CELL SURFACE GLYCOPROTEINS ASSOCIATED WITH HUMAN B CELL LYMPHOMAS - ULBP, DNA AND POLYPEPTIDES

Abstract: The invention is directed to purified and isolated novel ULBP polypeptides, the nucleic acids encoding such polypeptides, processes for production of recombinant forms of such polypeptides, antibodies generated against these polypeptides, fragmented peptides derived from these polypeptides, and the uses of the above. ULBP polypeptide can be found on the surface of human B cell lymphomas. Mammalian forms of ULBP polypeptide in isolated or purified forms are provided. In addition, isolated nucleic acids encoding ULBP polypeptides and expression vectors comprising a cDNA encoding ULBP poypeptides are provided. The ULBP polypeptides can be isolated or synthesized and used to prepare antibodies, and in particular monoclonal antibodies, against the polypeptides. The antibodies, in turn, are useful for detecting the presence of ULBP polypeptides in human cell samples, which can be correlated with the existence of a malignant condition in a patient. ULBP polypeptides stimulate IFN- gamma production, NK cell proliferation, and CTL activity.

Description: CELL SURFACE GLYCOPROTEINS ASSOCIATED WITH HUMAN B CELL LYMPHOMAS - ULBP, DNA AND POLYPEPTIDES CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application Ser. No. 60/069,857, filed December 17, 1997, and of U.S. Provisional Application Ser. No. 60/092,946, filed July 15, 1998, both of which are hereby incorporated by reference. BACKGROUND OF THE INVENTION Field of the Invention The invention is directed to purified and isolated polypeptides, the nucleic acids encoding the polypeptides, and processes for production of recombinant forms of the polypeptides. This invention is also directed to antibodies that specifically recognize the polypeptides, compositions containing the polypeptides, and to the use of these materials in various assays. In particular, the polypeptides, the nucleic acids encoding the polypeptides, and the antibodies are useful for the detection of human B cell lymphomas, enhancing the production of interferon γ (IFN-γ), enhancing the growth of NK cells, and enhancing the activity of cytotoxic T lymphocytes (CTLs). Description of Related Art 1. Lymphomas The malignant lymphomas represent multiple diseases with diverse morphologic and clinical expressions. Morphologic classification schemes have been shown to be useful in delineating natural history, prognosis, and response to therapy. However, in some instances, distinctive morphologic entities may be very closely related clinically or biologically or both, whereas other diseases that share morphologic similarities may be clinically and biologically quite distinct. Thus, immunological methods have become essential tools for the diagnosis and classification of certain human tumors, particularly for leukemias and lymphomas. More particularly, some patients carrying malignancies have been found to produce antibodies against rumor-associated or other malignant cell-associated surface determinants as well as against a variety of differentiation and other non-tumor antigens. Several very sensitive immunological procedures have been developed for the detection of such antibodies. A variety of other markers of hematopoietic malignancies allows clinically valuable categorizations, which are not possible by morphological or histochemical parameters. These markers can also be detected using immunological procedures. One of the main advantages of these procedures over other methods for the measurement of tumor-associated markers is their high level of sensitivity, with the ability often to detect a substance at nanogram or even picogram levels. In addition, immunological tests afford the ability to discriminate among substances with closely related structures and thereby to identify tumor-associated analogs found in normal, nonmalignant states. Many tumor-associated markers have been detected in the sera or other body fluids of patients with malignancies. A few of these markers have been sufficiently restricted to tumor-bearing individuals to aid in detection or differential diagnosis (or both) of malignant