA are well known in the art (see, e.g., Itakura et al., U.S. Patent No. 4,704,362; Hinnen et al., PNAS USA 75:1929-1933, 1978; Murray et al., U.S. Patent No. 4,801,542; Upshall et al., U.S. Patent No. 4,935,349; Hagen et al., U.S. Patent No. 4,784,950; Axel et al, U.S. Patent No. 4,399,216; Goeddel et al., U.S. Patent No. 4,766,075; and Sambrook et al. Molecular Cloning A Laboratory Manual, 2nd edition, Cold Spring Harbor Laboratory Press, 1989, all of which are incorporated herein by reference and see the detailed discussion below). Suitable host cells include a wide variety of prokaryotic and eukaryotic host cells, including bacterial, mammalian, yeast or other fungi, viral, plant, or insect cells. More particularly, bacterial host cells suitable for carrying out the present invention indude E. coli, B. subtilis, Salmonella typhimurium, and various species within the genus' Pseudomonas, Streptomyces, and Staphylococcus, as well as many other bacterial species well known to one of ordinary skill in the art. Representative examples of baderial host cells include DH5n (Stratagene, Lajolla, California), JM109 ATCC No. 53323, HB101 ATCC No. 33694, and MN294. Suitable bacterial expression vectors preferably comprise a promoter which functions in the host cell, one or more selectable phenotypic markers, and a bacterial origin of replication. Representative promoters include the β-ladamase (penicillinase) and lactose promoter system (see Chang et al., Nature 275:615, 1978), the trp promoter (Nichols and Yanofsky, Meth in Enzymology 101:155, 1983) and the tac promoter (Russell et al., Gene 20: 231, 1982). Representative selectable markers include various antibiotic resistance markers such as the kanamycin or ampicillin resistance genes. Suitable expression vectors include but are not limited to bacteriophages such as lambda derivatives or plasmids such as pBR322 (see Bolivar et al., Gene 2:9S, 1977), the pUC plasmids pUC18, pUC19, pUC118, pUC119 (see Messing, Meth in Enzymology 101:20-77, 1983 and Vieira and Messing, Gene 19:259-268, 1982), and pNH8A, pNH16a, pNH18a, and Bluescript M13 (Stratagene, La Jolla, Calif.). Yeast and fungi host cells suitable for carrying out the present invention include, among others Saccharomyces cerevisae, the genera Pichia or Kluyveromyces and various species of the genus Aspergillus. Suitable expression vectors for yeast and fungi include, among others, YC p 50 (ATCC No. 37419) for yeast, and the amdS cloning vector pV3 (Turnbull, Bio /Technology 7:169, 1989). Protocols for the transformation of yeast are also well known to those of ordinary skill in the art. For example, transformation may be readily accomplished either by preparation of spheroplasts of yeast with DNA (see Hinnen et al., PNAS USA 75:1929, 1978) or by treatment with alkaline salts such as LiCl (see Itoh et al., J. Bacteriology 153:163, 1983). Transformation of fungi may also be carried out using polyethylene glycol as described by Cullen et al. (Bio /Technology 5:369, 1987). Mammalian cells suitable for carrying out the present invention include, among others: COS (e.g., ATCC No. CRL 1650 or 1651), BHK (e.g., ATCC No. CRL 6281), CHO (ATCC No. CCL 61), HeLa (e.g., ATCC No. CCL 2), 293 (ATCC No. 1573) and NS-1 cells. Suitable expression vectors for directing expression in mammalian cells generally include a promoter, as well as other transcriptional and translational control sequences. Common promoters include SV40, MMTV, metallothionein-1, adenovirus Ela, CMV, immediate early, immunoglobulin heavy chain promoter and enhancer, and RSV-LTR. Protocols for the transfection of mammalian cells are well known to those of ordinary skill in the art. Representative methods indude caldum phosphate mediated electroporation, retroviral, and protoplast fusion-mediated transfection (see Sambrook et al., supra). Given the teachings provided herein, promoters, terminators, and methods for introducing expression vectors of an appropriate type into plant, avian, and insect cells may also be readily accomplished. For example, within one embodiment, Nuk or derivatives thereof may be expressed from plant cells (see Sinkar et al., J. Biosd (Bangalore) 11:47-58, 1987, which reviews the use of Agrobacterium rhizogenes vectors; see also Zambryski et al., Genetic Engineering, Principles and Methods, Hollaender and Setlow (eds.), Vol. VI, pp. 253-278, Plenum Press, New York, 1984, which describes the use of expression vectors for plant cells, including, among others, pAS2022, pAS2023, and pAS2034). Insect cells suitable for carrying out the present invention include cells and cell lines from Bombyx or Spodotera species. Suitable expression vectors for directing expression in insect cells include Baculoviruses such as the Autographa California nudear polyhedrosis, virus (Miller et al. 1987, in Genetic Engineering, Vol. 8 ed. Setler, J.K. et al., Plenum Press, New York) and the Bombyx mori nuclear polyhedrosis virus (Maeda et al., 1985, Nature 315:592). Alternatively, Nuk may be expressed in non-human trans genie animals such as, mice, rats, rabbits, sheep and pigs (see Hammer et al. (Nature 315:680-683, 1985), Palmiter et al. (Sdence 222:809-814, 1983), Brinster et al. (Proc Natl. Acad. Sd USA 82:44384442, 1985), Palmiter and Brinster (Cell. 41:343-345, 1985) and U.S. Patent No. 4,736,866). The Nuk protein or parts thereof may also be prepared by diemical synthesis using techniques well known in the chemistry of proteins such as solid phase synthesis (Merrifield, 1964, J. Am. Chem. Assoc. 85:2149-2154) or synthesis in homogenous solution (Houbenweyl, 1987, Methods of Organic Chemistry, ed. E. Wansch, Vol. 15 I and π, Thieme, Stuttgart). Conjugates of the Nuk protein of the invention, or parts thereof, with other molecules, such as proteins or polypeptides, may be prepared. This may be accomplished, for example, by the synthesis of N-terminal or C-terminal fusion proteins. Thus, fusion proteins may be prepared by fusing, through recombinant techniques, the N-terminal or C-terminal of Nuk protein or parts thereof, and the sequence of a selected protein or marker protein with a desired biological function. The resultant fusion proteins contain Nuk protein or a portion thereof fused to the selected protein or marker protein as described herein. Examples of proteins which may be used to prepare fusion proteins indude immunoglobulins and parts thereof sudi as the constant region of immunglobulin γl, and lymphokines such as gamma interferon, tumor necrosis factor, IL-1, IL-2,IL-3, H-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-11, GM-CSF, CSF-1 and G-CSF. Sequences which encode the above-described proteins may generally be obtained from a variety of sources, including for example, depositories which contain plasmids encoding sequences including the American Type Culture Collection (ATCC, Rockville Maryland), and the British Biotechnology Limited (Cowley, Oxford England). Examples of such plasmids include BBG 12 (containing the GM-CSF gene coding for the mature protein of 127 amino acids), BBG 6 (which contains sequences encoding gamma interferon), ATCC No. 39656 (which contains sequences encoding TNF), ATCC No. 20663 (which contains sequences encoding alpha interferon,) ATCC Nos. 31902 and 39517 (which contains sequences encoding beta interferon), ATCC No. 67024 (which contains a sequence which encodes Interleukin-lβ), ATCC Nos. 39405, 39452, 39516, 39626 and 39673 (which contains sequences encoding Interleukin-2), ATCC Nos. 59399, 59398, and 67326 (which contain sequences encoding Interleukin-3), ATCC Nos. 57592 (which contains sequences encoding Interleukin-4). ATCC Nos. 59394 and 59395 (which contain sequences encoding Interleukin-5), and ATCC No. 67153 (which contains sequences encoding Interleukin-6). Within a particularly preferred embodiment of the invention, Nuk is doned into an expression vector as a fusion gene with the constant region of human immunoglobulin γl. Briefly, the expression vectors pNUTΔGH and pVL1393 are prepared for cloning by digestion with Smal followed by dephosphorylation by calf intestinal alkaline phosphatase. The linear produd is isolated after agarose gel electrophoresis. The Nuk genes are then generated by polymerase chain reaction using the cloned Nuk cDNA as a template. In particular, the Nuk fusion protein is synthesized from the extracelluar domain of Nuk protein, preferably amino adds 26 to 548, SEQ ID NO: 2 and Figure 2. In another embodiment, the Nuk fusion protein is synthesized from the carboxy terminal tail of Nuk protein, preferably amino acids 601 to 994, SEQ ID NO:2 and Figure 2. The constant region of an immunoglobulin, such as human γl gene may be prepared, for example, from pUCB7Ig monomer. Briefly, the CH gene is isolated by digestion with Xbal which cuts at the 3' end of the gene followed by treatment with E. coli DNA polymerase I in the presence of all four dNTPs in order to create a blunt end. The plasmid is then digested with Bdl which cuts at the 5' end of the gene. The fragment containing the heavy chain gene is isolated after electrophoresis in an agarose gel. The fusion Nuk amplified fragment is inserted into each prepared vector along with the heavy chain fragment. Orientation of the resulting plasmids is determined by PCR with one priming oligo which anneals to the vector sequence and the other priming oligo which anneals to the insert sequence. Alternatively, appropriate restriction digests can be performed to verify the orientation. The sequence of the fusion Nuk immunoglobulin constant region gene can be verified by DNA sequencing. Phosphorylated receptor tyrosine kinase proteins of the invention, or parts thereof, may be prepared using the method described in Reedijk et al. The EMBO Journal 11(4):1365, 1992. For example, tyrosine phosphorylation may be induced by infecting bacteria harbouring a plasmid containing a nucleotide sequence of the invention or fragment thereof, with a λgtll bacteriophage encoding the cytoplasmic domain of the Elk tyrosine kinase. Bacteria containing the plasmid and bacteriophage as a lysogen are isolated. Following induction of the lysogen, the expressed receptor protein becomes phosphorylated. IV. Utility of the Nucleic Acid Molecules and Proteins of the Invention The nudeic add molecules of the invention or fragments thereof, allow those skilled in the art to construct nucleotide probes for use in the detection of nucleotide sequences in biological materials. A nucleotide probe may be labelled with a detectable substance such as a radioactive label which provides for an adequate signal and has suffident half-life sudi as 3 2P, 3H, 14C or the like. Other detectable substances which may be used include antigens that are recognized by a specific labelled antibody, fluorescent compounds, enzymes, antibodies specific for a labelled antigen, and luminescent compounds. An appropriate label may be selected having regard to the rate of hybridization and binding of the probe to the nudeotide to be detected and the amount of nucleotide available for hybridization. Labelled probes may be hybridized to nucleic acids on solid supports such as nitrocellulose filters or nylon membranes as generally described in Sambrook et al, 1989, Molecular Cloning, A Laboratory Manual (2nd ed.). The nucleotide probes may be used to detect genes, preferably in human cells, that encode proteins related to or analogous to the receptor tyrosine kinase protein of the invention. The nucleotide probes may therefore be useful in the diagnosis of disorders of the nervous system arising from mutations or alterations to the Nuk gene or a homologue thereof. The receptor tyrosine kinase protein of the invention and portions thereof, for example amino acids of the carboxy terminal tail, preferably amino acids 601 to 994; or amino acids of the extracellular domain, preferably amino adds 26 to 548 (SEQ ID NO: 2 and Figure 2), may be used to prepare antibodies. Antibodies having specifidty for Nuk protein may also be raised from fusion proteins created by expressing trpE-Nuk fusion proteins in baderia as described above. Within the context of the present invention, antibodies are understood to indude monoclonal antibodies, polyclonal antibodies, antibody fragments (e.g., Fab, and F(ab') 2 and recombinantly produced binding partners. Antibodies are understood to be reactive against Nuk protein if they bind with a K a of greater than or equal to 10- 7 M. As will be appredated by one of ordinary skill in the art, antibodies may be developed which not only bind to Nuk protein, but which bind to a ligand of Nuk protein, and which also block the biological activity of Nuk protein. Such antibodies will be useful in the diagnosis and treatment of disorders of the nervous system and nerve damage. Polyclonal antibodies may be readily generated by one of ordinary skill in the art from a variety of warm-blooded animals such as horses, cows, various fowl, rabbits, mice, or rats. Briefly, Nuk protein is utilized to immunize the animal through intraperitoneal, intramuscular, intraocular, or subcutaneous injections, in conjunction with an adjuvant such as Freund's complete or incomplete adjuvant. Following several booster immunizations, samples of serum are collected and tested for reactivity to Nuk protein. Particularly preferred polyclonal antisera will give a signal on one of these assays that is at least three times greater than bad ground. Once the titer of the animal has reached a plateau in terms of its reactivity to Nuk protein, larger quantities of antisera may be readily obtained either by weekly bleedings, or by exsanguinating the animal. Monoclonal antibodies may also be readily generated using conventional techniques (see U.S. Patent Nos. RE 32,011, 4,902,614, 4,543,439, and 4,411,993 which are incorporated herein by reference; see also Monodonal Antibodies, Hybridomas: A New Dimension in Biological Analyses, Plenum Press, Kennett, McKearn, and Bechtol (eds.), 1980, and