VERTEBRATE FRINGE PROTEINS

Abstract: Isolated DNA encoding a protein or polypeptide having at least one biological activity of a vertebrate fringe protein, as well as the protein or polypeptide encoded thereby, are described. Assays for identifying agents which alter the expression of the described fringe genes and assays for agents which alter the production of angiogenic precursors, the formation of the apical ectodermal ridge and the subdivisions of the neural tube are also described.

Description: Vertebrate fringe proteins Government Support Work described herein was supported by grant R01 HD/GM32443 from the National Institute of Health and funding from the Howard Hughes Medical Institute. Work described herein was also supported by the National Science Foundation and the American Cancer Society. The U.S. government has certain rights in the invention. Related Applications This application is a continuation-in-part application of U.S. Patent Application Serial No. 08/586,165, filed January 16, 1996, the entire teachings of which are incorporated herein by reference. Background of the Invention Pattern formation is the activity by which embryonic cells form ordered spatial arrangements of differentiated tissues. The physical complexity of higher organisms arises during embryogenesis through the interplay of cell-intrinsic lineage and cell-extrinsic signaling. Inductive interactions are essential to embryonic patterning in vertebrate development, from the earliest establishment of the body plan, to the patterning of the organ systems, to the generation of diverse cell types during tissue differentiation (Davidson, Developmen t 108 : 365 - 389 (1990) ; Gurdon, Cell 68 : 185 - 199 (1992) ; Jessell et al . , Cel l 68 : 251 - 210 (1992) ) . The effects of developmental cell interactions are varied. Typically, responding cells are diverted from one route of cell differentiation to another by inducing cells which differ from both the uninduced and induced states of the responding cells. Sometimes cells induce their neighbors to differentiate like themselves (homogenetic induction) ; in other cases a cell inhibits its neighbors from differentiating like itself. Cell interaction in early development may be sequential, such that an initial induction between two cell types leads to a progressive amplification of diversity. Moreover, inductive interactions occur not only in embryos, but also in adult cells, and can act to establish and maintain morphogenetic patterns as well as induce differentiation (Gurdon (1992) ) . These interactions are thought to explain how distinct cell types emerge from a group of cells that have otherwise equivalent potentials. Genetic and biochemical studies in invertebrates have shown that one means by which a signal may be transmitted is via the Notch/Lin-12/Glp-l family of transmembrane proteins (Artavnis-Tsakonas et al . , Science 268 : 225 - 232 (1995)) . These proteins are thought to serve as receptors which are activated upon binding a member of the emerging DSL (Delta-Serrate-Lag-2) family of putative ligands (Tax et al . , Na ture 368:150-154 (1994)) . Members of the DSL family are also transmembrane proteins, and activation of the receptor appears to require contact with an adjacent cell. Three highly conserved vertebrate Notch receptor family members have also been identified, each of which is widely expressed during embryogenesis (Artavnis- Tsakonas et al . , 1995) . Additionally, vertebrate members of the DSL family have recently been identified (Henrique et al . , Cell 80:909-918 (1995) ; Henrique et al . , Nature 375:787-790 (1995) ; Chitnis et al . , Nature 375:761-766 (1995) ; and Bettenhausen et al . , Developmen t (1995, in press)) , fueling the speculation that the process of lateral cell signaling occurs in vertebrates in a manner similar to invertebrates. However, the molecular signals which regulate the cell interactions, signalling and pattern formation in this pathway and others are currently not well understood. Sum arv of the Invention The present invention pertains to isolated DNA encoding a polypeptide having at least one biological activity of a vertebrate fringe protein, as well as to the polypeptide or protein encoded thereby. In one embodiment, the present invention pertains to isolated DNA encoding the fringe A [ radical fringe) and fringe B (lunatic fringe) proteins and comprising the nucleic acid sequences of SEQ ID NOS: 1 and 6, respectively (Figures 1A-B and 2A-B, respectively) . The present invention also pertains to the isolated fringe A protein comprising the amino acid sequence of SEQ ID NO: 5 (Figure 1A-B) and the isolated fringe B protein comprising the amino acid sequence of SEQ ID NO: 7 (Figure 2A-B) . In an alternate embodiment, the nucleic acid sequence of the isolated DNA encoding a polypeptide having at least one biological activity of a vertebrate fringe protein is at least 30 percent identical to the nucleic acid sequence of either SEQ ID NOS: 1 or 6. In another embodiment, the amino acid sequence of the subject fringe proteins is at least 30 percent similar to the carboxy-terminal 60-75% of the amino acid sequence of either SEQ ID NOS: 5 or 7. The present invention also makes available isolated proteins which are encoded by genes derived from vertebrate organisms, and which are capable of functioning either as an agonist of at least one biological activity of the described vertebrate fringe proteins or an antagonist of at least one biological activity of the described fringe proteins. Exemplary biological activity which the present vertebrate fringe A protein may have includes the ability to affect neural-specification of cell type identity, proliferative regulation of migratory paths, proliferation of neural crest (branchial arches) , definition of morphological boundaries, including limb specification of morphological boundaries, cell shape decisions, regulation of AER formation and indirect regulation of proliferative signals. Exemplary biological activity which the present vertebrate fringe B protein may have includes the ability to affect proliferative and fate specification signals of neural tissues, regulation of formation of components of ear, nose and facial structures, induction, patterning and proliferation of vasculature, and fate specification within somitic tissues. Another aspect of the present invention relates to a method of inducing and/or maintaining a differentiated state of a cell responsive to a fringe protein, by contacting the cell with a fringe protein or a fringe agonist. For instance, it is contemplated by the invention that, in light of the present finding of a broad involvement of fringe proteins in the formation of ordered spatial arrangements of differentiated tissues in vertebrates, the subject method could be used to generate and/or maintain an array of different vertebrate tissues both in vi tro and in vivo . The subject method can be practiced with, for example, the goal of inducing vasculature differentiation. Another embodiment of the invention relates to a method of reducing or decreasing a differentiated state of a cell responsive to a fringe protein, by contacting the cells with a fringe antagonist. For example, the method of the present invention can be practiced with the goal of preventing or altering the subdivision of the neural tube in order to determine the effects of altered neural tube development on the development of the embryo. The present invention also relates to an assay for identifying agents which alter the differentiation or development of an organism. For example, an organism can be treated with the agent to be tested, and the expression pattern of the fringe A and/or fringe B genes can be assessed and compared with their respective expression patterns in a control organism in the absence of the agent to be tested. An increase in the expression of a particular fringe gene indicates that the agent is an agonist of at least one biological activity of, or has a positive effect on the expression of, that fringe gene. Similarly, a decrease in the expression of the fringe gene indicates that the agent is an antagonist of at least one biological activity of, or has a negative effect on, that fringe gene. Alternatively, a similar assay is provided for identifying an agent which alters the formation of angiogenic precursors or angiogenic cells. For example, an agent to be tested can be administered to an organism, and the expression of fringe B in the test organism can be assessed in tissue or cells in which angiogenic precursors are formed, and the expression can be compared with the expression of fringe B in a control organism in the absence of the agent to be tested. A difference in the expression of fringe B indicates that the agent alters the formation of angiogenic precursors or angiogenic cells. The same assay can be used to identify agents which alter the formation of the apical ectodermal ridge (AER) by assessing the expression of fringe A in place of fringe B. The same assay can also be used to identify agents which alter the formation or subdivision of the neural tube by assessing the expression of fringe A and/or fringe B. Brief Description of the Drawings Figure 1A-B represents the partial nucleic acid sequence (SEQ ID NO: 1) of the chicken fringe A ( radical fringe) gene and the three potential encoded amino acid sequences (SEQ ID NOS: 2, 3 and 4, respectively) . There is a frame shift in the open reading frame between amino acids 27 and 28 (indicated with an X) ; as a result, the encoded protein includes part of SEQ ID NO: 2 (up to and including amino acid number 27) and part of SEQ ID NO: 3 (from amino acid 28 on) . The complete sequence of the encoded protein is underlined (SEQ ID NO: 5) . Conserved cysteine residues are indicated with circles at amino acid positions 160, 171, 189, 253, 357 and 366. Figure 2A-B represents the nucleic acid sequence (SEQ ID NO: 6) and the translated amino acid sequence (SEQ ID NO: 7) of the chicken fringe B ( luna tic fringe) gene. The coding region is indicated with capital letters, and conserved cysteine residues are indicated with circles at amino acid positions 152, 163, 181, 244, 348 and 359. The handwritten changes indicate corrections to the nucleotide sequence. Specifically, the corrected sequence indicates a "c" at nucleotide position 1182, an "ag" inserted between nucleotides 1232 and 1433, a "c" at nucleotide position 1443, a "c" inserted between nucleotides 1450 and 1451 and a "cccccg" inserted between nucleotides 2727 and 2728. Figure 3 represents the cDNA sequence (SEQ ID NO: 8) of chicken fringe A ( radical fringe) . The coding region is indicated with capital letters. There may also be an additional nucleotide between nucleotide positions 41 and 42 (indicated with an arrow) . Figure 4A-B is an illustration of the amino acid residues which are conserved between the amino acid sequences of the fringe A ( radical fringe) protein, the fringe B (lunatic fringe) protein and the Drosophila fringe (dFringe) protein (SEQ ID NO: 9) . The consensus amino acid sequence is indicated above the alignment of the three fringe sequences, and the beginning of the conserved region is indicated with a vertical line between amino acid residues 148 and 149 of the consensus sequence. Figure 5 is a schematic diagram comparing conceptual translations of avian and Drosophila fringe protein