disease; however, most markers have been found to lack sufficient specificity for such applications, with an appreciable frequency of elevated marker levels in patients with nonmalignant diseases (i.e., false positive results). In addition, malignant lymphomas are usually an admixture of a neoplastic element and normal elements or both. A cell suspension prepared from a malignant lymphoma will consist of a mixture of benign and malignant cells, and the malignant cells may not necessarily be in the majority. To determine the phenotype of malignant cells, it is necessary to identify those markers associated with neoplastic cells. It should be evident that there exists a need in the art for the identification of additional cell surface markers associated with malignant conditions. In particular, there is a continuing need for cell surface markers associated with malignant lymphomas. The identification of specific cell surface markers associated with malignant lymphomas will aid the detection of malignancies associated with these markers. 2. NK cells One of the major types of circulating mononuclear cells is that of the natural killer, or NK, cell (M. Manoussaka et al., Journal of Immunology 158:112-119, 1997). NK cells are a cell type derived from bone marrow precursors (O. Haller et al., Journal of Experimental Medicine 145:1411-1420, 1977). NK cells appear to be closely related to T cells and the two cell types share many cell surface markers (M. Manoussaka et al, 1997). Although NK cells are cytotoxic cells as are some T cells, unlike T cells, NK cells do not express the T-cell receptor or CD3 components (P. Scott and G. Trinichieri, Current Opinion in Immunology 7:34-40, 1995; G. Trinichieri, Adv. Immunology 47:187-376, 1989). NK cells commonly express CD16 and CD56 antigens (K. Oshimi, International Journal ofHematology 63:279- 290, 1996). Similar to cytotoxic T lymphocytes (CTL), NK cells are capable of exerting a cytotoxic effect by lysing a variety of cell types (G. Trinichieri, 1989). NK cells are capable of exerting cytotoxicity in a non-MHC restricted fashion (E. Ciccione et al., J. Exp. Med. 172:47, 1990; A. Moretta et al., J. Exp. Med. 172:1589, 1990; and E. Ciccione et al., J. Exp. Med. 175:709). NK cells mediate some of their functions through the secretion of cytokines, such as interferon γ (IFN-γ), granulocyte-macrophage colony-stimulating factors (GM-CSFs), tumor necrosis factor α (TNF-α), macrophage colony-stimulating factor (M-CSF), interleukin-3 (IL- 3), and IL-8 (P. Scott and G. Trinichieri, 1995). Cytokines including IL-2, IL-12, TNF-α, and IL-1 can induce NK cells to produce cytokines (P. Scott and G. Trinichieri, 1995). IFN-α and IL-2 are strong inducers of NK cell cytotoxic activity (G. Trinichieri et al., Journal of Experimental Medicine 160: 1147- 1 169, 1984; G. Trinichieri and D. Santoli, Journal of Experimental Medicine 147:1314-1333, 1977). NK cells can be stimulated and expanded in the presence of IL-2 (K. Oshimi, International Journal ofHematology 63:279-290, 1996). IL-12 has been shown to induce cytokine production from T and NK cells, and augment NK cell-mediated cytotoxicity (M. Kobayashi et al., Journal of Experimental Medicine 170:827-846, 1989). NK cells can lyse a variety of cell types, including normal stem cells, infected cells, and transformed cells (D. See et al., Scand. J. Immunol. 46:217-224, 1997). Cells that lack MHC class I are susceptible to NK cell-mediated lysis (H. Reyburn et al., Immunol. Rev. 155:119- 125, 1997). The lysis of cells occurs through the action of cytoplasmic granules containing proteases, nucleases, and perform (D. See et al., 1997). Antibodies directed against CD2 and CD1 l a inhibit the cytotoxic effect of NK cells (O. Ramos et al., J. Immunol. 142:4100-4104, 1989; C. Scott et al., J. Immunol. 