Antibodies: A Laboratory Manual, Harlow and Lane (eds.), Cold Spring Harbor Laboratory Press, 1988, which are also incorporated herein by reference). Briefly, within one embodiment a subject animal such as a rat or mouse is injected with Nuk protein. The Nuk protein may be admixed with an adjuvant such as Freund's complete or incomplete adjuvant in order to increase the resultant immune response. Between one and three weeks after the initial immunization the animal may be reimmunized with another booster immunization, and tested for reactivity to Nuk protein using assays described above. Once the animal has plateaued in its reactivity to Nuk protein, it is sacrificed, and organs which contain large numbers of B cells such as the spleen and lymph nodes are harvested. Cells which are obtained from the immunized animal may be immortalized by transfection with a virus such as the Epstein bar virus (EBV) (see Glasky and Reading, Hybridoma 8(4):377-389, 1989). Alternatively, within a preferred embodiment, the harvested spleen and/or lymph node cell suspensions are fused with a suitable myeloma cell in order to create a "hybridoma" which secretes monoclonal antibody. Suitable myeloma lines indude, for example, NS-1 (ATCC No. TIB 18), and P3X63 - Ag 8.653 (ATCC No. CRL 1580). Following the fusion, the cells may be placed into culture plates containing a suitable medium, such as RPMI 1640, or DMEM (Dulbecco's Modified Eagles Medium) (JRH Biosciences, Lenexa, Kansas), as well as additional ingredients, such as Fetal Bovine Serum (FBS, ie., from Hyclone, Logan, Utah, or JRH Biosdences). Additionally, the medium should contain a reagent which selectively allows for the growth of fused spleen and myeloma cells such as HAT (hypoxanthine, aminopterin, and thymidine) (Sigma Chemical Co., St. Louis, Missouri). After about seven days, the resulting fused cells or hybridomas may be screened in order to determine the presence of antibodies which are reactive against Nuk protein. A wide variety of assays may be utilized to determine the presence of antibodies which are reactive against Nuk protein, including for example Countercurrent Immuno-Electrophoresis, Radioimmunoassays, Radioimmunoprecipitations, Enzyme-Linked Immuno-Sorbent Assays (ELISA), Dot Blot assays, Inhibition or Competition Assays, and sandwich assays (see U.S. Patent Nos. 4,376,110 and 4,186,530; see also Antibodies: A Laboratory Manual, Harlow and Lane (eds.), Cold Spring Harbor Laboratory Press, 1988). Following several clonal dilutions and reassays, a hybridoma produdng antibodies reactive against Nuk protein may be isolated. Other techniques may also be utilized to construct monoclonal antibodies (see William D. Huse et al., "Generation of a Large Combinational Library of the Immunoglobulin Repertoire in Phage Lambda," Science 246:1275-1281, December 1989; see also L. Sastry et al., "Cloning of the Immunological Repertoire in Escherichia coli for Generation of Monoclonal Catalytic Antibodies: Construction of a Heavy Chain Variable Region-Specific cDNA Library," Proc Natl. Acad. Sci USA 86:5728-5732, August 1989; see also Michelle Alting-Mees et al., "Monoclonal Antibody Expression Libraries: A Rapid Alternative to Hybridomas," Strategies in Molecular Biology 3:1-9, January 1990; these references describe a commerdal system available from Stratacyte, La Jolla, California, which enables the production of antibodies through recombinant techniques). Briefly, mRNA is isolated from a B cell population, and utilized to create heavy and light chain immunoglobulin cDNA expression libraries in the λΙmmunoZap(H) and λΙmmunoZap(L) vectors. These vectors may be screened individually or co-expressed to form Fab fragments or antibodies (see Huse et al. supra; see also Sastry et al., supra). Positive plaques may subsequently be converted to a non-lytic plasmid which allows high level expression of monoclonal antibody fragments from I coli. Similarly, binding partners may also be constructed utilizing recombinant DNA techniques to incorporate the variable regions of a gene which encodes a specifically binding antibody. Within one embodiment, the genes which encode the variable region from a hybridoma producing a monoclonal antibody of interest are amplified using nucleotide primers for the variable region. These primers may be synthesized by one of ordinary skill in the art, or may be purchased from commerdally available sources. Primers for mouse and human variable regions including, among others, primers for Vπa VHb VHC/ VH ./ CHI/ V and CL regions are available from Stratacyte (La Jolla, Calif). These primers may be utilized to amplify heavy or light chain variable regions, which may then be inserted into vectors such as ImmunoZAP™H or ImmunoZAP™L (Stratacyte), respectively. These vectors may then be introduced into £. coli for expression. Utilizing these techniques, large amounts of a single-chain protein containing a fusion of the VH and VL domains may be produced (See Bird et al., Science 242:423-426, 1988). In addition, such techniques may be utilized to change a "murine" antibody to a "human" antibody, without altering the binding specifidty of the antibody. Once suitable antibodies or binding partners have been obtained, they may be isolated or purified by many techniques well known to those of ordinary skill in the art (see Antibodies: A Laboratory Manual, Harlow and Lane (eds.), Cold Spring Harbor Laboratory Press, 1988). Suitable techniques include peptide or protein affinity columns, HPLC or RP-HPLC, purification on protein A or protein G columns, or any combination of these techniques. The polyclonal or monoclonal antibodies may be used to detect the receptor tyrosine kinase protein of the invention in various biological materials, for example they may be used in an ELISA, radioimmunoassay or histochemical tests. Thus, the antibodies may be used to quantify the amount of a receptor tyrosine kinase protein of the invention in a sample in order to determine its role in particular cellular events or pathological states and to diagnose and treat such pathological states. In particular, the polyclonal and monoclonal antibodies of the invention may be used in immuno-histochemical analyses, for example, at the cellular and sub-subcellular level, to detect the novel receptor tyrosine kinase protein of the invention, to localise it to particular cells and tissues and to specific subcellular locations, and to quantitate the level of expression. Cytochemical techniques known in the art for localizing antigens using light and electron microscopy may be used to detert the novel tyrosine kinase of the invention. Generally, an antibody of the invention may be labelled with a detectable substance and the novel receptor tyrosine kinase of the invention may be localised in tissue based upon the presence of the detectable substance. Examples of detectable substances include various enzymes, fluorescent materials, luminescent materials and radioactive materials. Examples of suitable enzymes indude horseradish peroxidase, biotin, alkaline phosphatase, β-galactosidase, or acetylcholinesterase; examples of suitable fluorescent materials include umbelliferone, fluorescein, fluorescein isothiocyanate, rhodamine, dichlorotriazinylamine fluorescein, dansyl chloride or phycoerythrin; an example of a luminescent material includes luminol; and examples of suitable radioactive material include radioactive iodine I 125 , 1 1 31 or tritium. Antibodies may also be coupled to electron dense substances, such as ferritin or colloidal gold, which are readily visualised by electron microscopy. Radioactive labelled materials may be prepared by radiolabeling with 12 5 1 by the chloramine-T method (Greenwood et al, Biochem. J. £2:114, 1963), the lactoperoxidase method (Marchalonis et al, Biochem. J. 124:921. 1971), the Bolton-Hunter method (Bolton and Hunter, Biochem. J. 13_3_:529, 1973 and Bolton Review 18, Amersham International Limited, Buckinghamshire, England, 1977), the iodogen method (Fraker and Speck, Biochem. Biophys. Res. Commun. 8_Q_:849, 1978), the Iodo-beads method (Markwell Anal. Biochem. 125:427, 1982) or with tritium by reductive methylation (Tack et al., J. Biol. Chem. 255:8842, 1980). Known coupling methods (for example Wilson and Nakane, in "Immunofluorescence and Related Staining Techniques", W. Knapp et al, eds, p. 215, Elsevier /North-Holland, Amsterdam & New York, 1978; P. Tijssen and E. Kurstak, Anal. Biochem. 136:451. 1984) may be used to prepare enzyme labelled materials. Fluorescent labelled materials may be prepared by reacting the material with umbelliferone, fluorescein, fluorescein isothiocyanate, dichlorotriazinylamine fluorescein, dansyl chloride, derivatives of rhodamine such as tetramethyl rhodamine isothiocyanate, or phycoerythrin. Indirect methods may also be employed in which the primary antigen-antibody reaction is amplified by the introduction of a second antibody, having specifidty for the antibody reactive against the novel tyrosine kinase of the invention. By way of example, if the antibody having specificity against the novel tyrosine kinase protein of the invention is a rabbit IgG antibody, the second antibody may be goat anti-rabbit gamma-globulin labelled with a detectable substance as described herein. Where a radioactive label is used as a detectable substance, the novel tyrosine kinase of the invention may be localized by radioautography. The results of radioautography may be quantitated by determining the density of particles in the radioautographs by various optical methods, or by counting the grains. As discussed above, the expression patterns found for the novel tyrosine kinase of the invention indicate that it plays unique and important roles in the determination, migration and pathfinding of axons, in axogenesis and fasciculation, development and regeneration of the neural tube, and in the regulation of spedfic cell-cell interactions during early development of the nervous system. Therefore, the above described methods for detecting nucleic acid molecules and fragments thereof and Nuk protein and parts thereof, can be used to monitor vertebrate axonal migration, fasciculation and regeneration by detecting and localizing the novel tyrosine kinase protein of the invention in migrating axons and in the migrating membrane surface of cells of the developing nervous system. It would also be apparent to one skilled in the art that the above described methods may be used to study the developmental expression of Nuk and, accordingly, will provide further insight into the role of Nuk protein in neuronal celhcell interactions in embryogenic development and in axonogenesis and nerve regeneration. The finding of a novel receptor tyrosine kinase protein which is expressed in migrating axons and the developing neural tube permits the identification of substances which may affect axonogenesis, neural embryonic development and neuron regeneration. A substance which affects expression of Nuk protein may be assayed using the above described methods for detecting nudeic acid molecules and fragments thereof and Nuk protein and parts thereof, by comparing the pattern and level of expression of the Nuk protein or parts thereof, in the presence and absence of the substance. The invention also provides methods for identifying substances which are capable of binding to the Nuk protein, or isoforms and parts thereof. In particular, the methods may be used to identify ligands and natural and synthetic derivatives of such ligands, which are capable of binding to and in some cases activating the receptor tyrosine kinase protein of the invention, isoforms thereof, or part of the protein. Substances which can bind with the receptor tyrosine kinase protein of the invention may be identified by reacting the novel receptor tyrosine kinase protein which is expressed in migrating vertebrate axons isoforms thereof, or part of the protein, with a substance which potentially binds to the novel receptor tyrosine kinase protein, isoforms thereof, or part of the protein such as the extracelluar domain, and assaying for substance-receptor complexes, for free substance or for non-complexed receptor tyrosine kinase protein isoforms thereof or part of the protein, or for activation of the receptor tyrosine kinase protein. Conditions which permit the formation of substance-receptor protein complexes may be selected having regard to factors such as the nature and amounts of the substance and the receptor protein. The substance-receptor complex, free substance or non-complexed proteins may be isolated by conventional isolation techniques, for example, salting out, chromatography, electrophoresis, gel filtration, fractionation, absorption, polyacrylamide gel electrophoresis, agglutination, or combinations thereof. To facilitate the assay of the components, antibody against the receptor protein or the substance, or a labelled receptor protein, or a labelled substance may be utilized. Antibodies, receptor protein or substance may be labelled with a detectable substance as described above. The receptor tyrosine kinase protein, isoforms or parts thereof, or substance used in the method of the invention may be insolubilized. For example, the receptor protein or substance may be bound to a suitable carrier. Examples of suitable carriers are agarose, cellulose, dextran, Sephadex, Sepharose, carboxymethyl cellulose polystyrene, filter paper, ion-exchange resin, plastic film, plastic tube, glass beads, polyamine-methyl vinyl-ether-maleic acid copolymer, amino acid copolymer, ethylene-maleic acid copolymer, nylon, silk, etc. The carrier may be in the shape of, for example, a tube, test plate, beads, disc, sphere etc. The insolubilized receptor tyrosine kinase protein or substance may be prepared by reacting the material with a suitable insoluble carrier using known chemical or physical methods, for example, cyanogen bromide coupling. The receptor tyrosine kinase protein, parts thereof, or