sequences. Each contains a signal peptide (green) followed by a poorly conserved domain (yellow) and a more highly conserved region (blue) . The mature protein is predicted to be produced by proteolytic cleavage at a tetrabasic site (RXXR, arrowhead) within the poorly conserved domain. Each mature domain contains six cysteines (C) at conserved positions. For each domain the percentage of identical amino acids between the two chick proteins, and between the chick and Drosophila proteins are indicated. Figure 6 is an illustration of interactions regulating AER formation and dorso-ventral patterning. Expression of En -1 represses expression of radical fringe (R-Fng) and Wnt- 7a 16 in the ventral (blue) ectoderm. As a consequence Wnt - 7a is expressed exclusively in the dorsal (red) ectoderm, where it induces expression of Lmx- 1 in the underlying dorsal mesoderm and hence regulates dorsal-specific patterning 12"14 . Expression of radical fringe is similarly confined to the dorsal ectoderm by EN- 1 . EN- 1 also promotes ventral fate determination in the ectoderm and underlying mesoderm 16 . Interaction between cells expressing (solid arrow) and not expressing (dotted arrow) radical fringe results in the formation of the AER, perhaps via downsteam signals involving the Serrate (Ser) -Notch (N) pathway, in response to permissive signals from the underlying mesoderm (grey arrowheads) . Detailed Description of the Invention Inductive signals from discrete signaling centers regulate much of embryonic development . The apical ectodermal ridge (AER) is an inductive center which functions during amniote limb development 1 . In birds, the AER is a pseudostratified columnar epithelium that runs antero-posteriorly (AP) along the distal margin of the limb bud ectoderm, and has several distinct roles during limb ontogeny. It is required for: maintaining developmental plasticity within the underlying limb mesenchyme; signaling to the posterior mesenchyme to maintain expression of the signaling molecule sonic hedgehog in the zone of polarizing activity (ZPA) ; synergizing with the ZPA to pattern the limb mesenchyme; and controlling the antero-posterior width of the developing limb bud. The signals produced by the AER which mediate each of these activities are members of the fibroblast growth factor family, notably FGF2 , FGF4 , and FGF8. The correct spatial and temporal control of the formation of signaling centers such as the AER is critical for normal embryogenesis. However, as with most embryonic signaling centers, how AER positioning is regulated is understood only to a limited extent . The first gene known to be expressed where the AER will form is FGF8, which is detected at stage 16 2"4 in a stripe several cell diameters wide running AP within the limb field. During stage 18 the FGF8 expression domain narrows and becomes restricted to the AER as it forms at the bud apex. Tissue grafting experiments have shown that signals from the limb mesoderm induce AER formation in the overlying ectoderm 5"7 , and that the capacities of both the mesenchyme to signal and the ectoderm to respond diminish over time, essentially being lost from both tissues by stage 20 8 . While mesodermal signals trigger AER formation, the information specifying both the position and number of AERs is encoded within the ectoderm 9,10 . The AER is positioned at the junction of the dorsal (D) and ventral (V) limb ectoderm. Several genes are expressed in DV restricted domains within the limb bud prior to AER formation, and are thus candidates for regulating AER formation. WNT- 7a and LMX-1 are expressed in dorsal ectoderm and mesoderm, respectively 11"13 . They specify dorsal limb mesodermal fates, but do not appear to be directly involved in AER formation, since neither loss of Wn t - 7a function 14 nor gain of WNT- 7a or LMX- 1 function 12,13 alters normal AER formation. Engrailed- 1 ( EN- 1 ) is a ventral limb ectodermal marker that is also expressed in the ventral half of the AER 15 . EN-1 has been directly implicated in regulating AER formation or maintenance, as well as in controlling ventral cell fates, as limb buds of en-1- / ' - mice have ventrally extended and flattened AERs, in addition to bi-dorsal distal structures 16 . Two recessive mutant chick strains, limbl ess 11,1B and eudiplopodia 19 , are defective in normal AER formation and provide some insights into the mechanisms of AER positioning. In limbl ess homozygote embryos, limb buds initiate normally, but regress because they never form an AER. Recombination experiments have shown that it is the limbless ectoderm that is defective 20 . Recent molecular analysis has found that limbl ess ectoderm is uniformly dorsal: the dorsal marker WNT- 7a is ubiquitously expressed, the ventral marker EN- 1 is absent, and rescue of limbless limb outgrowth with FGF-soaked beads results in bi-dorsal limbs 21"23 . These results have led to the hypothesis that the dorso-ventral and AER induction pathways are linked, and that AERs form due to interactions between dorsally- and ventrally-specified ectoderm. Limbs of eudiplopodia homozygote embryos display a different phenotype. After the normal AER has formed and limb bud elongation begins, ectopic AERs form on the dorsal surface of eudiplopodia limb buds, giving rise to polydactylous limbs. The resulting extra digits have a morphologically bi-dorsal phenotype, suggesting that AERs can arise independently of dorso-ventral ectodermal identity. However, molecular analysis of eudiplopodia limbs has not been performed, and alternative explanations are plausible. There are intriguing functional parallels between the AER of the vertebrate limb bud and the margin cells of the Drosophila wing imaginal disc which suggest that they may be induced by similar mechanisms. Like the AER, the wing margin is established at the DV compartment boundary and is required for proxi odistal wing outgrowth and patterning 24 . Cells arising from the dorsal half of the margin have dorsal identities, while those in the ventral half have ventral identities 24 - 25 . Partial loss of wing margin, like surgically produced gaps in the AER, results in proximo-distal patterning defects (notches) , while complete loss results in loss of the wing blade 24 . Genetic analysis has identified a signaling pathway which regulates positioning of the wing margin. ' Dorsal cell identity in the Drosophila wing disc is specified by expression of the selector gene apterous 24 ' 25 . Fringe, a putative secreted factor, is expressed within the dorsal wing compartment, under the control of apterous 26 . Ectopic margins form at the boundary of either dorsal loss of function or ventral gain of function fringe clones. In addition, either ubiquitous fringe expression or complete loss of fringe expression abolishes margin formation. Thus, the juxtaposition of fringe-expressing and non-expressing cells appears to be necessary and sufficient for margin formation within the context of the wing disc. Figure 5 shows a schematic diagram comparing conceptual translations of avian and Drosophila fringe protein sequences. Each contains a signal peptide (green) followed by a poorly conserved domain (yellow) and a more highly conserved region (blue) . The mature protein is predicted to be produced by proteolytic cleavage at a tetrabasic site (RXXR, arrowhead) within the poorly conserved domain. Each mature domain contains six cysteines (C) at conserved positions. For each domain the percentage of identical amino acids between the two chick proteins, and between the chick and Drosophila proteins are indicated. Significantly, outgrowths induced by ectopic margins in the dorsal compartment have dorsal identity, while those in the ventral compartment have ventral identity, as assessed by the DV character of the bristles on either side of the ectopic margin. Thus, margin formation per se is independent of DV fate specification; DV fate is determined by whether or not the cells express apterous. Interactions between fringe-expressing and non-expressing cells, either at the DV boundary or in ectopic clones, induces a series of complex interactions among the ligands serra te and delta, and their receptor, Notch, ultimately giving rise to a functional margin 27"34 . The analysis of AER formation described herein indicates that positioning of the AER is indeed regulated similarly to that of the Drosophila wing margin. As described herein, two chick homologs of fringe, radical fringe and l una tic fringe, have been isolated. The expression patterns of the vertebrate fringe genes have been analyzed in both wild-type and AER-defective chicken strains. Grafting experiments juxtaposing fringe-expressing and non-expressing limb ectoderm have been performed, and the vertebrate fringe and engrailed- 1 genes have been misexpressed within developing chick limb buds. The results of these studies support the hypothesis that the AER forms at the juxtaposition of radical fringe expressing and non-expressing cells, and furthermore support the dissociation of the molecular control of AER formation from that of dorso-ventral tissue specification. They also suggest that engrail ed-1 is indirectly involved in AER formation through regulation of radical fringe expression. Three highly conserved vertebrate Notch family members have been identified, each of which is widely expressed during embryogenesis (Artavnis-Tsakonas et al . , 1995) . Additionally, vertebrate members of the DSL (Delta-Serrate-Lag-2) family have recently been identified (Henrique et el . , (1995) ; Chitnis et al . , (1995) ; and Bettenhausen et el . , (1995)) , fueling the speculation that the process of lateral cell signaling occurs in vertebrates in a manner similar to invertebrates. The vertebrate DSL family of transmembrane proteins has at least six members falling into two structural classes bearing homology to either Delta or Serrate, the two identified ligands for Notch in Drosophila (Nye and Kopan, Current Biology 5 ( 9 ) :966- 969 (1995)) . A homolog of Del ta was independently isolated and named Delta-1 or Delta-like 1 (Dill) (Henrique et al . , (1995) ; Chitnis et al . , (1995) ; Bettenhausen, (1995) ) . Cloning using the polymerase chain reaction has yielded three more Del a homologs and three Serrate homologs (Nye and Kopan, (1995) ) . The effects of Notch signaling on molecular and cellular events have been studied using constitutively activated derivatives of Notch, and these studies have demonstrated that the activated intracellular derivatives of Notch have significant inhibitory effects on myogenesis and neurogenesis during Xenopus development and in mammalian cell culture systems that undergo differentiation (Coffman et al . , Cell 73:659-661 (1993) ; Kopan et al . , Development 120:2385-2396 (1994) ; Nye et al . , Development 120:2421-2430 (1994)) . The present invention relates to the ability of the fringe A and B proteins encoded by the fringe genes described herein to signal to or activate members of the DSL family of transmembrane proteins, which in turn bind to the Notch receptor and affect the differentiation and development of embryonic tissue. Thus, the fringe genes and proteins described herein are early regulators of tissue differentiation and development, upstream from the DSL/Notch receptor signalling mechanism. The secreted fringe proteins induce members of the DSL protein family, which in turn