142:4105-41 12, 1989). NK cells can also lyse cells by antibody-dependent cellular cytotoxicity (D. See et al., 1997). NK cells have been shown to destroy both extracellular protozoa and the cells infected by protozoa (T. Scharton-Kersten and A. Sher, Current Opinion in Immunology 9:44-51 , 1997). In most instances, cytotoxic activity appears to be dependent upon lymphokine activation (T. Scharton-Kersten and A. Sher, 1997). NK cells have been implicated as mediators of host defenses against infection in humans with varicella zoster, herpes simplex, cytomegalovirus, Epstein-Barr virus, hepatitis B, and hepatitis C viruses (D. See et al., 1997). Many viruses induce NK cell cytotoxicity, including heφesvirus and cytomegalovirus (C. Biron, Current Opinion in Immunology 9:24-34, 1997). The induction of NK cell activity is a result of the induction of IFN-α/β by viral infection, and NK cells are important in the early defense against many viral infections (C. Biron, 1997). The NK1+CD3- population of NK cells is the subset activated by viral infection (C. Biron, 1997). The response of NK cells to viral infection involves direct cytotoxicity and production of various cytokines such as IFN-γ and TNF-α (C. Biron, 1997). A number of human lymphoproliferative disorders of NK cells are known. These include NK cell-lineage granular lymphocyte proliferative disorder (NK-GLPD), NK cell lymphoma, and acute leukemia of NK cell lineage (K. Oshimi, International Journal of Hematology 63:279-290, 1996). Most patients with aggressive type NK-GLPD die of the disease (K. Oshimi, 1996). NK cell lymphoma is resistant to combination chemotherapy (K. Oshimi, 1996). NK cells activated with IL-2 have been shown to have activity against human leukemia cells (L. Silla et al., Journal of Hematotherapy 4:269-279, 1995). Furthermore, NK cells appear to have a role in the treatment of chronic myeloid leukemia (K. Oshimi, 1996). NK cells are involved in both the resistance to and control of cancer spread (T. Whiteside and R. Herberman, Current Opinion in Immunology 7:704-710, 1995). Furthermore, the presence and activation of NK cells may be outcome determinative; low or non-existent NK activity is associated with a high frequency of viral disease and cancer (T. Whiteside and R. Herberman, 1995). In view of the important role that NK and T cells play in vivo, in host defenses, tumor cell surveillance, and autoimmune diseases, there exists a need in the art for polypeptides suitable for the in vivo and in vitro enhancement of NK and T cell activity. 3. Interferon γ The production of IFN-γ is a function of T cells and NK cells, and IFN-γ activates antiviral immune reactions (E. De Maeyer and J. De Maeyer-Guignard, in The Cytokine Handbook, A.W. Thompson (ed.), Academic Press, 1994, pp. 265-288). IFN-γ preferentially inhibits Th2 proliferation, but not Thl proliferation (T. F. Gajewski and F. W. Fitch, J. Immunology 140:4245-4252, 1988). IFN-γ also plays an important role in macrophage activation and promotes proliferation of activated B cells (De Maeyer and De Maeyer- Guignard). These, and other effects of IFN-γ, indicate that increased in vivo levels of IFN-γ production serve as a general immune modulator. IFN-γ has been used clinically in treating chronic granulomatous disease, atopic dermatitis, systemic achlerosis, lepratmatous leprosy, common warts, hepatitis B infection, myelogenous leukemia, and metastatic melanoma (J. Mordenti et al., in Therapeutic Proteins, A.H.C. Kung et al. (eds.), W.H. Freeman and Co., 1993, pp.187-199). In view of the important role that IFN-γ plays, in vivo, in immune modulation, there exists a need in the art for polypeptides suitable for the enhancement of tn vivo and in vitro IFN-γ levels. 4. Cytotoxic T Lymphocytes CTLs are an important in vivo defense against viral, and bacterial, and cancerous diseases, in that they lyse target cells bearing foreign antigens (G. Berke, in Fundamental Immunology, W. E. Paul (ed.) Raven Press Ltd., 1989, pp. 735-764. In view of the important role that CTLs plays, in vivo, in the immune response to infections and tumor surveilance, there exists a need in the art for polypeptides suitable for the enhancement of in vivo and in vitro CTL actvity. 