substance may also be expressed on the surface of a cell using the methods described herein. The above mentioned methods of the invention may be used to identify ligands which bind with and activate the novel receptor tyrosine kinase protein of the invention thereby affecting signalling pathways, particularly those involved in neuronal development and axonal migration and regeneration. Identification and isolation of such a Nuk protein ligand will permit studies of the role of the ligand in the developmental regulation of axonogenesis and neural regeneration, and permit the development of substances which affect these roles, such as functional or non-functional analogues of the ligand. It will be appredated that such substances will be useful as pharmaceuticals to modulate axonogenesis, nerve cell interactions and regeneration to treat conditions such as neurodegenerative diseases and cases of nerve injury. Ligands which bind to and activate the novel receptor tyrosine kinase protein of the invention may be identified by assaying for protein tyrosine kinase activity i.e. by assaying for phosphorylation of the tyrosine residues of the novel receptor tyrosine kinase protein. Receptor tyrosine kinase protein activity may be assayed using known techniques such as those using anti-phosphotyrosine antibodies and labelled phosphorous. For example, immunoblots of the complexes may be analyzed by autoradiography (32p-labelled samples) or may be blocked and probed with antiphosphotyrosine antibodies as described in Koch, CA. et al (1989) Mol. Cell. Biol. 9, 4131-4140. The ligands for many receptor tyrosine kinase proteins are cell-bound, either as they are associated with the cell surface via heparin and hepatocyte growth factor or because they are transmembrane proteins (Lyman et al. 1993, supra). Ligands for receptor tyrosine kinases of the EphlElklEck subfamily of receptor tyrosine kinases may require cell-to-cell contact to activate the receptor. Membrane attachment of the ligand could facilitate ligand dimerization or clustering, or both, which may promote receptor multimerization and activation. Accordingly, a ligand for Nuk protein may have a cell-bound form. A cell-bound ligand may be identified by reacting the receptor tyrosine kinase protein of the invention, an isoform or a part thereof with a cell suspeded of expressing the ligand on the surface of the cell following the procedures generally described in Lyman et al., 1993, (Cell 75:1157-1167). Thus, the invention provides a method for identifying cells expressing a surface bound ligand of Nuk protein and for specifically selecting for such cells. By way of example, a cDNA encoding a ligand for Nuk protein may be doned by first constructing a fusion protein. The fusion protein may consist of the extracelluar domain of Nuk protein (amino acids 26 to 548, SEQ ID NO: 2 and Figure 2). The fusion protein may be expressed and used as a probe to examine cells or cell lines (e.g. neuroblastoma or neuroepitheliomia cell lines) for their capacity to bind the extracellular domain of Nuk protein (determined by flow cytometry). The identification of cells and cell lines that bind the extracellular domain may be facilitated by incorporating in the fusion protein a sequence encoding a marker protein for example, the Fc portion of human IgG which may be detected with labelled anti-human IgG antibodies. Cells or cell lines which bind the extracellular domain are presumed to express a cell-bound form of the ligand. Following identification of a source of the Nuk ligand, a cDNA expression library is constructed, following known techniques, using mRNA from the cells /cell lines which have been identified as binding the fusion protein containing the extracellular domain of Nuk protein. cDNAs are then transfected into host cells (e.g. COS cells and see discussion herein re host cells) which are then screened for their capacity to bind the extracellular domain of Nuk protein. Individual clones whid are capable of binding the extracellular domain of Nuk protein are identified and the cDNAs are sequenced. The cDNAs may be used as hybridization probes to isolate genomic DNA encoding the ligand. The invention also contemplates a method for assaying for an agonist or antagonist of the binding of the novel receptor tyrosine kinase of the invention with a substance which is capable of binding with the novel tyrosine kinase protein, preferably a ligand. The agonist or antagonist may be an endogenous physiological substance or it may be a natural or synthetic drug. Substances which are capable of binding with the Nuk protein and preferably ligands, including cell-bound ligands may be identified using the methods set forth herein. Substances which bind to other receptor tyrosine kinases of the EphlElklEck subfamily may also be used in this assay. For example, the substance may be LERK-2, a binding protein for the receptor- tyrosine kinase ELK (Fletcher, F.A. et al., Oncogene (1994), 9, 3241-3247), or the cell-bound ligands B61 (also known as EFL-1), EHK1-L (also known as EFL-2) and ELK-L ( also known as EFL-3)(Davis, S. et al., Sdence Vol. 266, ρ.816, Nov. 4, 1994). In accordance with a preferred embodiment, a method is provided which comprises providing a known concentration of the novel receptor tyrosine kinase protein of the invention, incubating the protein with a ligand which can bind to and activate the protein, and a suspected agonist or antagonist under conditions which permit the formation of substance-receptor protein complexes, and assaying for substance-receptor protein complexes, for free substance, for non-complexed proteins, or for activation of the receptor tyrosine kinase protein. Conditions which permit the formation of substance-receptor protein complexes, and methods for assaying for substance-receptor protein complexes, for free substance, for non-complexed proteins, or for activation of the receptor tyrosine kinase protein are described herein. It will be understood that the agonists and antagonists that can be assayed using the methods of the invention may act on one or more of the binding sites on the receptor tyrosine kinase or the ligand, including agonist binding sites, competitive antagonist binding sites, non-competitive antagonist binding sites or allosteric sites. The invention also makes it possible to screen for antagonists that inhibit the effects of an agonist of the interaction of Nuk protein with a Nuk protein ligand. Thus, the invention may be used to assay for a substance that competes for the same ligand binding site of the novel receptor tyrosine kinase protein of the invention. The invention further contemplates a method for identifying a substance which is capable of binding to an activated receptor tyrosine kinase protein of the invention or an isoform or part of the activated protein, comprising reacting an activated receptor tyrosine kinase protein of the invention, or an isoform, or part of the protein, with at least one substance which potentially can bind with the receptor tyrosine kinase protein, isoform or part of the protein, under conditions which permit the formation of substance-receptor kinase protein complexes, and assaying for substance-receptor kinase protein complexes, for free substance, for non-complexed receptor kinase proteins, or for phosphorylation of the substance. An activated receptor tyrosine kinase protein of the invention, or isoform or part thereof may be prepared by binding of a ligand to the extracellular domain of a receptor tyrosine kinase protein of the invention which results in activation of the catalytic domain. Such a ligand may be identified using the methods hereinbefore described. An activated receptor or part thereof, may also be prepared using the methods described for example in Reedijk et al. The EMBO Journal, 11(4):1365, 1992 for producing a tyrosine phosphorylated receptor or part thereof. Conditions which permit the formation of substance-receptor protein complexes may be seleded having regard to factors such as the nature and amounts of the substance and the receptor protein. The substance-receptor complex, free substance or non-complexed proteins may be isolated by conventional isolation techniques described above. Phosphorylation of the substance may be determined using for example, labelled phosphorous as described above. In an embodiment of this method, intracellular ligands such as Src homology region 2 (SH2)-containing proteins which are capable of binding to a phosphorylated activated receptor tyrosine kinase protein of the invention may be identified. SH2-containing proteins refers to proteins containing a Src homology region 2 which is a noncatalytic domain of -100 amino acids which was originally identified in the Vfps and Vsrc cytoplasmic tyrosine kinases by virtue of its effects on both catalytic activity and substrate phosphorylation (T. Pawson, Oncogene 3, 491 (1988) and I. Sadowski et al., Mol. Cell. Biol. 6, 4396 (1986)). (See also Koch et al., Sdence 252:668, 1991; Moran et al., PNAS USA 87:8622 and Anderson et al., Science 250:979, 1990 for discussions on SH2-containing proteins and the role of SH2 domains). SH2-containing proteins may function downstream of the Nuk signalling pathway by binding to the activated receptor protein. For example, the cytoplasmic tyrosine kinases of the Src family may bind via their SH2 domains to the activated Nuk receptor protein thereby regulating cellular processes particularly in the nervous system. Intracellular ligands which may be phosphorylated by the novel receptor tyrosine kinase of the invention may also be identified using the method of the invention. SH2-domains of cytoplasmic signalling proteins have been found to bind to the phosphorylated receptor tyrosine kinase protein of the invention. In particular, the SH2 domains of p21«s GTPase-activating protein (GAP), Src, and phosphoinositide-spedfic phospholipase C (PLCγ) have been found to bind Nuk protein. The SH-2 domain binding site on the Nuk protein is a conserved tyrosine containing region which is located adjacent to the membrane and it corresponds to amino acids 600 to 618 as shown in SEQ. ID. NO:2 in the Sequence Listing. Therefore, the invention also contemplates a method for assaying for an agonist or antagonist of the binding of an activated receptor tyrosine kinase of the invention, or a portion thereof with an SH2 domain of an intracellular ligand. The agonist or antagonist may be an endogenous physiological substance or it may be a natural or synthetic drug. The activated receptor may be prepared as described herein or a portion of the ligand comprising the amino acid sequence 600 to 618 as shown in SEQ. ID. NO:2 in the Sequence Listing may be used in this method of the invention. Examples of SH2 domains of intracellular ligands indude the SH2 domains of GAP, Src, and PLCγ. It will be appreciated that the entire intracellular ligand may be used in this method. It has also been found that COS cells express large amounts of Nuk protein. Accordingly, COS cells may be used to identify in vivo, intracellular proteins or ligands which bind to the Nuk protein. The invention further provides a method for assaying for a substance that affects axonal migration, neural development, nerve cell interactions and nerve regeneration comprising administering to a non-human animal or to a tissue of an animal, a substance suspected of affecting axonal migration, and detecting, and optionally quantitating, the novel receptor tyrosine kinase of the invention in the non-human animal or tissue. In an embodiment of the invention, the method may be used to assay for a substance that affects axonal migration during embryogenesis. The novel receptor tyrosine kinase of the invention may be quantitated using the methods described herein. In another embodiment, the method may be used to assay for a substance that affects axonal migration in nerve regeneration, comprising administering a substance suspected of affecting axonal migration to a non-human animal having an injured peripheral nervous system and detecting, and optionally quantitating, the novel receptor tyrosine kinase of the invention in the non-human animal. Examples of non-human animals having an injured peripheral nervous system include animals having damaged axons, such as axotomized facial neurons (Sendtner et al. Nature, 345, 440-441, 1990), neurodegenerative conditions (for example, the MPTP model as described in Langston J.W. et al., Symposium of Current Concepts and Controversies in Parkinson's Disease, Montebello, Quebec, Canada, 1983 and Tatton W.G. et al., Can. J. Neurol. Sci. 1992, 19), and traumatic and non-traumatic peripheral nerve damage (for example, animal stroke models such as the one described in MacMillan et al. Brain Research 151:353-368 (1978)). Substances which are capable of binding to the Nuk protein of the invention or parts thereof, particularly ligands, and agonists and antagonists of the binding of ligands and Nuk protein, identified by the methods of the invention, may be used for stimulating or inhibiting neuronal development, regeneration and axonal migration. The ligands, agonists and antagonists may accordingly be used to stimulate or inhibit neuronal development, regeneration and axonal migration associated with neurodegenerative conditions and conditions involving trauma and injury to the nervous system, for example Alzheimer's disease, Parkinson's disease, Huntington's disease, demylinating diseases, such as multiple sclerosis, amyotrophic lateral sclerosis, bacterial and viral infections of the nervous system, deficiency diseases, such as Wernicke's disease and nutritional poly neuropathy, progressive supranuclear palsy, Shy Drager's syndrome, multistem degeneration and olivo ponto cerebellar atrophy, peripheral nerve damage, trauma and ischemia resulting from stroke. The invention also provides methods for studying the function of the Nuk protein. Cells, tissues, and non-human