bind to the Notch receptor to affect lateral cell signalling. The present invention also pertains to isolated DNA encoding a polypeptide having at least one biological activity of a vertebrate fringe protein, as well as to the polypeptide or protein encoded thereby. In one embodiment, the present invention pertains to DNA encoding the fringe A ( radical fringe) and fringe B { l una ti c fringe) proteins and comprising the nucleic acid sequences of SEQ ID NOS: 1 and 6, respectively (Figures 1A-B and 2A-B, respectively) . The cDNA sequence (SEQ ID NO: 8) of chicken fringe A ( radical fringe) is shown in Figure 3; the coding region is indicated with capital letters. The present invention also pertains to the fringe A protein comprising the amino acid sequence of SEQ ID NO: 5 (Figure 1A-B) and the fringe B protein comprising the amino acid sequence of SEQ ID NO: 7 (Figure 2A-B) . The present invention also relates to an isolated polypeptide having at least one biological activity of a vertebrate fringe protein and comprising the amino acid sequence of SEQ ID NO: 5 or SEQ ID NO: 7. As defined herein, "isolated" DNA or protein includes isolation from the native environment of the cell or organism, recombinantly produced, or chemically synthesized DNA or proteins. In another aspect of the invention, the nucleic acid sequence of the DNA encoding a polypeptide having at least one biological activity of a vertebrate fringe protein is at least 30 percent identical with the nucleic acid sequence of either SEQ ID NO: 1 or 6. Another subject of the invention are fringe proteinε having an amino acid sequence which is at least 30 percent similar to the carboxy-terminal 60-75% of the amino acid sequence of SEQ ID NOS: 5 or 7, as well as the DNA encoding such proteins. The present invention also relates to polypeptides having at least one biological activity of a vertebrate fringe protein and having an amino acid sequence comprising six conserved cysteine residues located within the conserved carboxy- terminal 60-75% of the amino acid sequence. As used herein, the term "similar" with respect to amino acids means amino acid residues which are either identical or which represent a conserved change. For example, it is reasonable to expect that an isolated replacement of a leucine with an isoleucine or valine, an aspartate with a glutamate, a threonine with a serine, or a similar replacement of an amino acid with a structurally related amino acid (i.e., conservative replacements) will not have a significant effect on the biological activity of the resulting molecule. Conservative replacements are those that take place within a family of amino acids that are related in their side chains (e.g., acidic, basic, nonpolar, uncharged polar, aliphatic, amide, sulfur-containing and aromatic) (see, for example, Biochemistry 2nd ed. , ed. L. Stryer (W.H. Freeman and Co., 1981) . Whether a change in the amino acid sequence of a polypeptide results in a functional fringe homolog can be readily determined by assessing the ability of the variant polypeptide to produce a response in cells in a manner similar to the wild-type or unchanged fringe . Polypeptides in which more than one replacement has taken place can also be tested in the same manner. The present invention also pertains to nucleic acid sequences which hybridize to the nucleic acid sequences of either SEQ ID NO: 1 or 6 under hybridization conditions of medium stringency. Stringency conditions which are appropriately termed "medium stringency" are known to those skilled in the art or can be found in standard texts such as Current Protocols in Molecular Biology, John Wiley & Sons, N.Y. (1989) , 6.3.1-6.3.6. Exemplary biological activity which the vertebrate proteins or polypeptides of the present invention, including the fringe A protein, may have includes the ability to affect neural-specification of cell type identity, proliferative regulation of migratory paths, proliferation of neural crest (branchial arches) , definition of morphological boundaries, including morphological boundaries of the limb, cell shape decisions and indirect regulation of proliferative signals. Other biological activity which the vertebrate proteins and polypeptides of the present invention, including the fringe B protein, may have includes the ability to affect proliferative and fate specification signals of neural tissues, regulation of formation of components of ear, nose and facial structures, induction, patterning and proliferation of vasculature, and fate specification within somitic tissues. The present invention also provides expression vectors containing a nucleic acid sequence encoding a fringe polypeptide or a polypeptide having at least one activity of a fringe polypeptide, operably linked to at least one regulatory sequence. Operably linked is intended to mean that the nucleotide sequence is linked to a regulatory sequence in a manner which allows expression of the nucleic acid sequence. Regulatory sequences are art-recognized and are selected to direct expression of the nucleic acid sequence to produce a polypeptide having at least one activity of a fringe protein. Accordingly, the term regulatory sequence includes promoters, enhancers and other expression control elements. Such regulatory sequences are described in Goeddel, Gene Expression Technology: Methods in Enzymology 185 , Academic Press, San Diego, CA (1990) ) . It should be understood that the design of the expression vector may depend on such factors as the choice of the host cell to be transformed and/or the type of protein desired to be expressed. Prokaryotic and eukaryotic host cells transfected by the described vectors are also a subject of this invention. For instance, cells which can be transfected with the vectors of the present invention include, but are not limited to, bacterial cells such as E. coli , insect cells (baculovirus) , yeast, or mammalian cells such as Chinese hamster ovary cells (CHO) . Thus, a nucleotide sequence derived from the cloning of the vertebrate fringe polypeptides or proteins of the present invention, encoding all or a selected portion of the protein, can be used to produce a recombinant form of fringe via microbial or eukaryotic cellular processes. Ligating the polynucleotide sequence into a gene construct, such as an expression vector, and transforming or transfecting into hosts, either eukaryotice (yeast, avian, insect or mammalian) or prokaryotic (bacterial cells) , are standard procedures used in producing other well-known proteins, e.g., insulin, interferons, human growth hormone, IL-1, IL-2 and similar proteins. Similar procedures, or modifications thereof, can be employed to prepare recombinant fringe proteins or polypeptides, by microbial means or tissue-culture technology in accord with the subject invention. Depending on the expression system chosen, the ability to obtain a recombinant protein which is either glycosylated or not can be controlled. The recombinant fringe protein can be produced by ligating the cloned gene, or a portion thereof, into a vector suitable for expression in either prokaryotic cells, eukaryotic cells or both (see, for example, Broach et al . , Experimental Manipula tion of Gene Expression, ed. M. Inouye (Academic Press, 1983) p. 83; Molecular Cloning: A Laboratory Manual , 2nd Ed., ed. Sambrook et al . , (Cold Spring Harbor Laboratory Press, 1989) Chapters 16 and 17) . Expression vehicles for production of recombinant fringe include plasmids and other vectors. For instance, suitable vectors for the expression of fringe include but are not limited to plasmids of the types: pBR322-derived plasmids, pEMBL- derived plasmids, pEX-derived plasmids, pBTac-derived plasmids, and pUC-derived plasmids for expression in prokaryotic cells, such as E. coli . The present invention further pertains to methods of producing polypeptides which have at least one biological activity of a fringe protein. For example, a host cell transfected with a nucleic acid vector directing expression of a nucleotide sequence encoding a polypeptide having an activity of a vertebrate fringe protein can be cultured under appropriate conditions to allow expression of the polypeptide to occur. The polypeptide may be secreted and isolated from a mixture of cells and medium containing the polypeptide having at least one biological activity of a vertebrate fringe protein. Alternatively, the polypeptide can be retained cytoplasmically and the cells harvested, lysed and the polypeptide isolated. The polypeptide can be isolated from cell culture medium, host cells, or both using techniques known in the art for purifying proteins including ion-exchange chromatography, gel filtration chromatography, ultrafiltration, electrophoresis and immunoaffinity purification with antibodies specific for a polypeptide having at least one biological activity of a vertebrate fringe protein. In another embodiment, fusion proteins can also facilitate the expression of proteins, such as the fringe proteins of the present invention. For example, the fringe protein can be generated as a glutathione-S- transferase (GST) fusion protein. Such GST fusion proteins can enable easy purification of the fringe protein, such as by the use of glutathione-derived matrices (see, for example, Current Protocols in Mol ecular Biology, eds: Ausabel et al . (N.Y. : John Wiley & Sons (1991) ) . The present invention also provides compositions which are suitable for pharmaceutical administration. The fringe proteins of the present invention, or polypeptides having at least one biological activity of a fringe protein, can be formulated with a physiologically acceptable medium to prepare a pharmaceutical composition. The particular physiological medium may include, but is not limited to, water, buffered saline, polyols (e.g., glycerol, propylene glycol, liquid polyethylene glycol) and dextrose solutions. The optimum concentration of the active ingredient (s) in the chosen medium can be determined empirically, according to procedures well known to medicinal chemists, and will depend on the ultimate pharmaceutical formulation desired. Methods of introduction of exogenous fringe at the site of treatment include, but are not limited to, intradermal, intramuscular, intraperitoneal, intravenous, subcutaneous, oral and intranasal. Other suitable methods of introduction can also include rechargeable or biodegradable devices and slow release polymeric devices. The pharmaceutical fringe composition can also be administered as part of a combinatorial therapy with other agents . Another aspect of the invention pertains to an antibody specifically reactive with a polypeptide having at least one biological activity of a vertebrate fringe protein. For example, by using peptides having an activity of a vertebrate fringe protein, anti- protein/anti-peptide antisera or monoclonal antibodies can be made using standard methods. A mammal such as a mouse, a hamster or rabbit can be immunized with an immunogenic form of the polypeptide (i.d., an antigenic fragment of the polypeptide which is capable of eliciting an antibody response) . Techniques for conferring immunogenicity on a protein or peptide include conjugation to carriers or other techniques will known in the art. A peptide having at least one biological activity of a fringe protein can be administered in the presence of an adjuvant. The progress of immunization