5. Protein Identification In another aspect, the identification of the primary structure, or sequence, of an unknown protein is the culmination of an arduous process of experimentation. In order to identify an unknown protein, the investigator can rely upon a comparison of the unknown protein to known peptides using a variety of techniques known to those skilled in the art. For instance, proteins are routinely analyzed using techniques such as electrophoresis, sedimentation, chromatography, sequencing and mass spectrometry. In particular, comparison of an unknown protein to polypeptides of known molecular weight allows a determination of the apparent molecular weight of the unknown protein (T.D. Brock and M.T. Madigan, Biology of Microorganisms 76-77 (Prentice Hall, 6d ed. 1991)). Protein molecular weight standards are commercially available to assist in the estimation of molecular weights of unknown protein (New England Biolabs Inc. Catalog:l 30-131 , 1995; j. L. Hartley, U.S. Patent No. 5,449,758). However, the molecular weight standards may not correspond closely enough in size to the unknown protein to allow an accurate estimation of apparent molecular weight. The difficulty in estimation of molecular weight is compounded in the case of proteins that are subjected to fragmentation by chemical or enzymatic means, modified by post-translational modification or processing, and/or associated with other proteins in non-covalent complexes. In addition, the unique nature of the composition of a protein with regard to its specific amino acid constituents results in unique positioning of cleavage sites within the protein. Specific fragmentation of a protein by chemical or enzymatic cleavage results in a unique "peptide fingeφrint" (D. W. Cleveland et al., J. Biol. Chem. 252:1102-1106, 1977; M. Brown et al., J. Gen. Virol. 50:309-316, 1980). Consequently, cleavage at specific sites results in reproducible fragmentation of a given protein into peptides of precise molecular weights. Furthermore, these peptides possess unique charge characteristics that determine the isoelectric pH of the peptide. These unique characteristics can be exploited using a variety of electrophoretic and other techniques (T.D. Brock and M.T. Madigan, Biology of Microorganisms 16-11 (Prentice Hall, 6d ed. 1991)). Fragmentation of proteins is further employed for amino acid composition analysis and protein sequencing (P. Matsudiara, J. Biol. Chem. 262: 10035-10038, 1987; C. Eckerskorn et al., Electrophoresis 1988, 9:830-838, 1988), particularly the production of fragments from proteins with a "blocked" N-terminus. In addition, fragmented proteins can be used for immunization, for affinity selection (R. A. Brown, U.S. Patent No. 5,151,412), for determination of modification sites (e.g. phosphorylation), for generation of active biological compounds (T.D. Brock and M.T. Madigan, Biology of Microorganisms 300-301 (Prentice Hall, 6d ed. 1991)), and for differentiation of homologous proteins (M. Brown et al., J. Gen. Virol. 50:309-316, 1980). In addition, when a peptide fingeφrint of an unknown protein is obtained, it can be compared to a database of known proteins to assist in the identification of the unknown protein using mass spectrometry (W.J. Henzel et al., Proc. Natl. Acad. Sci. USA 90:5011-5015, 1993; D. Fenyo et al., Electrophoresis 19:998-1005, 1998). A variety of computer software programs to facilitate these comparisons are accessible via the Internet, such as Protein Prospector (Internet site: prospector.uscf.edu), Multildent (Internet site: www.expasy.ch sprot multiident.html), PeptideSearch (Internet site: www.mann.embl- SUBSTΓΓUTE SHEET (RULE 26) heiedelberg.de...deSearch/FR_PeptideSearch Form.html), and ProFound (Internet site: www.chait-sgi.rockefeller.edu/cgi-bin/prot-id-frag.html). These programs allow the user to specify the cleavage agent and the molecular weights of the fragmented peptides within a designated tolerance. The programs compare these molecular weights to protein molecular weight information stored in databases to assist in determining the identity of the unknown protein. Accurate information concerning the number of fragmented peptides and the precise molecular weight of those peptides is required for accurate identification. Therefore, increasing the accuracy in determining the number of fragmented peptides and their molecular weight should result in enhanced likelihood of success in the identification of unknown proteins. In