animals lacking in Nuk expression or partially lacking in Nuk expression may be developed using recombinant molecules of the invention having specific deletion or insertion mutations in the Nuk gene. For example, the extracellular domain or parts thereof, such as the FN m and Ig domains; the transmembrane region or parts thereof; the tyrosine kinase domain or parts thereof, such as the ATP binding site and; the carboxy terminal tail may be deleted. A recombinant molecule may be used to inactivate or alter the endogenous gene by homologous recombination, and thereby create a Nuk defiάent cell, tissue or animal. Null alleles may be generated in cells, such as embryonic stem cells by deletion mutation. A recombinant Nuk gene may also be engineered to contain an insertion mutation which inactivates Nuk. Such a construct may then be introduced into a cell, such as an embryonic stem cell, by a technique such as transfection, electroporation, injection etc. Cells lacking an intart Nuk gene may then be identified, for example by Southern blotting, Northern Blotting or by assaying for expression of Nuk protein using the methods described herein. Such cells may then be fused to embryonic stem cells to generate transgenic non-human animals deficient in Nuk. Germline transmission of the mutation may be achieved, for example, by aggregating the embryonic stem cells with early stage embryos, such as 8 cell embryos, in vitro; transferring the resulting blastocysts into recipient females and; generating germline transmission of the resulting aggregation chimeras. Such a mutant animal may be used to define spedfic nerve cell populations, developmental patterns of axonogenesis, neural tube formation and nerve regeneration and in vivo processes, normally dependent on Nuk expression. The present inventors have generated a loss of function deletion mutation in Nuk, designated Nu in mouse embryonic stem cells, and have achieved germline transmission of this null allele. In particular, the Nuk mutation was obtained by deletion of exon 2, corresponding to codons 29 to 50, as shown in Figure 9. Adult animals homozygous for the mutation did not produce any Nuk protein. A second targeted mutation, designated Nuk 2 was generated in the Nuk gene as shown in Figure 10 using the pPNT-LOX-Nuk 2 gene trap vector to delete the GXGXXG ATP binding region of the kinase domain (amino adds 623-707, SEQ ID NO:2 and Figure 2) and to create a Nuk-lac Z fusion receptor. Chimeric animals expressing Nu k 2 were prepared. Animals generated with the Nuk 2 mutation provided Nuk 2 expressing cells staining for β-galactosidase activity, providing a convenient marker for Nuk -positive cells in both heterozygous and homozygous backgrounds as deteded by a blue/ green colour as shown in Figures 11 and 12. The invention also provides methods for preparing cells, tissues, and non-human animals lacking in Nuk expression or partially lacking in Nuk expression, and deficient in the expression of other genes. In accordance with one embodiment, an animal may be generated which is defident in Nuk and another tyrosine kinase of the EphlElklEck subfamily. Such animals could be used to determine how the members of the EphlElklEck subfamily co-operate in embryonic development, particularly development of the nervous system. For example, an animal lacking or partially lacking Nuk expression and Sek-4 expression (Becker N., et al., Mechanisms of Development 47 (1994) 3-17) may be generated to determine how the receptor tyrosine kinases co-operate in the segmental patterning of the hindbrain. Multiple deficient mice can also be generated to study the interaction of Nuk protein and other proteins such as the Src-family of cytoplasmic tyrosine kinases. For example, an animal may be generated which lacks or partially lacks Nuk expression, and expression of one or more Src family tyrosine kinases induding Src, Fyn, and/or Yes. The following non-limiting examples are illustrative of the present invention: EXAMPLES The following materials and methods were utilized in the investigations outlined in the examples: cDNA cloning, sequencing and chromosomal mapping To clone the Nuk coding region, the partial λQl Nuk cDNA insert described in Ben-David et al, 1991 (EMBO 10:317) was used to probe a λgtlO cDNA library constructed from 12.5 day mouse embryo mRNA (Logan et al, Dev. Genet. 13:345-358, 1992). Sequence analysis of a number of independent clones revealed that none reached as far as the ATG translation initiation codon. To obtain additional 5' Nuk coding sequences, rapid amplification of cDNA ends (RACE; Frohman et al, Proc. Nat. Acad. Sci. USA 85:8998-9002, 1988) was performed using Mft-specific oligonudeotides. 9.5 day embryonic poly(A)+ RNA was used for the RACE because Northern blot analysis determined this to be a source of abundant 4.0 and 4.5 Kb Nuk mRNA transcripts. A number of Nuk cDNA clones were generated with this approach, however, a strong stop in the majority of the RACE products just 5' of an Nhel restriction site prevented cloning of the complete coding region. One clone out of over 50 examined (pNukRACE 2.15) extended far enough such that an oligonucleotide could be used to colony screen further RACE products. Using this approach two independent clones out of over 1000 screened (pNukRACE A2 and K2) were both found to contain an additional 67 base pairs of Nuk coding sequences 5' of the Nhel site. Translation of combined RACE and cDNA dones revealed a single open reading frame of 993 codons. Although lacking an ATG translation initiation codon, the sequence of the RACE clones encode a 26 amino acid hydrophobic signal peptide. The 5'-most sequence of both RACE clones end with the dinucleotide TG. Given that the length of the Nuk signal peptide is longer than most Eph family members, this TG may represent the last two nucleotides of the ATG initiator codon. The DNAs generated in this study were subcloned into either the pGEM7 (Promega) or pCRTI (Invitrogen) plasmid vectors prior to double stranded sequencing using the Sequenase system (United States Biochemical Corporation). Sequencing reactions were primed using either the standard forward and reverse primers or custom oligonudeotides synthesized in house on a Pharmacia Gene Assembler Plus. Chromosome mapping was performed by probing Pst I restriction endonulcease digested DNAs from 28 different recombinant inbred mouse strains at high stringency with a 1.0 Kb EcoRI fragment of the λQl Nuk cDNA. Generation of antibodies A trpE-Nuk fusion protein was created by subcloning a NcoI-BamHI fragment of λQl containing the carboxy terminal 94 Nuk codons plus 170 additional nucleotides of 3' untranslated sequences into the bacterial expression vector pATHl. A GST-Nuk fusion protein containing Nuk amino acids 601-994 was also expressed in bacteria. The trpE-Nuk fusion protein was purified from induced bacterial cultures by SDS-PAGE and used to immunize rabbits. Resulting trpE-Nuk antiserum was affinity-purified by binding to immobilized glutathione agarose purified GST-Nuk. Immunoprecipitation. in vitro kinase. and western blotting For biochemical studies, natural matings of CD1 mice were used to obtain embryos at 10.5, 12.5, and 14.5 days development. The embryos were collected and washed twice in phosphate buffer saline (PBS; 150 mM NaCl, 3 mM KC1, 9 mM Na 2 HP0 -2H 2 0, and 2 mM KH 2 PO 4 ) prior to homogenization in PLC-lysis buffer on ice (50 mM Hepes, pH 7.5, 150 mM NaCl, 10% glycerol, 1% Triton X-100, 1.5 mM MgCl 2 , 1 mM EGTA, 10 mM NaPPi, 100 mM NaF, 10 μg/ml aprotinin and leupeptin, 1 mM Na 3 Vθ 4 , and 1 mM PMSF). The embryo lysates were clarified by centrifugation at 12,000 x rpm for 10 min at 4°C. The cleared supernatant was put on ice and an aliquot was used for protein assay (BCA protein assay, PIERCE). For immunopredpitations, lmg of total protein in 1 ml PLC-lysis buffer were incubated with approximately 4 mg of affinity-purified anti-Nuk antibody and 100 ml of a 10% solution of protein A-sepharose beads. Preimmune serum, as well as preincubation of the anti-Nuk antibodies with a competing trpE-Nuk peptide, were used as controls in the immunoprecipitations. After 1 hour incubation at 4°C, the immunoprecipitates were collected by centrifugation at 12,000 x rpm for 30sec and washed three times with ice cold HNTG buffer (20 mM Hepes, pH 7.5, 150 mM NaCl, 0.1% Triton X-100, 10% glycerol, and 1 mM Na 3 V0 ). For in vitro kinase assays, the washed immunoprecipitates were incubated at room temperature (RT) for 15 minutes in 20 ml of kinase buffer (20 mM HEPES, pH7.5, 25 mM MgCl 2 , 4 mM MnCl 2 , and 0.1 mM Na 3 V0 ) containing 10 μCi of γ32P-ATP (Dupont; 3000 Ci/mmole-i). The immune complex kinase reaction products were denatured at 100 β C for 5 min in SDS sample buffer and separated by SDS-polyacrylamide gel electrophoresis. Gels were fixed in acetic add, submerged in 1 M KOH at 55°C for 45 min to remove phosphoserine and phosphothreonine, refixed, dried, and then exposed to Kodak XAR film. For Western blotting, the washed immunoprecipitates were separated by SDS-polyacrylamide gel electrophoresis and electro-transferred to a nylon filter using a semi-dry protein blotting apparatus. The filter was blocked overnight at 4°C in PBS containing 5% dry milk prior to incubation in the same solution containing 1 μg/ml anti-Nuk antibodies for lhr at room temperature (RT). The filter was washed at RT 3 X10 min with TBSN (20 mM TrisHCl, pH 7.5, 150 mM NaCl, and 0.05% NP-40). Nuk immunoreactivity was detected by using affinity-purified goat anti-rabbit horseradish peroxidase (BioRad, 1:20,000 dilution) and Enhanced Chemi -Luminescence reagents (Amersham). The filters were exposed at RT to Kodak XRP-5 film for 30 sec to 5 min. Whole-Mount Im unnhistochemistry The immunohistochemical detection used in this study was based on the Vectastain ABC Elite-peroxidase system and Vectastain biotin/avidin blocking reagents (Vector Laboratories). The specificity of the anti-Nuk antibody staining was confirmed by a variety of control experiments induding those in which the primary antibody was either omitted or preabsorbed with a trpE-Nuk peptide. In addition, other antibodies including a monoclonal antibody raised against the 160 kD subunit of Neurofilament (anti-NF; AMAC Inc.) and a rabbit polyclonal antibody raised against the murine Engrailed (En) proteins were used to control for the specificity of the immunohistochemistry. All observations reported here have been derived from a number of independent experiments all of which gave similar results. The total number of anti-Nuk stained embryos observed at each stage was from 40 to over 100. Embryos used in this study were obtained from natural matings of CD1 mice and at the required stages of development were dissected and membranes were removed in ice cold 0.1 M phosphate buffer (pH 7.4). Embryos were then fixed with occasional mixing for 2 h on ice in fresh 4% paraformaldhyde in 0.1 M phosphate buffer. In some instances 0.2 to 1% glutaraldhyde was also added to the fixative. The fixative was washed out overnight in 0.1 M phosphate buffer at 4°C prior to gently dehydrating the embryos in a methanol/ phosphate buffered saline (PBS) series (15%/85%; 30/70; 50/50; 75/25% H 2 0; 100% methanol) on ice for 30 minutes each step. Once in 100% methanol the embryos can be stored at -20°C for at least 2 months. Prior to rehydration and further manipulation, the embryos were transferred into 80% methanol /20% H 2 0 2 for 4-6 h at RT to bleach embryos and inactivate endogenous peroxidases. After washing in 100% methanol, the embryos were rehydrated on ice for 30 minutes each with occasional mixing in 75% methanol/25% H 2 0, 50% methanol/50% PBS, 30% methanol/70% PBS, 15% methanol/85% PBS, and 100% PBS + 0.01% Triton X-100. The embryos were then gently mixed on a Nutator at RT 2 x 1 h in PBSMT (2% Dry Milk Powder, 0.01% Triton X-100 in PBS). At this step a small number of embryos were placed at 4°C in PBSMT for preabsorbtion of the biotinylated secondary antibodies as described by the manufacturer (Vector Laboratories). The embryos were then blocked overnight at 4 β C on Nutator using the Vedastain ABC Elite and blocking kits in PBSMT+3% normal goat serum (NGS) + 10% avidin blocking reagent. The avidin was then washed out in PBSMT 4 x 1 hour at 4°C followed by 2 x 1 hour at RT. The blocked embryos were then incubated on a Nutator for at least 16 h at 4 β C with affinity-purified anti-Nuk antibodies (0.5 to 1.0 mg/ml) in PBSMT containing 3% normal goat serum and 10% biotin blocking agent. Unbound primary antibodies and biotin were washed out in PBSMT 4 x 1 hour at 4 β C followed by 2 x 1 hour at RT. The preabsorbed biotinylated secondary antibodies were then added to the embryos and incubation was carried out overnight at 4°C on a Nutator. The secondary antibodies were washed out as described above and the Vectastain ABC elite avidin-biotin-HRP reagent in PBSMT+3% NGS was added and allowed to incubate overnight at 4°C on a Nutator. The ABC elite avidin-biotin-HRP reagent was washed out as described above ending with a final wash in PBT (0.2% BSA, 0.01% Triton X-100 in PBS) at RT. For FIRP detection, embryos were incubated in 0.3 mg/ml diaminobenzidine (DAB) in PBT at RT for at least 20 minutes. H 2 0 2 was added to 0.03% and the embryos were incubated at RT under a dissecting microscope until color density was suffident, usually about 1-10 minutes. The color of HRP-DAB reaction product can be changed from an orange to dark purple color by the addition of NiCl 2 to 0.5%. After staining, embryos were washed in PBT and dehydrated through a ethanol/PBS series: 30/70, 50/50, 80/20, 100% ethanol for 30 minutes each. For light microscopy, embryos were cleared in benzyl alcohokbenzyl benzoate (1:2). Photography was carried out with either a Wild M10 macroscope, or a Leitz DMRXE compound microscope using Kodak EPY 64 Tungston slide film or Kodak Ektar 100 print film. Whole-mount mRNA in