can be monitored by detection of antibody titers in plasma or serum. Standard ELISA or other immunoassays can be used with the immunogen as antigen to assess the levels of antibodies. Following immunization, anti-fringe antisera can be obtained, and if desired, polyclonal anti - fringe antibodies can be isolated from the serum. Monoclonal antibodies can also be produced by standard techniques which are well known in the art, such as the hybridoma technique (Kohler and Milstein, Na ture 256:495-497 (1975) ) , the human B cell hybridoma technique (Kozbar et al . , Immunology Today 4 : 12 (1983)) and the EBV-hybridoma technique (Cole et al . , Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp. 77-96 (1985) ) . In addition, nucleotide probes can be generated from the cloned sequences of the fringe proteins which allow for histological screening of intact tissue and tissue samples for the presence of fringe mRΝA. These probes can be used for both predictive and therapeutic evaluation of allelic mutations which might be manifest in, for example, birth defects. Used in conjunction with anti-fringe antibody immunoassays, the nucleotide probes can help facilitate the determination of the molecular basis for a developmental disorder which may involve some abnormality associated with expression (or decrease in or lack thereof) of a fringe polypeptide or protein. The present invention also features non-human transgenic embryos which ectopically express a fringe gene taken from another species (see, for example, U.S. Patent No. 4,873,316 (Meade e_t al. ) and U.S. Patent No. 5,215,904 (Goulde et al.)) . For example, a mouse embryo can be transfected with a DNA construct containing a chicken fringe gene. Such transgenic manipulations are useful to study the conservation in function of various fringe homologs across species. Results described herein indicate that AER induction at the distal tip of the limb bud is determined by the ventral boundary of the radical fringe expression domain. Ventrally expressed EN-1 also appears to act in this pathway by repressing radical fringe expression, thereby restricting it to the dorsal limb ectoderm. While AER formation can be affected by DV patterning genes such as EN-1 , it is independent of DV specification. Furthermore the similarities between the vertebrate AER and the arthropod wing margin suggest that conserved genetic pathways regulate the formation of these functionally analogous signaling centers. The radical fringe expression border regulates formation of the AER Data described herein indicate that the discrete radical fringe expression boundary within the limb ectoderm regulates AER formation. The radical fringe boundary correlates well with AER positioning in both wild type embryos and in eudiplopodia mutant embryos with ectopic AERs. Conversely in limbless mutant embryos, the lack of an AER correlates with the absence of a radi cal fringe expression border. Ventral to dorsal ectoderm grafts, which juxtapose radical fringe- expressing and non-expressing tissue, also induce ectopic AERs along a graft boundary. Finally, misexpression of radi cal fringe or l una ti c fringe can disrupt AER formation and/or induce ectopic AERs, as can misexpression of engrailed-1 , which represses radical fringe expression. Thus, several distinct lines of evidence indicate that the discontinuity in radical fringe expression which normally occurs at the DV boundary is important for AER positioning. A similar conclusion has been independently reached by Rodriguez, et al . 21 Misexpression of fringe within the chick limb bud results in AERs which are altered in three different ways: endogenous AERs are disrupted, ectopic ventral AERs are induced, and ventral bifurcations of endogenous AERs adjacent to disrupted AERs are observed. AER disruption appears to result from viral fringe expression extending across, and thereby altering, the normal radical fringe expression border. In the extreme scenario, the entire AP length of the bud expresses radi cal fringe, resulting in complete failure of AER development . Ectopic ventral AERs and ventrally bifurcated AERs appear to result from new boundaries created by ectopic ventral fringe expression domains. Perhaps unexpectedly, the ectopic AERs are not continuous rings, but rather discrete linear AER segments. This could be explained if AER formation requires a stable fringe border. Since the retroviral vectors used herein are replication-competent, the virus can continue to spread in infected limb buds, in many cases resulting in infection boundaries which are unstable. Ectopic AERs tend to be present either parallel or orthogonal to the endogenous AER. This phenomenon is observed under several different experimental conditions, including fringe or en-2 misexpression, following ectoderm grafts or in eudiplopodia mutant embryos. While this might be an artifact of either the nature of retroviral spread or the shape of the tissue grafts, neither explanation accounts for the ectopic eudipl opodia AERs. Thus, AER orientation might reflect inherent biases within the limb ectoderm, or a self- organizing behavior of the AER itself. The exclusively ventral location of the ectopic AERs following fringe misexpression is generally consistent with the normal expression pattern of radical fringe . Since radical fringe is normally expressed dorsally, ectopic AERs might be expected only on the non-expressing limb surfaces, as shown herein. However, eudiplopodia mutants form ectopic AERs on their dorsal limb surface, which correlate with late patches of ectopic dorsal radical fringe expression. Presumably, these ectopic dorsal domains define new radi cal fringe expression borders because they occur after radical fringe expression in the dorsal ectoderm has faded. Thus, viral fringe misexpression might be expected to also produce late stage ectopic dorsal AERs, but these have not been observed. This difference might be explained by the observation that eudiplopodia limb buds retain competence to form AERs until later stages than wild type limb buds 10,38 , and thus wild type limb ectoderm might be insensitive to late stage fringe misexpression. Disrupted AERs are observed coincident with viral fringe expression in either the ectoderm or mesoderm. It seems likely that the mesodermally produced fringe protein acts directly on the ectoderm, as fringe proteins are predicted to be secreted, and both Xenopus and avian fringe proteins can be readily detected extracellularly when expressed in cell culture 35 . Alternatively, fringe in infected mesoderm could act indirectly by interfering with the normal mesodermal AER-inducing signal. At present the answer to the question of how the fringe /non- fringe border acts to induce the AER remains unclear; however, clues may be found by extrapolation from the induction of the Drosophila wing margin which appears to be at least partially an analogous process. In the Drosophila wing disc, fringe acts in part by inducing the expression of serrate, a ligand in the Notch signaling pathway. Intriguingly, vertebrate serrate and Notch homologs, ser-2 and Notch- 139 , are expressed in the chick limb bud in a manner consistent with their being downstream of radical fringe in a similar genetic cascade (Figure 6; and D. Henrique, EL, CT and D. Ish-Horowicz, data not shown) . Engrailed function The AERs of en-1 - / ' - mice are elongated along the ventral limb ectoderm but have an appropriately positioned dorsal edge, suggesting that en -1 is involved in positioning the ventral margin of the normal AER 16. As shown by Logan et al . (C. Logan and A. Lumsden, personal communication) , and herein, ectopic en -1 can disrupt normal AER formation and give rise to dorsal AERs. It is shown herein that this likely occurs through repression of radical fringe expression. However, a border of EN-1 expression is not necessary for AER formation, since ectopic dorsal AERs form in eudiplopodia homozygote mutant limbs independently of EN- 1 expression. Taken together, these results suggest that EN-1 normally functions to position the ventral edge of the AER in the ventral ectoderm, possibly by regulating radical fringe expression. Nevertheless if a radical fringe expression boundary is established by an EN-1-independent mechanism, then an otherwise normal AER can develop. The AER forms independently of the dorso-ventral axis Meinhardt proposed 40 that AERs are induced by interactions occuring at the boundaries of the dorsal and ventral subdivisions of vertebrate limb fields. Experimental support for linkage between DV patterning and AER formation comes from analysis of limbless mutants embryos, in which loss of an AER and lack of DV boundary are coincident 21"23 . However, the ectopic AERs present in eudiplopodia mutant embryos clearly establish that AER formation per se can occur independently of a DV fate boundary. Thus, the dorsal markers WNT- 7a and LMX- 1 are expressed throughout eudiplopodia dorsal ectoderm where ectopic AERs form, while the ventral EN- 1 marker is excluded. The normal AER does form at the boundary between dorsal and ventral limb ectoderm, albeit through the expression of radi cal fringe. EN-1 apparently plays a key role in linking AER positioning and DV patterning, since it can repress both radical fringe and WNT- 7a 16 expression (C. Logan and A. Lumsden, personal communication) , thereby limiting them to the dorsal ectoderm. The same genetic pathway thus influences both DV patterning and AER positioning, although it subsequently separates into two distinct branches (Figure 6) . Previous grafting studies and induction of the AER In recombination experiments between limb mesoderm and flank ectoderm, the ectoderm has been found to respond by forming an AER until about stage 20 8 . This competence period corresponds well with the persistence of radical fringe expression within the limb ectoderm. However radical fringe expression is only weakly detected in the flank ectoderm following stage 17. One would expect that radi cal fringe is up-regulated following the grafting procedure, as the ectoderm becomes converted to a limb ectodermal fate 5"7,10 . Grafting of flank ectoderm to limb mesoderm can also induce novel AERs, which in some cases give rise to bi-dorsal digits, phenocopying eudiplopodia i L . Similar grafting experiments indicate that AERs are induced by a signal from the limb bud mesoderm. Since minced, recombined mesoderm works as effectively as intact mesoderm 5 , this appears to be a permissive, rather than an instructive, signal. An AER is apparently induced by the limb mesoderm as long as there also exists a radical fringe expression boundary along which the AER can form. This inductive signaling pathway likely involves a member of the FGF family, since FGF-soaked bead implants into stage 18 interlimb flank induce limb outgrowth and AER formation. Other FGFs might also be expressed within the nascent limb mesoderm. Evolutionarv Implications It is striking that radical fringe appears to act at the boundary of a continuous expression domain. This is remarkably similar to the function of Drosophila fringe within the wing disc, where fringe activity positions the wing margin along the edge of the fringe expression domain. Thus, in each case, fringe genes are acting within a two-dimensional epithelium to locate a linear signaling center that controls proximo-distal