addition, peptide digests of unknown proteins can be sequenced using tandem mass spectrometry (MS/MS) and the resulting sequence searched against databases (J.K. Eng, et al., J. Am. Soc. Mass Spec. 5:976-989 (1994); M. Mann and M. Wilm, Anal. Chem. 66:4390-4399 (1994); J.A. Taylor and R.S. Johnson, Rapid Comm. Mass Spec. 11:1067-1075 (1997)). Searching programs that can be used in this process exist on the Internet, such as Lutefisk 97 (Internet site: www.lsbc.com:70/Lutefisk97.html), and the Protein Prospector, Peptide Search and ProFound programs described above. Therefore, adding the sequence of a gene and its predicted protein sequence and peptide fragments to a sequence database can aid in the identification of unknown proteins using tandem mass spectrometry. Thus, there also exists a need in the art for polypeptides suitable for use in peptide fragmentation studies, for use in molecular weight measurements, and for use in protein sequencing using tandem mass spectrometry. SUMMARY OF THE INVENTION The invention aids in fulfilling these various needs in the art by providing isolated ULBP nucleic acids and polypeptides encoded by these nucleic acids. Specifically, one embodiment of this invention provides cell surface glycoproteins associated with human B cell lymphomas. In a broad sense, this invention pertains to novel polypeptides referred to herein as ULBP polypeptides, which are found on the surface of human B cell lymphomas. The present invention is specifically directed to mammalian forms of ULBP polypeptides in isolated and purified forms. Particular embodiments of the invention are directed to an isolated ULBP nucleic acid molecule comprising the DNA sequence of SEQ ID NO: 1 , SEQ ID NO:3, or SEQ ID NO:9 and an isolated ULBP nucleic acid molecule encoding the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, or SEQ ID NO: 10, as well as nucleic acid molecules complementary to these sequences. Both single-stranded and double-stranded RNA and DNA nucleic acid molecules are encompassed by the invention, as well as nucleic acid molecules that hybridize to a denatured, double-stranded DNA comprising all or a portion of SEQ ID NO: 1 , SEQ ID NO:3, and/or SEQ ID NO:9. Also encompassed are isolated nucleic acid molecules that are derived by in vitro mutagenesis of nucleic acid molecules comprising sequences of SEQ ID NO:l, SEQ ID NO:3, or SEQ ID NO:9, that are degenerate from nucleic acid molecules comprising sequences of SEQ ID NO:l, SEQ ID NO:3, or SEQ ID NO:9, and that are allelic variants of DNA of the invention. The invention also encompasses recombinant vectors that direct the expression of these nucleic acid molecules and host cells stably or transiently transformed or transfected with these vectors. In addition, the invention encompasses methods of using the nucleic acids noted above to identify nucleic acids encoding proteins having ULBP activity; to identify human chromosome number 6; to map genes on human chromosome number 6; to identify genes associated with certain diseases, syndromes, or other human conditions associated with human chromosome number 6, and to study cell signal transduction and the ULBP system. The invention also encompasses the use of sense or antisense oligonucleotides from the nucleic acid of SEQ ID NO:l, SEQ ID NO:3, or SEQ ID NO:9 to inhibit the expression of the polynucleotide encoded by the ULBP gene. The invention also encompasses isolated polypeptides and fragments thereof encoded by these nucleic acid molecules including soluble polypeptide portions of SEQ ID NO:l, SEQ ID NO:3, or SEQ ID NO:9. The invention further encompasses methods for the production of these polypeptides, including culturing a host cell under conditions promoting expression and recovering the polypeptide from the culture medium. Especially, the expression of these polypeptides in bacteria, yeast, plant, insect, and animal cells is encompassed by the invention. In general, the polypeptides of the invention can be used to study cellular processes such as immune regulation, cell proliferation, cell death, cell migration, cell-to-cell interaction, and inflammatory responses. In addition, these polypeptides