situ were performed as described in Canton & Rossant, (1992) Development: 116, 357-368. Paraffin section immunnhistochemistry For the immunohistochemical staining of sectioned embryos, dissected animals were fixed with occasional mixing for 2 h on ice in 4% paraformaldhyde + 1% glutaraldehyde in 0.1 M phosphate buffer. Embryos were then washed in phosphate buffer and dehydrated on ice for 30 minutes each with occasional mixing in 25% ethanol/75% PBS, 50% ethanol/50% PBS, 70% ethanol/30% H 2 0 prior to storage at 4 β C. Before embedding in paraffin, embryos were completely dehydrated in 85% ethanol/ 15% H 2 0 and then 100% EtOH. The ethanol was replaced with multiple changes of xylene at RT, and the embryos were then equilibrated to 60°C. The xylene was exchanged with many changes of melted paraffin for 2 to 4 hours at 60 β C prior to embedding. Embryos were sectioned at 4 to 6 μm and placed on slides freshly subbed in 1% aminopropyltriethoxysilane. Sections were placed on a slide dryer for 1 hour, allowed to dry overnight at RT, and then partially melted at 55"C for 20 min. The antibody staining was performed exactly as described in the Vectastain ABC Elite instructions. Anti-Nuk antibodies were used at 4 to 8 μg/ml. For Nuk+peptide control 8 μg/ml of trpE-Nuk fusion protein was added to the anti-Nuk antibodies and preincubated for 1 hour before adding to the sections. Anti-NF monoclonal antibodies were used at 4 μg/ml. In some instances, the HRP reaction was carried out in DAB only. These sections were counterstained in hematoxylin prior to mounting with coverslips. In other sections, the HRP reaction was carried out in DAB+NiCl 2 to produce a darker higher contrast product for black and white photography. These sections were not counterstained prior to mounting. Photography was performed on a Leitz DMRXE microscope using Kodak EPY 64 tungsten color slide film or Kodak Technical Pan black and white print film. Transmission electron microscopy Since the HRP-DAB reaction product produced in the immunohistochemical staining is election dense, the subcellular localization of Nuk protein using transmission electron microscopy (EM) of sectioned anti-Nuk protein whole mount stained embryos was performed. Similar ultrastructural immunoperoxidase localization studies of the mammalian embryonic central nervous system has been performed using antibodies directed against the glial fibrillary acid protein (Levitt et al, 1981) or stains specific for the extracelluar matrix (Nakanishi, 1983). Embryos were fixed in 4% paraformaldehyde + 1% glutaraldehyde and then immunoreacted as whole-mounts with anti-Nuk antibodies as described above. The stained embryos were postfixed in OSO 4 , dehydrated, and embedded in Spurr resin. Ultrathin sections were observed and photographed on a Philips EM430 transmission electron microscope. To render the HRP reaction product more visible, counterstaining of the ultrathin sections with uranyl acetate and lead citrate was ommitted. To control for fixing conditions and morphology, similar staged and fixed embryos were postfixed in OSO 4 and prepared immediately for EM analysis. Other than a loss of lipids from membranes due to extensive washing, the morphology of the immunoreacted samples was comparable to the controls. EXAMPLE 1 Cloning and chromosomal location of Nuk The initial Nuk cDNA clone, designated λQl, was isolated from an unamplified λgtll expression library constructed from a mouse erythroleukemia cell line by screening with anti-phosphotyrosine antibodies (Ben-David et al, EMBO 10:317-325, 1991). Sequence analysis of this done indicated it was a partial cDNA whose expected translation product was dosely related to members of the Eph family of receptor-like tyrosine kinases. Standard cDNA screening of a 12.5 day mouse embryo cDNA library and rapid amplification of cDNA ends (RACE) from 9.5 day embryo mRNA was used to clone the remainder of the Nuk coding region (except for the A of the presumptive ATG initiation codon) following the methods described above. Figure 2 shows that the translation of the combined Nuk RACE and cDNA sequences revealed a single open reading frame of 994 amino acids containing both a hydrophobic signal peptide and transmembrane domain (long underlines. Figure 2). All the hallmarks of a receptor tyrosine kinase of the Eph family are found in Nuk protein, including 20 cysteine residues whose position is conserved in the extracellular domain of Eph family members (bold type. Figure 2), an immunoglobulin-like domain near the amino terminus (Ig-like), and two fibronectin type HI repeats (FN HI; between Nuk amino acids residues 330-420 and 444-534). The Ig-like domain of Nuk contains specific residues (Cys 70 , Trp β o, Cys 1 ^) known to be conserved in the Ig superfamily (Williams and Barclay, Ann. Rev. Immunol 6:381-405, 1988). When compared to the Ig-like domains found in other receptor tyrosine kinases (O'Bryan et al, Mol. Cell. Biol. 11:5016, 1991), the Nuk Ig-like domain was found to contain a number of conserved residues (overlines. Figure 2). A repeat involving the Nuk Ig-like domain and residues 239 to 268 is apparent (underlines. Figure 2). Although significantly shorter than a normal Ig domain, residues involved in the Nuk-specific repeat correspond to conserved residues often found in Ig-like domains (residues that are both overlined and underlined in Figure 2). Following the transmembrane domain, the Nuk cytoplasmic region contains a tyrosine kinase catalytic domain (brackets. Figure 2) and a carboxy-terminal tail of 106 residues. The cartoon in Figure 2 shows the location of the various domains. The carboxy-terminal region used to raise the anti-Nuk antibodies is indicated in Figure 2. Following a 26 amino acid hydrophobic signal peptide, the Nuk protein extracelluar domain is composed of an Ig-like domain and two FN in repeats. The Nuk protein extracelluar domain also contains 20 cysteines whose position is conserved in the Eph family (Lhotak et al, Mol. Cell Biol. 11:2496-2502, 1991). A hydrophobic transmembrane domain divides the Nuk protein into approximately two halves, a 548 amino add extracelluar region and a 419 amino acid cytoplasmic region which contains a tyrosine kinase catalytic domain. The Nuk protein sequence was compared to other known members of the Eph family. Nuk was found to be most highly related to the full length amino acid sequence of chicken Cek5 (96% identity; Pasquale, Cell Regulation 2:523-534, 1991) and to short PCR products of mRNA from rats (Tyro 5; Lai and Lemke, Neuron 6:691-704, 1991) and humans (Erk; Chan and Watt, Oncogene 6:1057-1061 1991). The dose identity between Nuk and Cek5 suggest they represent the mammalian and avian orthologs of the same progenitor gene. The absence of full length cDNAs for Tyro 5 and Erk precludes the determination of whether these sequences correspond to the same or a dosely related but different gene. The chromosomal location of Nuk was determined by probing for restriction fragment length polymorphisms (RFLPs) in the DNA of a number of recombinant inbred mouse strains derived from matings between AKR/J and DBA/2J mice (B.A. Taylor, personal communication). The Nuk locus mapped to the distal end of mouse chromosome 4 near the ahd-1 mutation. EXAMPLE 2 Nuk tyrosine kinase activity in mouse embryos To investigate the biological function of Nuk protein, antibodies were raised against a bacterial fusion protein containing the C-terminal 94 amino acids of Nuk (trpE-Nuk). Anti-Nuk protein antibodies were affinity-purified by binding to a bacterial GST-Nuk fusion protein. The spedfidty of the anti-Nuk protein antibodies was assayed by immunoblotting of bacterial lysates expressing either βgal-Nuk or βgal-Elk carboxy-terminal domains. This experiment demonstrated that the anti-Nuk protein antibodies recognized only the β-gal-Nuk fusion protein. To confirm this result with mammalian protein extracts, Elk protein was immunoprecipitated from a rat brain protein lysate with anti-Elk antibodies. Western blot analysis of the immune-complexes verified that anti-Elk, but not anti-Nuk antibodies could recognize Elk. These experiments demonstrated that the affinity-purified anti-Nuk protein antibodies do not cross-react with the related Elk protein. To assay for Nuk tyrosine kinase activity in vitro, Nuk protein was immunoprecipitated from 10.5, 12.5, and 14.5 day mouse embryo protein lysates and then incubated in the presence of [^pjATP. The expression and tyrosine kinase activity of Nuk in the embryos is shown in Figure 3. Anti-Nuk protein antibodies immunoprecipitated a protein-tyrosine kinase of 135 kD, as shown in Figure 3A. A highly phosphorylated protein with a relative mobility of 135 kD was detected in protein extracts of 10.5 day (lane 3, Figure 3A), 12.5 day (lane 4, Figure 3A), and 14.5 day (lane 5, Figure 3A) embryos. The mobility of this protein is consistent with that reported for other Eph family members including Eck, Elk, and Cek5 (Lindberg et al, Mol Cell Biol. 10:6316-6324, 1990; Lhotak et al, Mol Cell Biol. 11:2496-2502, 1991; Pasquale et al, }. Neurosci. 12:3956-3967, 1992). No kinase signal was observed from 10.5 day embryo lysates when preimmune serum was used (lane 1, Figure 3A) or if the anti-Nuk antibodies were initially preincubated with a trpE-Nuk fusion protein (lane 2, Figure 3A). To enrich for phosphotyrosine, the gel was treated with KOH as described previously herein. Similar anti-Nuk protein immunoprecipitates were also subjected to Western blot analysis with anti-Nuk antibodies, as shown in Figure 3B. Immunoblotting identified Nuk protein as a 135 kD protein, (Figure 3B). The anti-Nuk protein antibodies deteded Nuk protein as a 135 kD protein in 10.5 day (lane 3, Figure 3B), 12.5 day (lane 4, Figure 3B), and 14.5 day (lane 5, Figure 3B) embryos. No signal was detected if the 10.5 day embryo lysate was immunoprecipitated with preimmune serum (lane 1, Figure 3B), or with anti-Nuk antibodies pre-incubated with a trpE-Nuk fusion protein (lane 2, Figure 3B). The abundant low molecular weight signals are due to binding by the goat anti-rabbit secondary antibodies to the anti-Nuk protein antibodies and the IgG in the preimmune serum used in the immunoprecipitation. In agreement with the in vitro kinase studies, the Western blot revealed highest levels of the 135 kd Nuk protein in 10.5 day mouse embryos (Figure 3B, lane 3). EXAMPLE 3 Segmental expression of Nuk The biochemical data presented above and Northern mRNA analysis indicated Nuk is highly expressed in 9.5 to 10.5 day mouse embryos. To investigate this expression in detail, Nuk protein was visualized in 7.5 to 13 day mouse embryos in situ using the anti-Nuk protein antibodies in whole-mount and paraffin section immuno-histochemical techniques. To confirm the protein studies, Nuk mRNA in situs on whole-mounts (Conlon and Rossant, Development 116:357-368, 1992) and frozen sections were also performed with similar staged embryos. Nuk protein localization in whole-mount and paraffin sections of 7.5 to 10.5 day mouse embryos is shown in Figure 4, comprising whole-mount preparations showing Nuk mRNA (Figure 4A) and protein (Figures 4B to 4E) at early postimplantation stages of embryonic development. Unless otherwise stated, in Figure 4, dorsal is left and anterior is up. Figure 4A represents whole-mount mRNA studies which detected Nuk transcripts enriched in the neural ectoderm of an embryo at 7.5 days development (note dorsal surface is up). By the 6 somite stage (8 days) Nuk protein is detected as dark orange horseradishperoxidase (HRP) staining as shown in Figure 4B. This is most apparent in the neural groove where relatively high levels are observed in specific hindbrain rhombomeres and in the midbrain region. The arrowhead in Figure 4B points to rhombomeres r2, r3 that express high levels of Nuk protein. The sinus venosis of the developing circulatory system also stains positive for Nuk. Figure 4C shows an embryo at 12 somite stage (8.75 days) and reveals elevated levels of Nuk protein in specific regions of the anterior neural tube. The most anterior structures immunoreactive for Nuk include the ventral diencephalon followed by the ventral mesencephalon/midbrain. Rhombomeres r2, r3, and r5 of the hindbrain are also immunoreactive for Nuk protein. The arrow in Figure 4C points to rhombomere 5. The expression of Nuk was confirmed by performing mRNA in situs on similar staged embryos. Figure 4D shows 24 somite (9.5 day) stage embryos immunoreaded with either anti-Nuk protein antibodies (left) or, as a control, anti-Nuk protein antibodies that were preincubated with a trpE-Nuk fusion protein (right). Nuk protein is most abundant in the ventral midbrain, diencephalon, and optic stalk. Along the spinal cord, high levels of Nuk protein is detected at the dorsal surface of the neural tube (see Figure 5 for more detail). Note the absence of specific staining in the control embryo. Figure 4E shows whole-mount immunolocalization of Nuk protein in the developing brain of a 9.5 day embryo in greater detail. Nuk protein is most highly expressed in the ventral midbrain encompassing the flexure region. Nuk protein is also detected in the ventral diencephalon, optic chiasm, optic stalk, retinal cells (out of focus), and basal telencephalon. At this stage, Nuk protein is still detected in the hindbrain and is localized to the floorplate (arrow in Figure 4E points to ventral region of rhombomere 2). Note the small patch of Nuk protein now detectable in the lateral region of rhombomere 4 just anterior of the otic vesicle. Figures 4F to 4J show immunohistochemical detection of Nuk protein in paraffin sections of 9.5 and 10.5 day mouse embryos. Figures 4F and 4G show serial transverse sections of a 9.5 day embryo immunoreacted with anti-Nuk antibodies which detect Nuk protein (brown stain) in the ventral midbrain. Nuk protein is localized to all layers of the neural tube including the proliferative ventricular