outgrowth. This apparent conservation of fringe gene function in vertebrates and insects, as well as the similar roles of the AER and wing margin as signaling centers, suggests that these are functionally homologous structures formed by the action of conserved genetic pathways. This is the second example of conserved signaling systems acting similarly during the development of both fly wings and amniote limbs. The AP axis of each is patterned through the action of the conserved hedgehog/ Sonic hedgehog genetic pathways 42"44 . The use of both the fringe and hedgehog pathways in patterning the limbs of members of these divergent phyla might be serendipitous. However, these two apparently independent genetic pathways might have already been used together to regulate the outgrowth and patterning of secondary fields in the last common ancestor of vertebrates and invertebrates. Both sets of signals are also present in the developing branchial arches 45 , and the arch epithelium has AER-like activity 46,47 , suggesting that their joint use in chordates at least predates the evolution of paired appendages. Molecular descriptions of the genetic networks which pattern other appendicular outgrowths will be necessary to determine how ancient this relationship is. The present invention will also facilitate further determination of the potential role of the fringe proteins in cell fate and development. In vi tro cell cultures can be used for the identification, isolation, and study of genes and gene products that are expressed in response to the presence of fringe; these genes would be downstream of the fringe signal. For example, if new transcription is initiated by the fringe signal, a subtractive cDNA library (Wang et a_l. PNAS USA 88.11505- 11509 (1991) ) prepared with control cells and cells treated with fringe can be used to isolate genes that are turned on or off by the fringe signal. Once isolated, the genes regulated by fringe induction can be sequenced and their embryonic distribution can be determined by in si tu hybridization approaches. If their embryonic expression is in agreement with the particular differentiation pattern being studied, they can be tested for that differentiating activity in cell culture and in embryos or organisms. The following criteria can be used, alone or in combination, to determine whether a particular gene, which is associated with development or differentiation, is regulated by the fringe gene or genes: (1) the RNA of the putatively regulated gene appears quickly following application of fringe; (2) the induction of the putatively regulated gene does not require previous protein synthesis; (3) where fringe is provided as a soluble factor, immediate early response genes should be expressed as a result of contact with fringe and not from a secondary cell-cell interaction; and (4) the putatively regulated gene should be present and activated in the appropriate tissue during the particular differentiation being studied. A further use of the invention concerns the therapeutic application of a fringe protein, or an agonist or antagonist thereof, to enhance survival of or increase the differentiation of particular tissues or cells in various systems throughout the body. The ability of fringe to regulate differentiation during development and also presumably in the adult state indicates that fringe can be reasonably expected to facilitate control of adult cells with regard to maintenance, functional performance and aging of normal cells; repair and regeneration processes in chemically or mechanically lesioned cells; and prevention of degeneration and premature death which result from a loss of differentiation in certain pathological conditions. In light of this knowledge, the present invention can be used in the treatment of, prevention and/or reduction of the severity of various disorders, including disorders of the vasculature system such as varicosities and atherosclerosis. Particularly since the subject fringe proteins appear to be involved in committing cells to form blood vessels, the fringe proteins of the present invention can also be used in the treatment of tumor pathogenesis, i.e., the ability to block the activity of the fringe proteins can be useful in preventing the establishment or maintenance of the blood supply to solid tumors. The fringe proteins of the present invention are also useful in wound healing, in that the augmentation of fringe activity, and thus the augmentation of angiogenesis, may aid in speeding the recovery of wounded regions of the body. The present invention, by making available purified or isolated fringe proteins, allows the development of assays which can be used to identify drugs or agents with are either agonists or antagonists of fringe activity. By mutagenesis, and other structural surveys of the fringe protein family, rational drug design can be employed to manipulate fringe A or B or portions thereof, as either agonists or antagonists, as well as to facilitate design of small molecule agonists and antagonists. The present invention also provides assays for identifying agents which alter the expression of a fringe gene or which alter the differentiation or development of an organism which is associated with the expression of a fringe gene. For example, the assay can be used to identify agents which alter the formation of angiogenic precursors or angiogenic cells or the AER or which alter the formation or subdivision of the neural tube. In the context of the present invention, the term "alters" includes both increasing and decreasing the differentiation of the organism, as well as changing the differentiation pattern of the organism. For example, an organism can be treated with the agent to be tested, and the expression pattern of the fringe A and/or fringe B genes can be assessed and compared with their respective expression patterns in a control organism in the absence of the agent to be tested. An increase in the expression of a particular fringe gene indicates that the agent is an agonist of at least one biological activity of, or has a positive effect on the expression of, that fringe gene. Similarly, a decrease in the expression of the fringe gene indicates that the agent is an antagonist of at least one biological activity of, or has a negative effect on, that fringe gene. Alternatively, a similar assay is provided for identifying an agent which alters the formation of angiogenic precursors or angiogenic cells. For example, an agent to be tested can be administered to an organism, and the expression of fringe B in the test organism can be assessed in tissue or cells in which angiogenic precursors are formed, and the expression can be compared with the expression of fringe B in a control organism in the absence of the agent to be tested. A difference in the expression of fringe B indicates that the agent alters the formation of angiogenic precursors or angiogenic cells. The same assay can be used to identify agents which alter the formation of the apical ectodermal ridge (AER) by assessing the expression of fringe A in place of fringe B. The same assay can be used to identify agents which alter the formation or subdivision of the neural tube by assessing the expression of fringe A and/or fringe B. As defined herein, "expression" of the fringe genes includes both the location and extent or quantitative amount of expression in the organism. Furthermore, the comparison between the test organism and the control organism can be in terms of the whole organism (i.e., the expression as a whole) or in terms of comparable portions or tissues of the organisms. As described in the following examples, the DNA clones provided by the present invention were obtained by first screening a chick genomic library with a partial human fringe cDNA clone. Positive plaques were identified and two chicken clones were selected. These clone were then used as probes to obtain genomic clones containing the coding sequence of the chicken fringe A and B genes . In order to identify other chicken fringe homologs, the chicken fringe A and B clones were used to probe a genomic Southern blot containing chicken DNA. These results indicate that there is an additional chicken fringe gene ( fringe C) . This methodology can also be used to identify other vertebrate fringe homologs. As described in the Examples, Northern blots and in situ hybridization demonstrated that the chicken fringe A gene is expressed early (from approximately HH stage 12 on (Hamburger & Hamilton, J. Exp . Morph . 88:49-92 (1951) ) ) in anterior neural tube/brain, surface ectoderm, ectodermal borders between branchial arches, otocyst, dermomyotomal compartment of somites, and dorsal limb ectoderm, with most concentration at the dorsal/ventral boundary just prior to apical ectodermal ridge (AER) formation, then restricted to the AER through at least stage 25. Northern blots and in si tu hybridization also demonstrated that the chicken fringe B gene is expressed in the early (stage 6+, just post-gastrulation) stripe in pre-somitic mesoderm, which may indicate presumptive somite, and also in the dorsal neural fold. Neural tube expression evolves into a series of six stripes running longitudinally the length of the CNS from tail to rostral forebrain, at least through day 10 of development. Expression of fringe B is restricted primarily to proliferative layers and is excluded from absolute dorsal and ventral regions. Fringe B is also expressed in a single stripe within the otocyst, within the complete nasal placode, within a subset of migrating branchial neural crest. Somitic expression evolves to the dermomyotome, which then devolves to a dorsal and ventral component. The ventral expression is restricted to the lateral epithelium which gives rise to epaxial muscle and dermis . The dorsal component may be blood vessel precursors, as later (day 4 and on) expression is seen in a meshwork of smaller blood vessels throughout the body. From the expression pattern results detailed in the Examples and described above, it appears that fringe A functions in the neural specification of cell type identity, proliferative regulation of migratory pathways, neural crest/branchial arch proliferation, definition of morphological boundaries, cell shape, limb specification of morphological boundaries and indirect regulation of proliferative signals. As described herein, fringe A has been shown to be an early marker for development of the AER, as it is expressed both in the formed AER and in cells which are AER precursors, i.e., destined to form the AER. Further, it appears that fringe B functions in proliferative and fate specification signals of neural tissues, regulation of the development of components of the ear, nose and facial structure, induction, patterning and proliferation of vasculature, and fate specification within somitic tissues. It has been shown that the pattern of fringe B gene expression is similar to the expression pattern of the Delta homolog Delta-1 (Dill) in the primary neurons of the central nervous system and posterior mesoderm of the developing chicken embryo. This similarity of expression may indicate that fringe B has a direct role in the regulation of the Delta-1/Notch signalling pathway. It has also been shown that the pattern of fringe A expression is complementary to the pattern of expression of the Serrate homolog Serrate-1, producing a striped