can be used to identify proteins associated with ULBP ligands and ULBP receptors. In yet another aspect, the invention includes assays utilizing these polypeptides to screen for potential inhibitors of activity associated with polypeptide counter-structure molecules, and methods of using these polypeptides as therapeutic agents for the treatment of diseases mediated by ULBP polypeptide counter-structure molecules. Further, methods of using these polypeptides in the design of inhibitors thereof are also an aspect of the invention. The invention further provides a method for using these polypeptides as molecular weight markers that allow the estimation of the molecular weight of a protein or a fragmented protein, as well as a method for the visualization of the molecular weight markers of the invention thereof using electrophoresis. The invention further encompasses methods for using the polypeptides of the invention as markers for determining the isoelectric point of an unknown protein, as well as controls for establishing the extent of fragmentation of a protein . Further encompassed by this invention are kits to aid in these determinations. Another aspect of this invention is the use of the ULBP nucleic acid sequences, predicted amino acid sequences of the polypeptide or fragments thereof, or a combination of the predicted amino acid sequences of the polypeptide and fragments thereof for use in searching an electronic database to aid in the identification of sample nucleic acids and/or proteins. Isolated polyclonal or monoclonal antibodies that bind to these polypeptides are also encompassed by the invention, in addition to the use of these antibodies to aid in purifying the ULBP polypeptide. The antibodies, in turn, are useful for detecting the presence of ULBP polypeptides in human cell samples, which can be correlated with the existence of a malignant condition in a patient. More generally, this invention provides methods of detecting B cell lymphomas using the polypeptides, DNA, and antibodies of the invention. The methods are based, for example, on immunological and DNA hybridization and amplification techniques. Furthermore, this invention provides in vitro and in vivo methods of increasing IFN-γ production, increasing NK cell proliferation, and increasing CTL activity. BRIEF DESCRIPTION OF THE FIGURES Figure 1 illustrates the effect of ULBP polypeptides on the production of IFN-γ by NK cells. Figure 2 illustrates the effect of ULBP polypeptides on the proliferation of NK cells, as indicated by Thymidine uptake. Figure 3 illustrates the effect of ULBP- 1 polypeptide on CTL activity. Figure 4 illustrates the effect of ULBP-2 polypeptide on CTL activity. DETAILED DESCRIPTION OF THE INVENTION ULBP DNAs are human DNAs encoding novel cell-surface glycoproteins. cDNAs encoding human ULBP polypeptides have been isolated and cloned from human cells. The discovery of these DNAs encoding human ULBP polypeptides enables construction of expression vectors comprising nucleic acid sequences encoding these polypeptides; host cells transfected or transformed with the expression vectors; biologically active human ULBP polypeptides as isolated and purified proteins and polypeptides; and antibodies immunoreactive with the polypeptides. In one particular embodiment of the invention, a ULBP gene designated ULBP-1 was expression cloned from a Namalwa (human B cell lymphoma) cDNA library. This was accomplished by binding to a UL16-Fc fusion protein. UL16 is a type 1 membrane glycoprotein encoded by human cytomegalovirus (Kaye et al., J. Virol. 