zone, the internal mantel layer, and the outer marginal layer which is adjacent to the surrounding mesenchyme. Note Nuk protein is detected only in the ventral aspect of the midbrain and its limit of expression marks a spedfic morphological bulge/constriction of the neural tube that separates ventral from dorsal components (arrowhead). Figure 4H shows a sagittal section along the midline of a 10.5 day embryo showing Nuk protein (dark stain) concentrated in ventral regions of the midbrain and in the optic chiasm (asterisk) and footplate of the hindbrain (thick arrow). The orientation of this embryo is opposite to the one shown in Figure 4E. Note the appearance of a morphological constriction (arrowhead) which separates the midbrain from the diencephalon. Since this is a section along the midline, the infundibulium or ventral most region of the diencephalon is exposed between the midbrain and optic chiasm. This region of the diencephalon does not express Nuk. Figures 41 and 4J show adjacent transverse sections of a 10.5 day embryo immunoreacted with either anti-Nuk protein antibodies (41) or anti-Nuk antibodies preincubated with a trpE-Nuk peptide (4J), and illustrate the specificity of the immunohistochemistry. Staining of Nuk protein at 10.5 days persists in the ventral midbrain and is also detected at lower levels in the floorplate of the hindbrain (arrowhead). The scale bars shown in Figure 4 represent the following measurements: (4A), 100 μm; (4B and 4C), 120 μ ; (4E, 4F, 4G, 41, 4J), 150 μm; (3H), 300 μm. As described above, by the six somite stage (8 days) high levels of Nuk protein are detected in the neural ectoderm prior to fusion of the dorsal edges of the neural plate (Figure 4B). Highest levels of Nuk immunoreactivity are detected in the presumptive midbrain and in specific rhombomere segments of the hindbrain. Nuk protein is also detected in the sinus venosis of the developing circulatory system. By the 12 somite stage (8.75 days) the dorsal edges of the neural plate have fused forming the neural tube. High levels of Nuk protein are localized to specific compartments of the neural tube including hindbrain rhombomeres r2, r3, and r5, the ventral midbrain (mesencephalon) encompassing the flexure, and the ventral diencephalon (Figure 4C). By 9.5 to 10.5 days of development, Nuk expression intensifies in specific ventral regions of the developing brain structures (Figures 4D and 4E). In the hindbrain at this stage, Nuk protein becomes concentrated along the floor plate. When anti-Nuk protein antibodies are used to stain transverse sections at the midbrain of 9.5 day (Figures 4F and 4G) and 10.5 day (Figure 41) embryos, it is apparent that Nuk protein is restricted to the morphological bulge corresponding to the ventral/basal plate of the mesencephalon. Preincubating the anti-Nuk protein antibodies with a Nuk peptide abolished the signal, demonstrating the specificity of the antibody in both whole-mount and paraffin section immunohisto-chemistry (Figure 4D and 4J). Staining of additional transverse sections determined that Nuk protein in the diencephalon of 9 to 10.5 day embryos is also restrided to ventral structures, including the hypothalamus, thalamus, and the optic stalk. Very high levels of Nuk protein were also detected in cells of the primitive optic chiasm at day 10.5. Sagittal sections confirmed that high levels of Nuk mRNA and protein (Figure 4H) are deteded specifically in ventral cells of the developing brain. Consistent with the in vitro kinase and Western analysis, anti-Nuk immunoreactivity in 11.5 to 13.5 day embryos decreases significantly in these brain structures as discussed below. EXAMPLE 4 Subcellular localization of Nuk The immunohistochemical reaction product observed for the anti-Nuk antibodies often appears as if the Nuk protein is localized to specific surfaces of the plasma membrane. One clear example of this is at the dorsal surface of the posterior neural tube which contains elevated amounts of the Nuk protein as early as 9.5 days of development (see Figure 4D). The subcellular localization of Nuk protein is shown in more detail in Figure 5. Figures 5 A and 5B are adjacent transverse sections of an 11.5 day embryo at the level of the caudal/posterior spinal cord immimoreacted with either anti-Nuk antibodies (5A) or anti-Nuk antibodies preincubated with a trpE-Nuk peptide (5B). Nuk protein is concentrated in the dorsal region of the neural tube along the basement membrane (arrow). This localization of Nuk protein is observed as early as 9.5 days of development in whole-mount preparations (see Figure 4D). Antibodies to Neurofilament (anti-NF) and Engrailed (anti-En) proteins did not stain the basement membrane demonstrating the specificity of the anti-Nuk antibodies. Figures 5C and 5D show transmission electron microscopy (EM) immunolocalization of Nuk protein in ventral midbrain cells of a 9.5 day embryo. Immunoperoxidase anti-Nuk stained whole-mount embryos were ultrathin sectioned and observed under EM. Figure 5C shows that, at the proliferative ventricular zone (VZ), Nuk-positive signals are concentrated at sites of cell-cell contact (arrows). Notice that Nuk protein immunostaining is often localized on the membranes of both cells that are making contact. At this stage of development there is no obvious ultrastructural differences between neuronal and glial cell types. Therefore, it cannot be distinguished if Nuk protein is associated with neurons, glial, or both cell types. Note the condensed d romatin evident in the nudeus of a cell that is in late prophase (asterisk). Figure 5D shows that cells within the postmitotic mantal region of the neural tube also exhibit Nuk protein localization at sites of cell-cell contact. The photomicrograph shows a site of contact between two cells labelled strongly for Nuk protein (filled arrow). Note that other sites of cell-cell contad do not contain detectable amounts of Nuk protein (open arrows). The scale bars in Figure 5 represent the following measurements: (A and B) 100 μm; (C), 2 μm; (D), 1 μm. Transverse sections of 11.5 day embryos shows in greater detail that Nuk protein is concentrated where the dorsal-most membrane of the neural tube make contact with the surrounding mesenchyme/neural crest cells (Figures 5A and B). Lateral and ventral regions of the neural tube do not exhibit this localized Nuk protein. Since the HRP-DAB reaction product produced in the immunohistochemical staining is electron dense, the subcellular localization of Nuk protein was investigated using transmission electron microscopy (EM), following the methods described above. The ventral midbrain of 9.5 day embryos was chosen for EM analysis because of its intense Nuk immunoreactivity. Figure 5C shows a section of the midbrain at the mitotically active ventricular zone. Nuk immuno-reactivity associated with these cells is usually localized to the plasma membranes at specific sites of cell-cell contact. As shown in Figure 5D, Nuk immunoreactivity was not found to be localized to all sites of cell-cell contad. In addition, small punctate deposits throughout the ventral midbrain were immunoreactive for Nuk protein. These structures appear to be plasma membrane material lost from post-mitotic cells during their radial migration from the ventricular zone. EXAMPLE 5 Nuk localization in pioneer cranial PNS axons and an early CNS axon tract and in spinal axons Axonogenesis in mouse embryos commences at approximately 10 days of development when neurons associated with both the central and peripheral nervous systems extend axon projections toward their targets. Anti-Nuk antibody staining of 10 to 12 day embryos revealed that Nuk protein is highly concentrated within most if not all early axonal projections of the PNS and to at least one early pathway of the CNS. An example of Nuk protein localization in a specific cranial nerve, the oculomotor nerve (III), is shown in Figure 6. Figure 6 A is a photomicrograph of the head of an 11 day anti-Nuk whole-mount immunostained embryo with one of the pair of oculomotor nerve fibers in focus showing strong labelling of Nuk protein (filled arrow). A clearer view of Nuk protein staining in the oculomotor axons fibers can be obtained when the whole-mount staged embryo was filleted down the midline to minimize tissue thickness (Figure 6B). This view shows that the Nuk protein positive oculomotor in axons exit the neural tube from the ventral aspect of the midbrain. An anti-Nuk stained frontal section shows that these axons originate from the Nuk-expressing cells in the ventral midbrain, at a region that is consistent with the position of the oculomotor nuclei (Figure 6E). Anti-Nuk antibodies also label an early pathway of the CNS (open arrow in Figure 6). Figures 6C to 6F show paraffin section immunohistochemistry of 10.5 to 11 day mouse embryos. Adjacent transverse sections bisecting the pair of oculomotor nerve fibers were immunostained with either anti-Nuk (6C) or anti-Neurofilament (6D) antibodies. The arrows in both Figures 6C and 6D point to the darkly stained axon bundles of the two oculomotor nerves. To verify the specificity of the immunohistochemistry, additional control experiments were performed in which the anti-Nuk antibodies were either omitted, preincubated with a trpE-Nuk peptide, or substituted with antibodies directed against the Engrailed (anti-En) nuclear homeodomain proteins. Figure 6E is a frontal section immunostained with anti-Nuk antibodies which label the ventral midbrain and oculomotor axon fibers as they exit the neural tube (filled arrow) and extend (open arrow) towards their target tissue, the pre-optic musde mass. Figure 6F shows a sagittal section immunostained with anti-Nuk antibodies which label the oculomoter axon fascicule as it enters the pre-optic muscle mass (open arrow). This section also shows the expression of Nuk protein in the developing retinal cells (r). The orientation of this section is the same as Figure 4H. The scale bar for Figures 6 (A) is 180 μm; (B) 120 μm ; and (C-F) 100 μm. To confirm that the observed Nuk protein localization is in axons, adjacent transverse sections that bisect the oculomotor nerve fibers of a 10.5 day embryo were immunoreacted with either anti-Nuk (Figure 6C) or neuron-specific anti-Neurofilament antibodies (anti-NF; Figure 6D). Both antibodies labelled the same structure demonstrating the axon-specific localization of Nuk protein. The main target tissue of oculomotor axons is the premuscle mass of cells adjacent to the developing retina, whose function in the adult is to control certain eye movements. An anti-Nuk stained sagittal section labelled the oculomotor axon fascicule at the point of entry into the premuscle mass (Figure 6F). This section also shows the high level of Nuk protein associated with the developing retinal cells. Other cranial PNS axons contain high levels of Nuk protein early during their elongation. Nuk protein is detected in the trigeminal (V) and facial (VII) nerve fibers in 10.5 day embryos (Figures. 7A to 7C). The vagus (X) parasympathetic autonomic nerve, and the accessory (XI) and hypoglossal (XII) somatomotor nerves also contain localized Nuk protein (Figures. 7D and 7E). As observed by the anti-Nuk labelling, these fibers enter a common region, the cardiac/pulmonary plexus, where they then elongate to their targets such as the cardiac muscle and other visceral organs (vagus) or the upper torso (accessory) and tongue (hypoglossal). Nuk protein localization in these cranial axons is very transient and is not detected after 12.5 days development. The embryo in Figure 6A and B also exhibits specific anti-Nuk labelling in the developing CNS of a connection between the telencephalon and the midbrain. Information describing the naming and position of early tracts in the developing mammalian forebrain is sparce. The location of Nuk immunoreactivity is consistent with the location of axonal projections originating from the ventral midbrain tegmentum such as those of the red nuclei or the reticular activating system (Carpenter, 1985, Core Text on Neuroanatomy, Baltimore: Williams & Wilkins). Alternatively, this pathway may correspond to the telencephalic/supraoptic tract described in zebra (Chitnis and Kuwanda, J. Neurosci. 