expression pattern. This complementary expression pattern may indicate that fringe A is an indirect regulator of Serrate-1. However, the expression pattern of fringe A has been shown to be similar to the expression pattern of Serrate homolog Serrate-2 in the AER of the developing chicken embryo, perhaps indicating a direct regulation, as in the fringe B/Delta-1 relationship. Fringe proteins, including the chicken fringe A and B proteins described herein and the Drosophila fringe protein described in Irvine & Wieschaus, Cell 79 : 595-606 (1994) , appear to be divided into 3 portions. The first portion, comprising 30-35 amino acids, appears to define a signal peptide in each protein. The second portion, which extends in fringe A through amino acid 104, in fringe B through amino acid 97 and in Drosophila fringe through amino acid 147, shows little or no conservation of sequence. The third portion, herein called the "conserved region", comprises the carboxy-terminal 60- 75% of the protein, and this portion of each protein comprises six conserved cysteines at equivalent positions (Figure 4A-B) . In fringe A, these positions are amino acid residues 160, 171, 189, 253, 357 and 366 (Figure 1A-B) ; in fringe B, these positions are amino acid residues 152, 163, 181, 244, 348 and 359 (Figure 2A-B) . This conserved region, and in particular the conserved cysteines, appear to be important to the function of the fringe protein, and the presence or absence of the conserved region and/or the conserved cysteine residues is a particularly helpful characteristic which can be used to identify other members of the vertebrate or invertebrate fringe protein family. Accordingly, this invention also relates to polypeptides or proteins which have greater than about 30% identity to the amino acid sequence of the conserved region of fringe A or fringe B. This invention also pertains to polypeptides or proteins which have conserved cysteine residues at positions equivalent to the positions of the conserved cysteine residues in the conserved region (Figure 4A-B) . Terms used herein are intended to have their art- recognized meaning unless otherwise defined. Teachings of the references cited herein are hereby incorporated herein by reference. The following examples are offered for the purpose of illustrating the present invention and are not to be construed to limit the scope of the present invention: Examples Chick embryos Fertile White Leghorn chicken eggs were purchased from SPAFAS, Inc., (Norwich CT, USA) , for all experiments involving retroviral infection. Ectoderm grafting experiments were done using White Leghorn embryos from a flock maintained at the University of Wisconsin-Madison. Eudiplopodia mutant embryos were produced by crossing heterozygous Single-comb White Leghorn chickens. In each cross, one parent was highly inbred (back-crossed to UCD line 003) . Mutant embryos were identified by ectopic dorsal limb outgrowths visible from stage 22. Limbless mutant embryos were obtained from a heterozygous mating flock maintained at the University of Wisconsin-Madison Poultry Science Dept. 22 . Limbless mutant embryos were identified by lack of AERs at stage 18/19. All embryos were incubated at 38°C in a humidified atmosphere. Tissue grafts Stage 18/19 donor embryos were lightly stained in ovo with neutral red to mark cells of the AER, limb buds were amputated, and ectodermal jackets isolated as described 48 . Ventral ectodermal tissue grafts were dissected free of the ectodermal hull in 1XEBSS + 10% horse serum, carefully excluding neutral red-retaining apical ridge cells from the graft tissue. Stage 18/19 host leg bud dorsal surface graft beds were prepared, and the grafting technique performed, as described ' '-. Cloning of Vertebrate fringe genes 600,000 plaques of a HH stage 12-15 whole chick embryo library cloned in the lambda ZAP vector were screened with the inserts of expressed sequence tags c- 32hl0 (Genbank F11888) and c-3md09 (Genbank F13368) . The filters were hybridized at 30°C in 50% deionized formamide, 2 x SSC, 10% dextran sulfate, 1% SDS and washed at 50°C in 2 x SSC , 1% SDS. A total of 32 l una ti c fringe and 5 radical fringe clones were isolated and recovered in pBluescript SK+ (Stratagene, La Jolla, CA USA) . Plaques containing partial cDNA clones of both fringe A and fringe B were isolated, and their inserts used to rescreen the library under the same conditions. Overlapping cDNAs for each gene were sequenced by the dideoxy chain termination method (Sanger et al . , PNAS USA 74:5463-5467 (1977)) , resulting in 3.5 kb of radical fringe and 2.8 kb of luna tic fringe sequence. Individual clones cFR30 and cFR42 contained the entire open reading frames of lunatic and radical fringe, respectively, and were used for retroviral construction. Computer analysis of DNA and protein sequences was performed using the DNAstar software suite (Madison, WI USA) . Standard recombinant DNA methodologies 50 were used, and enzymes were purchased from Boehringer Mannheim Biochemicals (Indianapolis, IN) . Whole-Mount In Situ Hybridization Whole-mount in situ hybridization was performed using procedures modified from Parr et al . (Development 125:247-261 (1993)) , Sasaki and Hogan (Development 228:47-59 (1993)) and Rosen and Beddington ( Trends Genet. 9:162-167 (1993) . Chick embryos were removed from the egg, and extraembryonic membranes were dissected in calcium-free and magnesium-free phosphate- buffered saline (PBS) at room temperature. Unless otherwise noted, all washes are for 5 minutes at room temperature. Embryos were fixed overnight at 4°C with 4% paraformaldehyde in PBS, washed twice with PBT (PBS with 0.1% Tween 20) at 4°C, and dehydrated through an ascending methanol series in PBT (25%, 50%, 75%, 2 x 100% methanol) . Embryos were stored at -20°C until further use. Both prelimb bud and limb bud stage embryos were rehydrated through a descending methanol series followed by two washes in PBT. Limb bud stage embryos were bleached in 6% hydrogen peroxide in PBT, washed three times with PBT, permeabilized with proteinase K (Boehringer Mannheim, 2 μg/ml) for 15 minutes, washed with 2 mg/ml glycine in PBT for 10 minutes and washed twice with PBT. Prelimb bud stage embryos were permeabilized (without prior incubation with hydrogen peroxide) by three 30 minute washes in RIPA buffer (150 mM NaCl, 1% Nonidet p-40, 0.5% deoxycholate, 0.1% SDS, 1 mM EDTA, 50 mM Tris-HCl [pH 8.0]) . In all subsequent steps, prelimb bud and limb bud stage embryos were treated equivalently. Embryos were fixed with 4% paraformaldehyde plus 0.2% gluteraldehyde in PBT, washed four times with PBT, washed once with prehybridization buffer (50% formamide, 5 x SSC, 1% SDS, 50 μg/ml total yeast RNA, 50 μg/ml heparin [pH 4.5]) , and incubated with fresh prehybridization buffer for 1 hour at 70°C. The prehybridization buffer was then replaced with hybridization buffer (prehybridization buffer with digoxigenin-labeled riboprobe at 1 μg/ml) and incubated overnight at 70°C. Following hybridization, embryos were washed three times for 3 minutes each time at 70°C with solution 1 (50% formamide, 5 x SSC, 1% SDS [pH 4.5]) , three times for 30 minutes each time at 70°C with solution 3 (50% formamide, 2 x SSC [pH 4.5] ) , and three times at room temperature with tris-buffered saline (TBS, with 2 mM levamisole) containing 1% Tween 20. Nonspecific binding of antibody was prevented by preblocking embryos in TBS plus 1% Tween 20 containing 10% heat-inactivated sheep serum for 2.5 hours at room temperature and by preincubating anti-digoxigenin Fab alkaline phosphatase conjugate (Boehringer Mannheim) in TBS pliϊs 1% Tween 20 containing heat-inactivated 1% sheep serum and approximately 0.3% heat-inactivated chick embryo powder. After an overnight incubation at 4°C with the preadsorbed antibody in TBS plus 1% Tween 20 containing 1% sheep serum, embryos were washed three times for 5 minutes each time at room temperature with TBS plus 1% Tween 20, five times for 1.5 hours each time at room temperature with TBS plus 1% Tween 20, and overnight with TBS plus 1% Tween 20 at 4°C. The buffer was exchanged by washing three times for 10 minutes each time with NTMT (100 mM NaCl, 100 mM Tris-HCl, 50 mM MgCl 2 , 1% Tween 20, 2 mM levamisole) . The antibody detection reaction was performed by incubating embryos with detection solution (NTMT with 0.25 mg/ml nitroblue tetrazolium and 0.13 mg/ml 5-bromo-4-chloro-3-indolyl- phosphate toluidinium) . In general, prelimb bud stage embryos were incubated for 5-15 hours and limb bud stage embryos for 1-5 hours. After the detection reaction was deemed complete, embryos were washed twice with NTMT, once with PBT (pH 5.5) , postfixed with 4% paraformaldehyde/O .1% glutaraldehyde in PBT, and washed several times with PBT. In some cases, embryos were cleared through a series of 30%, 50%, 70% and 80% glycerol in PBT. Whole embryos were photographed under transmitted light using a Nikon zoom stereo microscope with Kodak Ektar 100 ASA film. Selected embryos were processed for frozen sections by dehydration in 30% sucrose in PBS followed by embedding in gelatin and freezing. Cryostat sections (25 μm) were collected on superfrost plus slides (Fisher) , rehydrated in PBS, and mounted with gelvatol. Sections were photographed with Nomarski optics using a Zeiss Axiophot microscope and Kodak Ektar 25 ASA film. Several hundred bases near the 5' end of each fringe ORF have very high GC content (85-95%) . Clones cFR8 ( luna tic fringe, 2.8 kb insert, Clal digest, T3 polymer¬ ase) and cFRlO ( radical fringe, 3.5 kb, Clal, T3) which do not contain the GC-rich domains were used as templates for in si tu hybridization probes. The probes for LMX-2 13 , WZT-7a 51 , FGF8 4 , and Engrailed-2 21 have been described. RCAS retroviruses Radical fringe and luna tic fringe cDNAs encoding the entire open reading frame of each gene with no 5' or 3' untranslated sequences were fused in frame with the initiator ATG of the pSLAX-13 shuttle vector. Influenza hemagglutinin (HA) or Flag (Kodak, Rochester NY, USA) epitope tags were then added to the COOH terminus of each protein. The tagged cDNAs were then transferred as Clal fragments into the RCASBP(B) retroviral vector. The RCASBP (A) en-2 virus has been described 36 . Retrovirus was prepared, concentrated and titered essentially as described 52,53 . Retroviral titers ranged from 5xl0 8 to 2xl0 9 cfu/ml. Chick embryo fibroblasts infected with the tagged viruses were stained with antibodies to both retroviral pl9gag (3C254) and HA (12CA555) or Flag, and detected by indirect immunohistochemical methods. In all cases viral infection and fringe gene expression were found to be >99% coincident . Stage 7-11 embryos were injected with concentrated viral supematants, supplemented with 80 ng/ml polybrene for B-coat viruses, along the flank and in the neural tube, allowing significant amounts to spill on the lateral plate ectoderm. Embryos were allowed to develop to stages 17-28, then fixed for further analysis. Results AERs can be induced bv juxtaposing dorsal and ventral limb ectoderm The AER normally forms at the dorsal-ventral ectodermal boundary. To test whether the apposition of dorsal and ventral limb bud ectoderm is sufficient to lead to AER formation, a series of ectodermal grafting experiments were performed. Ectoderm, removed from either the dorsal or ventral surface of a stage 18/19 donor limb bud, was grafted onto the ectoderm-denuded dorsal surface of a stage 18/19 host leg bud. Twenty to twenty-six hours later, the embryos were harvested and analyzed for the presence of ectopic AERs by both morphological and molecular criteria. In 77% (23/29) of limb buds that received a ventral graft, ectopic AERs were detected. In transverse section they appear as pseudostratified columnar epithelia, similar to the endogenous AER. They also express the AER markers FGF8, MSX-2 and radi cal fringe and stain positive with neutral red, a vital dye concentrated by AER cells. Surprisingly, while these structures developed along the anterior edge of the graft/host tissue boundary, abutting the endogenous AER at approximately a 90 degree angle, they never formed along any other edge of the graft. Dorsal versus dorsal grafts (n=7) and control experiments removing dorsal ectoderm (n=6) or removing . dorsal ectoderm and surgically disrupting the endogenous AER (n=7) never gave rise to ectopic AERs. These data demonstrate that the juxtaposition of dorsal and ventral limb bud ectoderm results in AER formation. Cloning and expression of chicken fringe homologs The results of these grafting experiments support the idea of linkage between dorso-ventral patterning and AER formation. The analogy between AER and Drosophila wing margin function, and the parallel between their locations at their respective DV boundaries, raised the possibility that a vertebrate fringe homolog might function in AER formation, perhaps downstream of the genetic pathway leading to DV fate specification. A search of the expressed sequence tag database (dbEST) with the Drosophila fringe gene sequence identified two ESTs encoding human fringe homologs. These were used to clone cDNAs for two different chick fringe genes, radi cal fringe and lunatic fringe . Each gene has a single large open reading frame encoding a protein of approximately 370 amino acids with the hallmarks of post-translationally- processed secreted factors. The ORFs contain putative signal peptides, followed by a poorly conserved pro-domain and then a highly conserved mature region of about 265 amino acids. Approximately 20-25 amino acids amino terminal to the conserved sequences is a "tetrabasic" sequence, which likely defines a cleavage site for the production of the mature protein. The tetrabasic sites as well as six cysteines are conserved among the vertebrate and Drosophila proteins, suggesting they all share a proposed 'cysteine knot' structure. Each ORF also contains one putative N-linked glycosylation site. Analysis of both in vi tro translation and in vivo misexpression products of these genes are consistent with their encoding secreted, glycosylated, processed factors. These results are similar to those reported for the Xenopus lunatic and radi cal fringe genes 35 . To investigate whether either of these vertebrate fringe genes might be involved in AER formation, their RNA expression patterns were examined by in si tu hybridization. At stage 15/16, prior to limb bud initiation, radical fringe mRNA expression is detectable in the dorsal ectoderm along the primary body axis and in two broad lateral domains situated at axial levels corresponding to the presumptive limb fields. Within the limb field domains, a sharp demarcation of expression is seen such that the more lateral regions of the ectoderm, which will give rise to ventral limb ectoderm, have greatly reduced levels of radical fringe expression. As the limb buds emerge at stage 17, high levels of radical fringe expression remain confined to the dorsal ectoderm, with the sharp ventral boundary maintained. Still farther laterally, expression is detected in ectoderm overlying the presumptive body wall mesoderm. As the limb bud continues to grow, there are a series of changes in radical fringe expression which presage overt AER formation. Expression becomes elevated on the dorsal side of the DV border, is downregulated throughout the rest of the dorsal ectoderm, and becomes undetectable in the ventral limb ectoderm. Thus radi cal fringe expression becomes confined primarily to the mature AER, where it is maintained until at least stage 27, while low levels of dorsal ectodermal expression are detected until stage 24. Radical fringe expression is also detected in the somites, notochord and branchial arches. Somitic expression during early limb bud stages is restricted to the dermomyotome at more anterior axial levels. Expression is localized to the anterior branchial arch ectoderm prior to branchial ridge formation, and then becomes restricted to the ridges running along the posterior edge of each arch. Lunatic fringe has a complex and dynamic expression pattern throughout embryogenesis. Limb expression of luna tic fringe is confined to a population of cells which appear to be migrating into the limb mesenchyme from stage 17, and subsequently establish patterns suggestive of the limb vasculature. It is likely that these are angioblasts, since l unatic fringe is expressed in a subset of vascular cells throughout the embryo. Lunatic fringe expression is not detectable within the limb ectoderm, and thus probably does not normally contribute to AER formation. The expression of radical fringe in the dorsal limb ectoderm allowed its use as a marker to investigate whether the ectopic AERs induced in the grafting experiments arose precisely at the juxtaposition of ventral and dorsal ectoderm. In si tu hybridization with the radical fringe probe verified that the AERs formed at the border between radical fringe-expressing and non-expressing cells. Radical fringe expression patterns reflect abnormal AER formation in mutant limb buds The position of the AER in normal limb buds corresponds to both the juxtaposition of dorsal and ventral ectodermal tissue and the radical fringe expression boundary. To investigate whether either of these parameters could be dissociated from the positioning of the AER, the expression of marker genes were examined in the two mutant chick strains limbless and eudiplopodia . In limbless homozygote limb buds, which have previously been shown to be bi-dorsal in character and lack an AER, radical fringe expression is detected throughout the limb bud ectoderm. This result is consistent with both possibilities, that the AER fails to form due to lack of either DV or radical fringe boundaries. Ectopic AERs in eudiplopodia homozygote embryos are initially visible at stages 22-23 and produce very obvious dorsal outgrowths by stage 25. Ectopic patches of upregulated radical fringe expression are detected on the dorsal surface of eudiplopodia homozygote limb buds from stage 22 and are larger than, and associated with, nascent outgrowths. By stages 25-26 radical fringe expression is restricted to the ectopic and endogenous AERs . Intriguingly, the ectopic AERs in eudiplopodia mutant embryos tend to form along one of two orthogonal axes . As judged by expression of the AER specific marker FGF8 , the ectopic AERs form primarily either parallel or perpendicular to the position of the normal AER. The outgrowths which result from ectopic eudiplopodia AERs are morphologically bi-dorsal, suggesting that they may not arise from the juxtaposition of dorsal and ventral ectoderm. This was tested by examining the expression of genes known to specify DV cell fate, and which additionally serve as dorsal and ventral molecular markers. The dorsal ectoderm marker WNT- 7a is expressed throughout eudiplopodia homozygote dorsal ectoderm, but excluded from the ectopic AERs. The dorsal mesodermal marker LMX- 1 is similarly expressed throughout eudiplopodia homozygote dorsal mesoderm. In contrast the ventral ectodermal marker EN- 1 is expressed only in eudiplopodia homozygote ventral ectoderm and the ventral half of the endogenous AER. It is not detectable in the ectopic AERs or the ectodermal surfaces of the ectopic outgrowths. Thus, in eudiplopodia , the specification of ectopic AER position occurs independently of the expression pattern boundaries of genes known to impart DV cell fate but correlates precisely with radi cal fringe expression boundaries. Ectopic expression of vertebrate fringes can alter AER formation The expression pattern of radical fringe in wild type and limbl ess and eudiplopodia mutant limb buds suggests that the border between radical fringe-expressing and non-expressing cells may be normally involved in specifying the position of the AER. If true, then abolishing this border by misexpression of fringe genes should prevent AER formation, and lead to a lack of outgrowth of the underlying mesoderm, while ectopic fringe expression domains might induce ectopic AERs. To test directly these predictions, both luna tic and radical fringe were misexpressed in developing limb buds using replication-competent retroviral vectors. Embryos were infected at stages 7-11 and allowed to develop until stages 22 to 28, after AER formation and considerable limb outgrowth had occurred. Viruses encoding either fringe gene produced limb buds with similar mutant phenotypes, which were not observed with control retroviruses encoding alkaline phosphatase. Resultant phenotypes can be divided into three distinct classes based on limb bud morphology and expression of the AER marker FGF8. The first class includes limb buds with disrupted AERs. Those segments of the distal ectoderm lacking a morphological AER also do not express FGF8. The mesoderm underlying these regions fails to grow at the same rate as regions subjacent to an AER, resulting in limbs with a dramatically notched appearance. The second phenotypic class has gaps in the endogenous AER as well as ectopic AER segments on the ventral surface supporting ectopic distal outgrowths. No ectopic AERs were observed on the dorsal surface of any infected limb bud. A subset of these ectopic AERs are perpendicular to the normal AERs, and adjacent to gaps within the normal AER, suggesting that a domain of misexpression simultaneously disrupted the endogenous fringe boundary and created a new one. These perpendicular AERs are remarkably similar to those seen on both the eudiplopodia mutant limbs and following ventral to dorsal ectoderm grafts. In the third class, embryos are missing an entire limb. These phenotypes are consistent with predictions based on the model that borders of radi cal fringe expression play a role in AER formation. It was important to determine whether, in the ectopic case, the alterations in AER patterning indeed correlate with boundaries of fringe misexpression. Several infected embryos with disrupted AERs were sectioned and stained for expression of FGF8 as an AER marker and a retroviral protein epitope to delineate the domain of viral infection. In each case boundaries of viral expression were found to correlate with the gain or loss of an AER. For example, an embryo was examined in which the anterior AER is disrupted, a small ectopic ventral AER was induced and in the posterior the normal AER is intact. In the anterior, where the AER is lost, viral infection is observed throughout the DV extent of the anterior limb ectoderm, abolishing the fringe expression border. The first serial section with a disrupted AER is also the first section showing viral expression on both the dorsal and ventral sides of the DV border. Where the ectopic ventral AER is present, it is found at the ventral edge of a patch