10, 1892-1905, 1990; Wilson et al., (1990) Development, 108, 121-143, 1990). Closer definition of the origin and termination sites as well double labelling with other antibody probes should help determine the identity of this Nuk-positive tract EXAMPLE 6 Nuk Localization in Spinal Axons Figures 7 A to 7E show Nuk protein localization in cranial nerves of 10.5 to 12 day mouse embryos. Figure 7A is a 10.5 day whole-mount embryo immunostained with anti-Nuk antibodies which label the trigeminal nerve V and facial nerves VII. Figures 7B and 7C show transverse sections of a 10.5 day embryo showing Nuk-positive trigeminal nerve V axon fascicules labelled with the anti-Nuk antibodies. Note that Nuk protein is also present throughout the caudal hindbrain region of the neural tube. Figures 7D and E are 11.5 day anti-Nuk whole-mounts showing localization of Nuk protein in the vagus nerve (X) associated fibers as they pathfind to their target visceral organs (curved open arrows). Other cranial axons induding the accessory (XI) and hypoglossal (XH) nerve fibers are immunoreactive for Nuk protein. These cranial nerves initially extend to the plexus region (open arrow in D) before they pathfind to the heart and other target tissues. Figures 7F to 7L show Nuk protein localization in spinal nerves of 10 to 12 day embryos. Figure 7F is a whole-mount 10 day embryo showing Nuk protein concentrated within the earliest spinal nerve fibers exiting the neural tube (arrows). Figure 7G shows a slightly later stage embryo from that shown in Figure 7F. Notice that the nerve fibers exiting the neural tube have thickened due to fasciculation of additional axons. The insert is a dose-up of a single spinal nerve showing Nuk protein is localized throughout its length and can be observed at the leading tips of the growth cones. Figures 7H and 71 show transverse sections bisecting the rostral spinal cord of an 11 day embryo demonstrating that the darkly stained Nuk-positive fibers shown in Figures 7F and 7G are ventral motor axons (open arrows). At this stage, the accesory nerve also stains positive for Nuk protein as evident by the strong labelling of the axon fibers (filled arrows). The uniform expression of Nuk throughout the spinal region of the neural tube is apparent in these sections. Figures 7J and 7K show both sides of an 11 day whole-mount embryo demonstrating that Nuk protein is localized within the spinal motor nerves as they elongate to the plexus regions (open arrows in Figure 7J). Between the motor fibers, the appearance of Nuk-positive DRG cell bodies and axons can be observed in this embryo (filled arrows in Figure 7K). Note that the Nuk-positive DRG axons are more apparent in the posterior/caudal segments of the spinal cord. Figure 7L shows a close-up of a 12 day whole-mount embryo immunostained with anti-Nuk antibodies which label the nerve fibers of the parasympathetic chain ganglion. Note the ganglia form a chain of interganglionic axonal connections with each other and that each ganglion unit forms connections with two segments of the neural tube. Scale bars in Figures 7B, 7C, 7H, and 71 represent 200 μm. Immunostained whole-mount 10-10.5 day embryos revealed high levels of Nuk protein in the earliest spinal motor axons as they exit the neural tube (Figures 7F and G). The close-up photomicrograph in Figure 7G shows that Nuk protein is localized throughout the length of the axons induding the growth cones. Transverse sections immunostained with anti-Nuk antibodies confirmed that Nuk protein is highly localized to the ventral motor axons and the axons of the DRG (Figures 7H and I). Note detectable levels of Nuk protein throughout the spinal cord. By 11 days of development (Figures 7J and K) the Nuk-positive nerve fibers have elongated and thickened considerably due to the fasciculation of additional axons along the initial axons which, by this time, have reached the plexus regions (Tosney and Landmesser, /. Neurosci. 4:2518-2527, 1984; Tosney and Landmesser, Dev. Biol 109:193-2141985a; Tosney and Landmesser, /. Neurosci. 5:2336-2344 1985b; Landmesser and Swain, Neuron 8:291-305, 1992). Nuk protein in the rostral DRG cell bodies and their fibers connecting to the neural tube is also deteded (Figures 7J and 7K). By 12 days of development only low levels of Nuk immunoreactivity are detected in the motor and DRG axons. At this stage, Nuk protein is deteded in axon fibers of the sympathetic chain ganglion (Figure 7L) and nerve fibers surrounding the heart and diaphram regions. EXAMPLE 7 Nuk protein early in ear development Very high levels of Nuk protein were found in specific structures of the developing ear and associated vestibulocochlear (Vπi) ganglion in 10.5 to 12.5 day embryos as is shown in Figure 8. As shown in Figure 8, anti-Nuk immunoreacted whole-mount 10.5 day embryo detects Nuk protein localization (arrow) at the dorsal region of the otic vesicle (ov) surrounding the budding endolymphatic duct (Figure 8A). Branchial arches 1 and 2 are indicated. By 11.5 days of development the endolymphatic duct has elongated approximately 200μm dorsally (Figure 8B). Nuk protein is observed to envelope this structure (arrow). Nuk protein localization within cranial nerve VII as it enters the second branchial arch can also be seen in Figure 8B. Figure 8C shows a slightly different focal plane from (B) showing that the vestibulocochlear sensory fibers connecting to the developing ear stain positive for Nuk protein (arrow). Nuk protein assodated with the developing eye is also shown in Figure 8C. Transverse section of an 11.5 day embryo detects high levels of Nuk protein localized to the basement membrane of the endolymphatic duct cells where they contact the surrounding mesenchymal /neural crest cells (open arrow, Figure 8D). Nuk protein associated with the vestibulocochlear ganglion is also visible in this section (filled arrow). The scale bars in Figure 8 represent the following: (8A), 100 μm; (8B and 8C), 200 μm; (8D), 100 μm. At 10.5 days Nuk immunostaining associated with the ear is first observed at the dorsal region of the otic vesicle surrounding the initial bulge of the endolymphatic duct (Figure 8 A). By 11.5 days Nuk immunoreactivity encapsulates the endolymphatic duct during its dorsal elongation (Figure 8B). A different focal plane of the same embryo shows Nuk is also localized to the vestibulocochlear ganglion and its axon fibers connecting to the developing ear (Figure 8C). Transverse sections detected Nuk protein specifically localized to the basement membrane of the endolymphatic duct cells (Figure 8D). By 12.5 days of development only low levels of Nuk were detected in the endolymphatic duct and the acoustic ganglion. EXAMPLE S Generation of Loss of Function Nuk mutant A loss of function mutation in Nuk, designated Nwfc 1 was generated in embryonic stem cells, and germline transmission of the null allele was obtained as described in more detail below. A null allele was generated in mouse embryonic stem cells generallly following the methodology described in Capecchi M.R., Science 244:1288-1292, 1989. The null mutation was obtained by deletion of exon 2, corresponding to codons 29 to 50, as shown in Figure 9. To obtain germ line transmission of the mutation Nuk+I- embryonic stem cell lines (ES) were aggregated with 8 cell embryos in vitro and the resulting blastocysts were transferred into redpient females. Upon birth, animals chimeric for ES and embryonic stem cells were recovered by scoring for eye pigment and coat colour. Breeding of these "aggregation chimeras" confirmed that the germ line of at least one founder mouse is derived completely from the ES cells. Adult mice homozygous for the mutation did not express Nuk protein. EXAMPLE 9 Generation of a Nuk-lac Z fusion chimeric receptor mutant A targeted mutation, designated Nuk 2 was generated in the Nuk gene as shown in Figure 10. A pPNT-LOX-Nuk 2 gene trap vector was used to delete the GXGXXG ATP binding region of the kinase domain (amino acids 623-707, SEQ ID NO:2 and Figure 2) to create a Nuk-lac Z fusion receptor in ES cells. Chimeric animals were prepared as described above, by aggregating the ES cells with 8 cell CD1 embryos. Animals generated with the Nuk 2 mutation provided Nu k expressing cells staining for β-galactosidase activity, providing a convenient marker for Nuk-positive cells in both heterozygous and homozygous backgrounds. The Nu k 2 mutation led to the expression of a Nuk-lac Z fusion protein in mouse heterozygous embryos, detected by a blue/green colour as shown in Figures 11 and 12. Figure 11 A shows an embryo at the six somite stage (8.25 days development) expressing the Nuk 2 mutation in the brain and developing heart. Figure 11B shows that at the 14 somite stage (8.75 days development) expression continues in the hindbrain rhombomeres, the midbrain and diencephalon and persists in the heart. Figure 12 shows Nu k 2 expression in a 10.5 day old embryo. Figure 12A illustrates the very high levels of expression in the ventral midbrain, dienchephalon and retinal cells (which are out of focus in the photomicrograph). Figure 12B illustrates expression in the brain and spinal cord. EXAMPLE 10 Autophosphorylation of Nuk protein by ELK-ligand Fusion proteins consisting of the extracellular domain of Elk-ligand (Davis, S., et al. Sdence Vol. 266, Nov. 4, 1994, p.816) linked to the Fc portion of human immunoglobulin Gl were made in COS cells S/N following the methods outlined in Davis et al, 1994, supra. The expressed ligand was aggregated with anti-human Fc antibody and incubated with COS cells for 5 minutes, 15 minutes, 30 minutes or 1 hour. Following incubation, the COS cells were lysed and the lysate was immunoprecipitated with anti-Nuk antibodies. The immunoprecipitates were subjected to Western blotting analysis with anti- phosphotyrosine antibodies. Figure 13 shows the autophosphorylation of Nuk on tyrosine from 15 minutes to one hour following Elk-ligand stimulation. Therefore, Nuk autophosphorylation is induced by stimulation with the Elk- ligand. EXAMPLE 11 Nuk expression in Cell Lines Various cell lines were screened for expression of Nuk protein. Cells were homogenized and the homogenate was incubated with anti-Nuk antibody and the immunopredpitates were collected by centrifugation. The washed immunopreάpitates were separated by SDS-polyacrylamide gel electrophoresis and subjected to Western blotting with anti-Nuk antibodies or anti- phosphotyrosine antibodies. As shown in Figures 14 and in Table 1, COS cells expressed high levels of Nuk protein. TABLE 1 MΩS L Z ς___ ___z PCi2 » Nuk protein + ++++ + + (*NB/ can't stimulate with elk-ligand) EXAMPLE 12 GST-fusion mixing experiments with Nuk in vitro phosphorylation GST-fusion proteins containing SH2 domains of p85N, p85C, PLCγN, PLCγC, GAPN, Src, Crk, Abl, and Grb2 were made in E. coli. Nuk protein was immunoprecipitated from lysates of COS cells using Nuk antisera, and phosphorylated by incubation with ATP. Bacterial lysates containing the GST fusion proteins were incubated with phosphorylated/non-phosphorylated Nuk protein. Western blots were prepared using anti-GST antibodies. The results are shown in Figures 15 and 16, and they are summarized below in Table 2. Table 2 Fusion Nuk Protein Non-phosphorylated Phosphorylated GST p85N +/- +/- p85C - + PLCγN - ++ PLCγC ++ ++++ GAPN + +++ Src + ++++ Crk - + Abl - + SH2 Grb2 No No As shown in Table 2, phosphorylated Nuk protein bound to fusion proteins containing SH2 domains of PLCγN, PLCγC, Src, and GAPN. EXAMPLE 13 GST-fusion mixing experiments with Nuk in vitro phosphorylation The experiments described in Example 12 were also carried out in the pre se n ce of a com petin g ph o sph o r yl ated pe pti de - GMKTpYIDPFTpYEDPNEAVR(K) from the Nuk membrane proximal region. In particular, Nuk from COS cells obtained with Nuk antisera was phosphorylated by incubation with ATP. The phosphorylated Nuk was incubated with bacterial lysates from SH2-GST fusion protein expressing bacteria, in the presence or absence of the competing peptide. Western blots for were prepared using anti-GST antibodies. As shown in Figure 17, the Nuk phosphorylated peptide competes in vitro with the interaction of GAP, PLCγ, and Src SH2 fusion proteins. Unphosphorylated peptide did not interfere with the interaction nor did a nonspecific Trk SHC-binding peptide (HLAENPQY p FSD ). EXAMPLE 14 The phosphorylation of proteins after Elk-ligand stimulation in Nuk expressing COS cells was investigated. Anti-phosphotyrosine Western blots were prepared from whole cell lysates of COS cells stimulated with Elk- ligand. The results are shown in Figure 18. Proteins of about 160, 130 (probably Nuk), 58 and 40 KD were found to be phosphorylated on tyrosine after Elk- ligand stimulation. Having illustrated and described the principles of the invention in a preferred embodiment, it should be appredated to those skilled in the art that the invention can be modified in arrangement and detail without departure from such principles. We claim all modifications coming within the scope of the following claims. All publications, patents and patent applications referred to herein are incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety. The following sequence listings form part of the application. SEQUENCE LISTING (1) GENERAL INFORMATION: (i) APPLICANTS: (A NAME: Mount Sinai Hospital (B STREET: 600 University Avenue (C CITY: Toronto (D STATE: Ontario (E COUNTRY: Canada (F POSTAL CODE (ZIP): (G TELEPHONE NO.: (416) 586-8225 (H TELEFAX NO.: (416) 586-8844 (A NAME: Anthony Pavson (B STREET: 34 Glenvood Avenue (C CITY: Toronto (D STATE: Ontario (E COUNTRY: Canada (F POSTAL CODE (ZIP): M6P 3C6 (A NAME: Mark Henkemeyer (B STREET: 263 Adelaide Street West, Apt. 503 (C CITY: Toronto (D STATE: Ontario (E COUNTRY: Canada (F POSTAL CODE (ZIP): M5T 1K5 (A NAME: Kenneth Letvin (B STREET: 20 Carluke Crescent, Apt. 1010 (C CITY: Willo dale (D STATE: Ontario (E COUNTRY: Canada (F POSTAL CODE (ZIP): M2L 2J1 (ii) TITLE OF INVENTION: NOVEL NEURAL KINASE AND RECEPTOR TYROSINE KINASE (iii) NUMBER OF SEQUENCES: 2 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Bereskin & Parr (B) STREET: 40 King Street West, Box 401 (C) CITY: Toronto (D) STATE: Ontario (E) COUNTRY: Canada (F) ZIP: M5H 3Y2 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: Patentln Release #1.0, Version #1.25 (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: PCT ( B ) FILING DATE : (C) CLASSIFICATION: (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: Kurdydyk, Linda M (B) REGISTRATION NUMBER: 34,971 (C) REFERENCE/DOCKET NUMBER: 3153-148 (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: (416) 364-7311 (B) TELEFAX: (416) 361-1398 (C) TELEX: 06-23115 (2) INFORMATION FOR SEQ ID NO:l: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 3105 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (vi) ORIGINAL SOURCE: (A) ORGANISM: Mus musculus (D) DEVELOPMENTAL STAGE: Embryo (vii) IMMEDIATE SOURCE: (A) LIBRARY: lambda gtlO cDNA library (B) CLONE: Combined PnUKRACE A2 and K2 AND cDNA clones (viii) POSITION IN GENOME: (A) CHROMOSOME/SEGMENT: Distal end of chromosome 4 (B) MAP POSITION: near the ahd-1 mutation (xi) SEQUENCE DESCRIPTION: SEQ ID NO:l: ATGGGAGCCC GGGTCCCCGT TCTGCCCGGG CTGGATGGCT CATTCTGCTG GCTGCTGCTG 60 CTGCCGCTGC TAGCCGCCGT GGAAGAAACC CTGATGGACT CTACGACAGC AACGGCTGAG 120 CTGGGCTGGA TGGTACATCC CCCATCAGGG TGGGAAGAGG TGAGCGGCTA CGACGAGAAC 180 ATGAACACTA TCCGTACCTA CCAGGTGTGC AATGTCTTTG AGTCAAGCCA GAACAACTGG 240 CTGCGGACCA AATTCATCCG GCGCCGTGGC GCCCACCGTA TCCACGTGGA GATGAAGTTC 300 TCGGTGCGTG ACTGCAGCAG CATTCCCAGC GTGCCGGGCT CCTGCAAGGA GACCTTCAAC 360 CTCTACTACT ATGAGGCTGA TTTTGACTTA