of ventral ectodermal infection, which created a new border of fringe-expressing and non-expressing cells. More posteriorly, where the normal AER remains intact, the dorsal ectoderm is infected, in some regions including portions of the AER itself, but the ventral ectoderm is sparsely infected, if at. all. In another embryo which has only a single hind limb, heavy infection is detected throughout the mesoderm and ectoderm of the affected side, while only the dorsal ectoderm is infected in the contralateral normal limb, suggesting that total limb bud infection completely abolished the AER and hence abolished limb outgrowth. In another instance, loss of the AER was found to correlate with heavy mesodermal infection subjacent to the expected position of the AER, while the ventral and distal ectoderm itself was only lightly infected. Since the fringe proteins are secreted factors, it is believed that in this case AER disruption was mediated by fringe signaling from the mesoderm to the overlying ectoderm. These results are consistent with the hypothesis that the observed phenotypes result from ectopic ventral fringe expression altering the normal radi cal fringe expression boundary. Engrail ed specifies AER position bv repressing radical Fringe expression EN- is normally expressed in the ventral limb bud ectoderm, and the abnormal AERs found in en-1 - / '- mice indicate that it normally is involved in AER formation. The effects of ectopic EN-1 expression were therefore investigated. Stage 7-10 chick embryos were infected with a replication-competent retrovirus expressing murine en-2 36 , harvested from stages 17 to 27, and analyzed for AER formation. The phenotypes observed are similar to those seen following infection with the fringe viruses. They include disrupted AERs and notched limbs, ectopic AERs and bifurcated endogenous AERs. Notably, the ectopic AERs were found on the dorsal limb surface, unlike the ventrally located AERs induced by fringe misexpression. These phenotypes are similar to those first observed in en - 1 misexpression studies designed to examine DV limb patterning (C. Logan and A. Lumsden, personal communication) . Analysis of viral epitope expression in affected limb buds showed dorsal ectodermal expression correlating with AER loss. Prior to AER formation, the ventral limb ectodermal EN-1 expression domain is essentially complementary to that of radical fringe, suggesting that EN-1 might repress radical fringe expression. Radical fringe expression was therefore assessed at stages 17 and 18, prior to and during AER formation, in embryos infected with the en-2 virus. In en-2 infected limb buds large patches of the normal dorsal radical fringe expression domain were absent. Repressed radi cal fringe expression was detected at a frequency approximately equivalent to that of severely disrupted AERs in embryos allowed to develop to later stages within the same experiment. These results are consistent with the idea that EN-2 normally functions to position the AER by repressing radical fringe expression. Eguivalents Those skilled in the art will recognize or be able to ascertain, using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed in the scope of the following claims. REFERENCES 1. Tickle, C. Curr. Opin. Genet. Dev. 5:478-484 (1995) . 2. Hamburger S. Hamilton, J Exp Morph 88:49-92 (1951) . 3. Crossley et al . , Cell 84:127-136 (1996) . 4. Vogel et al . , Development 122:1737-1750 (1996) . 5. Kieny, Embryol Exp Morphol 8:457-467 (1960) . 6. Kieny, M. Arch d'Anat Micro 57:401-418 (1968) . 7. Saunders & Reuss, Dev Biol 138:, 41-50 (1974) . 8. Carrington & Fallon, J. Embryol Exp Morph 84:19-34 (1984) . 9. Goetinck, P. Dev Biol 20:71-91 (1964) . 10. Fraser & Abbott, J Exp Zool 276:237-48 (1971) . 11. Parr et al . , Development 229:247-61 (1993) . 12. Vogel et al . , Nature 378:716-720 (1995) . 13. Riddle et al . , Cell 83:631-640 (1995) . 14. 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Claims: CLAIMS We claim: 1. Isolated DNA encoding a polypeptide having at least one biological activity of a vertebrate fringe protein. 2. Isolated DNA according to Claim 1, said DNA comprising a nucleic acid sequence selected from the group consisting of: a) SEQ ID NO: 1; and b) SEQ ID NO: 6. 3. Isolated DNA according to Claim 1, said DNA comprising a nucleic acid sequence which hybridizes under conditions of medium stringency to a nucleic acid sequence selected from the group consisting of: a) SEQ ID NO: 1; and b) SEQ ID NO: 6. 4. Isolated DNA according to Claim 1, wherein the verte¬ brate organism is a chicken. 5. An isolated protein encoded by the DNA of Claim 1. 6. An isolated protein encoded by the DNA of Claim 2. 7. An isolated protein encoded by the DNA of Claim 3. 8. An isolated polypeptide having at least one biological activity of a vertebrate fringe protein. 9. An isolated polypeptide according to Claim 8, comprising an amino acid sequence selected from the group consisting of: a) SEQ ID NO: 5; and b) SEQ ID NO: 7. 10. An isolated polypeptide according to Claim 8, wherein said biological activity is selected from the group comprising a vasculature-inducing activity, a somite- inducing activity and an apical ectodermal ridge- inducing activity. 11. An isolated polypeptide according to Claim 10, wherein said vasculature-inducing activity comprises inducing the formation of angioblasts, blood vessels and blood cells. 12. An isolated polypeptide according to Claim 10, wherein said somite-inducing activity comprises inducing a subset of mesodermal cells to form somites. 13. An isolated polypeptide according to Claim 10, wherein said apical ectodermal ridge-inducing activity comprises inducing the formation of the apical ectodermal ridge or determining the position of the apical ectodermal ridge. 14. An isolated polypeptide according to Claim 8, comprising an amino acid sequence which is greater than about 30% identical to the amino acid sequence of the carboxy-terminal 60% of either SEQ ID NO: 5 or SEQ ID NO: 7. 15. An isolated polypeptide according to Claim 8, comprising an amino acid sequence comprising six conserved cysteine residues in the carboxy-terminal 60% of the polypeptide. 16. An immunogen comprising a portion of the polypeptide of Claim 8. 17. An expression vector comprising the DNA of Claim 1. 18. A host cell transfected with the expression vector of Claim 17 capable of directing the expression of a polypeptide having at least one biological activity of a fringe protein. 19. A method of producing a polypeptide having at least one biological activity of a fringe protein, comprising culturing a host cell of Claim 18 in a cell culture medium to express said polypeptide and isolating said polypeptide from said cell culture. 20. A composition suitable for pharmaceutical administration comprising: a) at least one polypeptide capable of functioning in one of either role of an agonist of at least one biological activity of a fringe protein or an antagonist of at least one biological activity of said fringe protein; and b) a physiologically acceptable carrier. 21. An antibody which specifically binds to the polypeptide of Claim 8. 22. A non-human transgenic animal comprising an expressible form of the DNA of Claim 1. 23. A method for treating tumor pathogenesis, comprising administering to a patient a therapeutically effective amount of a pharmaceutical preparation of the polypeptide of Claim 8. 24. A method of inducing the formation of an apical ectodermal ridge in an organism, comprising contacting the organism with a polypeptide of Claim 8. 25. A method of inducing the formation of a somite in an organism, comprising contacting the organism with a polypeptide of Claim 8. 26. A method of inducing the formation of angiogenic precursors or angiogenic cells in an organism, comprising contacting the organism with a polypeptide of Claim 8. 27. An assay for identifying an agent which alters the formation of an apical ectodermal ridge in an organism, comprising the steps of: a) administering to the organism an agent to be tested, thereby producing a test organism; b) determining the expression of fringe A in the test organism in tissue or cells in which the apical ectodermal ridge is formed; and c) comparing the expression of fringe A determined in step (b) with the expression of fringe A in tissue or cells in which the apical ectodermal ridge is formed in a control organism to which the agent to be tested is not administered, wherein a difference in the expression of fringe A between the test organism and the control organism indicates that the agent alters the formation of the apical ectodermal ridge . 28. An assay for identifying an agent which alters the formation of angiogenic precursors or angiogenic cells in an organism, comprising the steps of: a) administering to the organism an agent to be tested, thereby producing a test organism; b) determining the expression of fringe B in the test organism in tissue or cells in which angiogenic precursors are formed; and c) comparing the expression of fringe B determined in step (b) with the expression of fringe B in tissue or cells in which angiogenic precursors are formed in a control organism to which the agent to be tested is not administered, wherein a difference in the expression of fringe B between the test organism and the control organism indicates that the agent alters the formation of angiogenic precursors or angiogenic cells. 29. The assay of Claim 28, wherein step (a) is carried out pre-natally. 30. An assay for identifying an agent which alters the formation or subdivision of the neural tube in an organism, comprising the steps of: a) administering to the organism an agent to be tested, thereby producing a test organism; b) determining the expression of fringe B in the test organism in tissue or cells in which the neural tube is formed; and c) comparing the expression of fringe B determined in step (b) with the expression of fringe B in tissue or cells in which the neural tube is formed in a control organism to which the agent to be tested is not administered, wherein a difference in the expression of fringe B between the test organism and the control organism indicates that the agent alters the formation or subdivision of the neural tube. 31. An assay for identifying an agent which alters the expression of a fringe gene in an organism, comprising the steps of: a) administering to the organism an agent to be tested, thereby producing a test organism; b) determining the expression of the fringe gene in the test organism; and c) comparing the expression of the fringe gene determined in step (b) with the expression of the fringe gene in a control organism to which the agent to be tested is not administered, wherein a difference in the expression of the fringe gene between the test organism and the control organism indicates that the agent alters the expression of the fringe gene. 32. The assay of Claim 31, wherein the fringe gene is fringe A. 33. The assay of Claim 31, wherein the fringe gene is fringe B.

Inventors: LAUFER EDWARD M, OROZCO OLIVIA E, TABIN CLIFFORD J, LAUFER, EDWARD, M., OROZCO, OLIVIA, E., TABIN, CLIFFORD, J.
Applicants: HARVARD COLLEGE, LAUFER EDWARD M, OROZCO OLIVIA E, TABIN CLIFFORD J, LAUFER, EDWARD, M., OROZCO, OLIVIA, E., PRESIDENT AND FELLOWS OF HARVARD COLLEGE, TABIN, CLIFFORD, J.
Application year: 1997
Main classification: C07K14
IPC codes: A61K38, C07K14