GCCACCAAAA CCTTTCCCAA CTGGATGGAG 420 AATCCGTGGG TGAAGGTGGA CACCATCGCG GCCGATGAGA GCTTCTCTCA GGTGGACCTG 480 GGTGGCCGCG TCATGAAAAT CAACACTGAG GTGCGAAGCT TCGGTCCTGT GTCCCGCAAT 540 GGTTTCTACC TGGCCTTCCA GGACTΔCGGC GGCTGTATGT CCCTCΔTTGC TGTGCGCGTC 600 TTCTACCGGA AGTGCCCCCG AATCATCCAG AATGGTGCCA TCTTCCAGGA GACACTATCG 660 GGGGCTGAGA GCACTTCGCT GGTGGCAGCT CGGGGCAGCT GCATCGCCAA TGCTGAAGAA 720 GTGGACGTGC CCATCAAACT CTACTGTAAC GGGGACGGCG AATGGCTGGT GCCCATCGGT 780 CGCTGCATGT GCAAGGCGGG CTTCGAGGCT GTGGAGAACG GCACCGTCTG CCGAGGTTGT 840 CCATCAGGAA CCTTCAAGGC CAACCAAGGG GACGAAGCCT GCACCCACTG TCCCATCAAC 900 AGCCGCACCA CCTCTGAGGG TGCCACCAAC TGTGTATGCC GCAACGGCTA CTACAGGGCC 960 GACCTGGACC CCTTAGACAT GCCTTGCACA ACCATCCCCT CTGCGCCCCA GGCTGTGATC 1020 TCCAGCGTCA ACGAGACATC CCTCATGCTA GAGTGGACCC CACCCCGAGA CTCCGGGGGT 1080 CGCGAGGATC TTGTTTACAA CATCATCTGC AAGAGCTGTG GCTCCGGCCG GGGCGCATGC 1140 ACGCGCTGCG GGGACAACGT GCAGTACGCG CCCCGCCAGC TGGGCCTGAC TGAGCCGCGC 1200 ATCTACATCA GTGACCTGCT GGCACACACG CAGTACACCT TCGAGATCCA GGCCGTGAAT 1260 GGTGTGACCG ACCAGAGTCC CTTCTCACCT CAGTTCGCCT CTGTGAACAT CACCACCAAC 1320 CAAGCAGCAC CATCGGCCGT GTCCATCATG CACCAGGTGA GCCGCACTGT GGACAGCATC 1380 ACCCTGTCGT GGTCCCAGCC AGACCAGCCC AACGGTGTGA TCCTGGACTA CGAGCTGCAG 1440 TACTATGAGA AGGAGCTCAG TGAGTACAAC GCCACGGCCA TAAAAAGCCC CACCAACACA 1500 GTCACTGTGC AGGGCCTCAA AGCCGGCGCC ATCTATGTCT TCCAGGTGCG GGCACGCACC 1560 GTTGCAGGCT ATGGGCGCTA CAGTGGCAAG ATGTACTTCC AAACCATGAC AGAAGCCGAG 1620 TACCAGACCA GCATCAAGGA AAAGCTACCC CTCATCGTTG GCTCCTCCGC CGCCGGCTTΔ 1680 GTCTTCCTCA TCGCTGTGGT CGTCATTGCC ATCGTATGTA ACAGACGGGG GTTTGAGCGT 1740 GCCGACTCAG AGTACACGGA CAAGCTACAG CACTACACCA GCGGACACAT GACCCCAGGC 1800 ATGAAGATCT ΔTATAGATCC TTTCACCTAT GAAGATCCTA ATGΔGGCAGT GCGGGAGTTT - 1860 GCCAAGGAAA TTGACATCTC CTGTGTCAAG ATTGAGCAGG TGΔTTGGAGC ΔGGGGAATTT 1920 GGTGAGGTCT GCAGTGGCCA TTTGAAGCTG CCAGGCAAGA GAGΔGATCTT TGTAGCCATC 1980 AAGACCCTCA AGTCAGGATA CACGGAGAAA CAGCGCCGGG ACTTCCTGAG TGAGGCATCC 2040 ATCATGGGCC AGTTCGACCA CCCCAATGTC ΔTCCATCTGG AAGGGGTTGT CACCAAGAGC 2100 ACACCTGTCA TGATCATCAC TGAATTCATG GAGAATGGAT CTCTGGΔCTC CTTCCTCCGG 2160 CAAAATGATG GGCAGTTCAC AGTCATCCAA CTGGTGGGCA TGCTGΔGGGG CATTGCAGCC 2220 GGCATGAAGT ACCTGGCGGA CATGAACTAC GTGCACCGTG ACCTTGCTGC TCGAAACATC 2280 CTCGTCAACA GTAACCTGGT GTGTAAGGTG TCTGACTTTG GGCTCTCACG CTTCCTGGAG 2340 GATGACACGT CTGACCCCAC CTATACCAGC GCTCTGGGTG GGAAGATCCC CATCCGTTGG 2400 ACGGCACCGG AAGCCATCCA GTACCGGAAA TTCACCTCGG CCAGTGATGT GTGGAGCTAT 2460 GGCATCGTCA TGTGGGAGGT GATGTCCTAC GGGGAACGAC CCTACTGGGA CATGACCAAT 2520 CAAGACGTAA TCAACGCCAT TGAACAGGAC TACAGACTAC CTCCGCCCAT GGACTGCCCT 2580 AGCGCCCTGC ACCAGCTCAT GCTGGACTGC TGGCAGAAGG ACCGCAACCA CCGGCCCAAG 2640 TTCGGCCAGA TTGTCAACAC GCTGGACAAG ATGATCCGAA ACCCCAACAG CCTCAAAGCC 2700 ATGGCACCCC TGTCCTCTGG CATCAΔCCTG CCACTGCTGG ACCGCACGAT ACCGGACTAC 2760 ACCAGCTTTA ACACAGTGGA TGAGTGGCTA GAGGCCATCA AGATGGGCCA GTACAAGGAG 2820 AGCTTTGCCA ACGCCGGCTT CACCTCTTTC GACGTTGTAT CTCAGΔTGAT GΔTGGAGGAC 2880 ATTCTCCGCG TTGGGGTCAC TCTΔGCTGGC CACCΔGAAAA AAATCCTGAA CAGTΔTCCAG 2940 GTGATGCGGG CCCAGATGAA CCAGATCCAG TCTGTAGAGG TTTGΔCATTC GCCTGCCTCG 3000 GTTCTCCTCT TCCTCCACGC CGCCCCTGΔG CCCCTACGTC GGTCCCTGCT GCTCTGTCAC 3060 TGCAGGTCAG CACTGCCAGG AGGCCACAGA CAACAGGAAG ACCAA 3105 ( 2 ) INFORMATION FOR SEQ ID NO : 2 : ( i ) SEQUENCE CHARACTERISTICS : (A) LENGTH : 994 amino acids ( B ) TYPE : amino acid (C ) STRANDEDNESS : single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (vi) ORIGINAL SOURCE: (A) ORGANISM: Mus musculus (D) DEVELOPMENTAL STAGE: Embryo (vii) IMMEDIATE SOURCE: (A) LIBRARY: lamda gtlO cDNA library (B) CLONE: Combined pNukRACE A2 and K2 and cDNA clones (viii) POSITION IN GENOME: (A) CHROMOSOME/SEGMENT: Distal end of chromosome 4 (B) MAP POSITION: near the ahd-1 mutation (xi) SEQUENCE DESCRIPTION: SEQ ID N0:2: Met Gly Ala Arg Val Pro Val Leu Pro Gly Leu Asp Gly Ser Phe Cys 1 5 10 15 Trp Leu Leu Leu Leu Pro Leu Leu Ala Ala Val Glu Glu Thr Leu Met 20 25 30 Asp Ser Thr Thr Ala Thr Ala Glu Leu Gly Trp Met Val His Pro Pro 35 40 45 Ser Gly Trp Glu Glu Val Ser Gly Tyr Asp Glu Asn Met Asn Thr lie 50 55 60 Arg Thr Tyr Gin Val Cys Asn Val Phe Glu Ser Ser Gin Asn Asn Trp 65 70 75 80 Leu Arg Thr Lys Phe lie Arg Arg Arg Gly Ala His Arg lie His Val 85 90 95 Glu Met Lys Phe Ser Val Arg Asp Cyβ Ser Ser lie Pro Ser Val Pro 100 105 110 Gly Ser Cys Lys Glu Thr Phe Asn Leu Tyr Tyr Tyr Glu Ala Asp Phe 115 120 125 Asp Leu Ala Thr Lys Thr Phe Pro Asn Trp Met Glu Asn Pro Trp Val 130 135 140 Lys Val Asp Thr lie Ala Ala Asp Glu Ser Phe Ser Gin Val Asp Leu 145 150 155 160 Gly Gly Arg Val Met Lys He Asn Thr Glu Val Arg Ser Phe Gly Pro 165 170 175 Val Ser Arg Asn Gly Phe Tyr Leu Ala Phe Gin Asp Tyr Gly Gly Cys 180 185 190 Met Ser Leu He Ala Val Arg Val Phe Tyr Arg Lys Cys Pro Arg He 195 200 205 He Gin Asn Gly Ala He Phe Gin Glu Thr Leu Ser Gly Ala Glu Ser 210 215 220 Thr Ser Leu Val Ala Ala Arg Gly Ser Cys He Ala Asn Ala Glu Glu 225 230 235 240 Val Asp Val Pro He Lys Leu Tyr Cys Asn Gly Asp Gly Glu Trp Leu 245 250 255 Val Pro He Gly Arg Cys Met Cys Lys Ala Gly Phe Glu Ala Val Glu 260 265 270 Asn Gly Thr Val Cys Arg Gly Cys Pro Ser Gly Thr Phe Lys Ala Asn 275 280 285 Gin Gly Asp Glu Ala Cys Thr His Cys Pro He Asn Ser Arg Thr Thr 290 295 300 Ser Glu Gly Ala Thr Asn Cys Val Cys Arg Asn Gly Tyr Tyr Arg Ala 305 310 315 320 Asp Leu Asp Pro Leu Asp Met Pro Cys Thr Thr He Pro Ser Ala Pro 325 330 335 Gin Ala Val He Ser Ser Val Asn Glu Thr Ser Leu Met Leu Glu Trp 340 345 350 Thr Pro Pro Arg Asp Ser Gly Gly Arg Glu Asp Leu Val Tyr Asn He 355 360 365 He Cys Lys Ser Cys Gly Ser Gly Arg Gly Ala Cys Thr Arg Cys Gly 370 375 380 Asp Asn Val Gin Tyr Ala Pro Arg Gin Leu Gly Leu Thr Glu Pro Arg 385 390 395 400 He Tyr He Ser Asp Leu Leu Ala His Thr Gin Tyr Thr Phe Glu He 405 410 415 Gin Ala Val Asn Gly Val Thr Asp Gin Ser Pro Phe Ser Pro Gin Phe 420 425 430 Ala Ser Val Asn He Thr Thr Asn Gin Ala Ala Pro Ser Ala Val Ser 435 440 445 He Met His Gin Val Ser Arg Thr Val Asp Ser He Thr Leu Ser Trp 450 455 460 Ser Gin Pro Asp Gin Pro Asn Gly Val He Leu Asp Tyr Glu Leu Gin 465 470 475 480 Tyr Tyr Glu Lys Glu Leu Ser Glu Tyr Asn Ala Thr Ala He Lys Ser 485 490 495 Pro Thr Asn Thr Val Thr Val Gin Gly Leu Lys Ala Gly Ala He Tyr 500 505 510 Val Phe Gin Val Arg Ala Arg Thr Val Ala Gly Tyr Gly Arg Tyr Ser 515 520 525 Gly Lys Met Tyr Phe Gin Thr Met Thr Glu Ala Glu Tyr Gin Thr Ser 530 535 540 He Lys Glu Lys Leu Pro Leu He Val Gly Ser Ser Ala Ala Gly Leu 545 550 555 560 Val Phe Leu He Ala Val Val Val He Ala He Val Cys Asn Arg Arg 565 570 575 Gly Phe Glu Arg Ala Asp Ser Glu Tyr Thr Asp Lys Leu Gin His Tyr 580 585 590 Thr Ser Gly His Met Thr Pro Gly Met Lys He Tyr He Asp Pro Phe 595 600 605 Thr Tyr Glu Asp Pro Asn Glu Ala Val Arg Glu Phe Ala Lys Glu He 610 615 620 Asp He Ser Cys Val Lys He Glu Gin Val He Gly Ala Gly Glu Phe 625 630 635 640 Gly Glu Val Cys Ser Gly His Leu Lys Leu Pro Gly Lys Arg Glu He 645 650 655 Phe Val Ala He Lys Thr Leu Lys Ser Gly Tyr Thr Glu Lys Gin Arg 660 665 670 Arg Asp Phe Leu Ser Glu Ala Ser He Met Gly Gin Phe Asp His Pro 675 680 685 Asn Val He His Leu Glu Gly Val Val Thr Lys Ser Thr Pro Val Met 690 695 700 He He Thr Glu Phe Met Glu Asn Gly Ser Leu Asp Ser Phe Leu Arg 705 710 715 720 Gin Asn Asp Gly Gin Phe Thr Val He Gin Leu Val Gly Met Leu Arg 725 730 735 Gly He Ala Ala Gly Met Lys Tyr Leu Ala Asp Met Asn Tyr Val His 740 745 750 Arg Asp Leu Ala Ala Arg Asn He Leu Val Asn Ser Asn Leu Val Cys 755 760 765 Lys Val Ser Asp Phe Gly Leu Ser Arg Phe Leu Glu Asp Asp Thr Ser 770 775 780 Asp Pro Thr Tyr Thr Ser Ala Leu Gly Gly Lys He Pro He Arg Trp 785 790 795 800 Thr Ala Pro Glu Ala He Gin Tyr Arg Lys Phe Thr Ser Ala Ser Asp 805 810 815 Val Trp Ser Tyr Gly He Val Met Trp Glu Val Met Ser Tyr Gly Glu 820 825 830 Arg Pro Tyr Trp Asp Met Thr Asn Gin Asp Val He Asn Ala He Glu 835 840 845 Gin Asp Tyr Arg Leu Pro Pro Pro Met Asp Cys Pro Ser Ala Leu His 850 855 860 Gin Leu Met Leu Asp Cys Trp Gin Lys Asp Arg Asn His Arg Pro Lys 865 870 875 880 Phe Gly Gin He Val Asn Thr Leu Asp Lys Met He Arg Asn Pro Asn 885 890 895 Ser Leu Lys Ala Met Ala Pro Leu Ser Ser Gly He Asn Leu Pro Leu 900 905 910 Leu Asp Arg Thr He Pro Asp Tyr Thr Ser Phe Asn Thr Val Asp Glu 915 920 925 Trp Leu Glu Ala He Lys Met Gly Gin Tyr Lys Glu Ser Phe Ala Asn 930 935 940 Ala Gly Phe Thr Ser Phe Asp Val Val Ser Gin Met Met Met Glu Asp 945 950 955 960 He Leu Arg Val Gly Val Thr Leu Ala Gly His Gin Lys Lys He Leu 965 970 975 Asn Ser He Gin Val Met Arg Ala Gin Met Asn Gin He Gin Ser Val 980 985 990 Glu Val

Claims: WE CLAIM: 1. An isolated and purified nudeic add molecule comprising: (a) a nucleic acid sequence encoding a protein having the amino acid sequence as shown in SEQ ID NO:2 and Figure 2; (b) nucleic acid sequences complementary to (a); (c) nudeic add sequences which are at least 97% homologous to (a); or, (d) a fragment of (a) or (b) that is at least 15 bases and which will hybridize to (a) or (b) under stringent hybridization conditions. 2. A purified and isolated nucleic add molecule as daimed in claim 1 comprising: (a) a nudeic acid sequence as shown in SEQ ID NO:l and Figure 1, wherein T can also be U; (b) nucleic acid sequences complementary to (a); (c) nudeic add sequences which are at least 85% homologous to (a); or, (d) a fragment of (a) or (b) that is at least 15 bases and which will hybridize to (a) or (b) under stringent hybridization conditions. 3. A purified and isolated receptor tyrosine kinase protein having an amino acid sequence as shown in SEQ ID NO:2 or a sequence having at least 97% homology thereto, or an isoform or a part of the protein having at least 20 amino acids. 4. A part of the protein as daimed in daim 3 comprising an extracellular domain of a tyrosine kinase having the amino add sequence as shown in SEQ ID NO: 2 from amino add number 26 to 548 or a sequence having at lease 97% homology thereto. 5. A part of the protein as daimed in daim 3 comprising a carboxy terminal of a tyrosine kinase protein having the amino acid sequence as shown in SEQ ID NO: 2 from amino add number 601 to 994 or a sequence having at lease 97% homology thereto. 6. A recombinant molecule comprising a nucleic add molecule as daimed in daim 1 and one or more transcription or translation elements operatively linked thereto. 7. A transformant host cell induding a recombinant molecule comprising a nucleic acid molecule as claimed in daim 1 and one or more transcription or translation elements operatively linked thereto. 8. A method for preparing a receptor tyrosine kinase protein comprising expressing a purified and isolated nucleic add molecule as claimed in daim 1 in a host cell. 9. A method for identifying a substance which is capable of binding to a receptor tyrosine kinase protein or an isoform or part thereof as daimed in daim 3 comprising reacting the receptor tyrosine kinase protein or an isoform or part thereof, under conditions which permit the formation of a complex between the substance and the receptor tyrosine kinase protein, or isoform or part thereof, assaying for substance-receptor complexes, for free substance, for non-complexed receptor tyrosine kinase protein, or isoforms or parts thereof, or for activation of the receptor tyrosine kinase protein. 10. The method as claimed in claim 9, wherein the substance is a ligand of the receptor tyrosine kinase protein which is capable of activating the receptor tyrosine kinase. 11. The method as claimed in claim 9 wherein the part of the receptor tyrosine kinase protein has the amino acid sequence as shown in SEQ ID NO: 2 from amino acid number 26 to 548 or a sequence having at least 97 % homology thereto. 12. A method for assaying a medium for the presence of an agonist or antagonist of the interaction of a receptor tyrosine kinase protein as claimed in daim 3 and a substance which is capable of binding to the receptor tyrosine kinase protein, which comprises providing a known concentration of the receptor tyrosine kinase protein, reacting the receptor tyrosine kinase protein with a substance which is capable of binding to the receptor tyrosine kinase protein and a suspected agonist or antagonist under conditions which permit the formation of substance-receptor tyrosine kinase protein complexes, and assaying for substance-receptor tyrosine kinase protein complexes, for free substance, for non-complexed proteins, or for activation of the receptor tyrosine kinase protein. 13. A method for identifying a substance which is capable of binding to an activated receptor tyrosine kinase protein as claimed in claim 3, or isoform or part thereof which is activated comprising reacting a receptor tyrosine kinase protein as claimed in claim 3 which is activated, or an isoform or part of the protein, with at least one substance which potentially can bind with the receptor tyrosine kinase protein, isoform or part of the protein, under conditions which permit the formation of substance-receptor kinase protein complexes, and assaying for substance-receptor kinase protein complexes, for free substance, for non-complexed receptor kinase proteins, or for phosphorylation of the substance. 14. A monoclonal or polydonal antibody specific for an epitope of a protein as daimed in claim 3. 15. A recombinant DNA molecule adapted for transformation of a host cell comprising a nucleic add molecule as daimed in claim 2 wherein a portion of the nucleic add sequence is deleted, and one or more transcription or translation elements operatively linked thereto. 16. A transgenic non-human mammal all of whose germ cells and somatic cells contain a recombinant DNA molecule as claimed in daim 15 introduced into the animal, or an ancestor of the mammal at an embryonic stage.

Inventors: HENKEMEYER MARK, LETWIN KENNETH, PAWSON ANTHONY, HENKEMEYER, MARK, LETWIN, KENNETH, PAWSON, ANTHONY
Applicants: HENKEMEYER MARK, LETWIN KENNETH, MOUNT SINAI HOSPITAL CORP, PAWSON ANTHONY, HENKEMEYER, MARK, LETWIN, KENNETH, MOUNT SINAI HOSPITAL CORPORATION, PAWSON, ANTHONY
Application year: 1995
Main classification: C12N15
IPC codes: A01K67, C07K14, C07K16, C12N9, C12N15, C12P21, C12Q1