66:6609, 1992). UL16-Fc fuses the extracellular domain of UL16 to the IgGl Fc (mutein form) as previously described for OX40-Fc (Baum et al., EMBO J. 13:3992-4001, 1994). Expression cloning of ULBP-1 was carried out using standard techniques previously described for CD27L and OX4OL (Cell 73:447, 1993, EMBO J. 13:3992-4001, 1994). The entire disclosure of each of these references is relied upon and incoφorated by reference herein. The nucleotide sequence of the ULBP-1 gene (coding region only) in Staden format is as follows: ATGGCAGCGGCCGCCAGCCCCGCCTTCCTTCTGTGCCTCCCGCTTCTGCA CCTGCTGTCTGGCTGGTCCCGGGCAGGATGGGTCGACACACACTGTCTTT GCTATGACTTCATCATCACTCCTAAGTCCAGACCTGAACCACAGTGGTGT GAAGTTCAAGGCCTGGTGGATGAAAGGCCTTTTCTTCACTATGACTGTGT TAACCACAAGGCCAAAGCCTTTGCTTCTCTGGGGAAGAAAGTCAATGTCA CAAAAACCTGGGAAGAACAAACTGAAACACTAAGAGACGTGGTGGATTTC CTTAAAGGGCAACTGCTTGACATTCAAGTGGAGAATTTAATACCCATTGA GCCCCTCACCCTGCAGGCCAGGATGTCTTGTGAGCATGAAGCCCATGGAC ACGGCAGAGGATCTTGGCAGTTCCTCTTCAATGGACAGAAGTTCCTCCTC TTTGACTCAAACAACAGAAAGTGGACAGCACTTCATCCTGGAGCCAAGAA GATGACAGAGAAGTGGGAGAAGAACAGGGATGTGACCATGTTCTTCCAGA AGATTTCACTGGGGGATTGTAAGATGTGGCTTGAAGAATTTTTGATGTAC TGGGAACAAATGCTGGATCCAACAAAACCACCCTCTCTGGCCCCAGGCAC AACCCAACCCAAGGCCATGGCCACCACCCTCAGTCCCTGGAGCCTTCTCA TCATCTTCCTCTGCTTCATTCTAGCTGGCAGATGA [SEQ ID NO:l]. The equivalent amino acid sequence of ULBP-1 is: ULBP-1 POLYPEPTIDE MAAAASPAFL LCLPLLHLLS GWSRAG VDT HCLCYDFIIT PKSRPEPQWC EVQGLVDERP FLHYDCVNHK AKAFASLGKK VNVTKTWEEQ TETLRDVVDF LKGQLLDIQV ENLIPIEPLT LQAR SCEHE AHGHGRGSWQ FLFNGQKFLL FDSNNRKWTA LHPGAKKMTE K EKNRDVTM FFQKISLGDC KMWLEEFLMY WEQMLDPTKP PSLAPGTTQP KAMATTLSPW SLLIIFLCFI LAGR [SEQ ID NO:2]. The isolated and purified ULBP-1 polypeptide of the invention has a molecular weight of approximately 31 kD as determined by SDS-polyacrylamide gel electrophoresis (SDS- PAGE). The signal peptide of ULBP-1 polypeptide (amino acids 1-21) can be cleaved to provide a mature protein beginning at aa 22. In another distinct embodiment of the invention, a ULBP gene designated ULBP-2 was recognized as an EST homologous to the ULBP-1 gene (GenBank accession number R25716). The sequence in the EST database was not correct. Moreover, this EST was not recognized as encoding a protein related to any other protein. The EST clone was obtained and sequenced completely, revealing that it encoded a full length protein sequence. The nucleotide sequence ofthe ULBP-2 gene (coding sequence only) is as follows: ATGGCAGCAGCCGCCGCTACCAAGATCCTTCTGTGCCTCCCGCTTCTGCT CCTGCTGTCCGGCTGGTCCCGGGCTGGGCGAGCCGACCCTCACTCTCTTT GCTATGACATCACCGTCATCCCTAAGTTCAGACCTGGACCACGGTGGTGT GCGGTTCAAGGCCAGGTGGATGAAAAGACTTTTCTTCACTATGACTGTGG CAACAAGACAGTCACACCTGTCAGTCCCCTGGGGAAGAAACTAAATGTCA CAACGGCCTGGAAAGCACAGAACCCAGTACTGAGAGAGGTGGTGGACATA CTTACAGAGCAACTGCGTGACATTCAGCTGGAGAATTACACACCCAAGGA ACCCCTCACCCTGCAGGCAAGGATGTCTTGTGAGCAGAAAGCTGAAGGAC ACAGCAGTGGATCTTGGCAGTTCAGTTTCGATGGGCAGATCTTCCTCCTC TTTGACTCAGAGAAGAGAATGTGGACAACGGTTCATCCTGGAGCCAGAAA GATGAAAGAAAAGTGGGAGAATGACAAGGTTGTGGCCATGTCCTTCCATT ACTTCTCAATGGGAGACTGTATAGGATGGCTTGAGGACTTCTTGATGGGC ATGGACAGCACCCTGGAGCCAAGTGCAGGAGCACCACTCGCCATGTCCTC AGGCACAACCCAACTCAGGGCCACAGCCACCACCCTCATCCTTTGCTGCC TCCTCATCATCCTCCCCTGCTTCATCCTCCCTGGCATCTGA [SEQ ID NO:3]. The equivalent amino acid sequence of ULBP-2 is: ULBP-2 POLYPEPTIDE MAAAAATKIL LCLPLLLLLS GWSRAGRADP HSLCYDITVI PKFRPGPRWC AVQGQVDEKT FLHYDCGNKT VTPVSPLGKK LNVTTAWKAQ NPVLREVVDI LTEQLRDIQL ENYTPKEPLT LQARMSCEQK AEGHSSGSWQ FSFDGQIFLL FDSEKRMWTT VHPGARKMKE KWENDKVVAM SFHYFSMGDC IGWLEDFLMG MDSTLEPSAG APLAMSSGTT QLRATATTLI LCCLLIILPC FILPGI [SEQ ID NO: ] . The isolated and purified ULBP-2 polypeptide according to the invention has a molecular weight of approximately 31 kD as determined by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). In the case of this polypeptide, cleavage of the signal peptide occurs between amino acids 21 and 22. The ULBP-1 and ULBP-2 polypeptides are 60% identical at the amino acid level. The similarities and differences in their sequences are evident from the following sequence alignment of amino acids 1-243 of SEQ ID NO:2 with amino acids 1-245 of SEQ ID NO:4: ULBP-1 1 MAAAASPAFLLCLPLLHLLSGWSRAGWVDTHCLCYDFIITPKSRPEPQWC Mill.. : I I I I I I I I I I II I I I I : . I . I : I I I I : . : . I I I I : I . II ULBP-2 1 MAAAAATKILLCLPLLLLLSGWSRAGRADPHSLCYDITVIPKFRPGPRWC 51 EVQGLVDERPFLHYDCVNHKAKAFASLGKKVNVTKTWEEQTETLRDVVDF .III I I I :. I I I I I I • I : ... I I I I : I I I .. I .. I ... I I : I I I : 51 AVQGQVDEKTFLHYDCGNKTVTPVSPLGKKLNVTTAWKAQNPVLREVVDI 101 LKGQLLDIQVENLIPIEPLTLQARMSCEHEAHGHGRGSWQFLFNGQKFLL I . : II I I I : I I .. I I II I I I I I II I I : . I . I I : - I I I I I I : I I Ml 101 LTEQLRDIQLENYTPKEPLTLQARMSCEQKAEGHSSGSWQFSFDGQIFLL 151 FDSNNRKWTALHPGAKKMTEKWEKNRDVTMFFQKISLGDCKMWLEEFLMY I I I : . I . I I . : I I I I : I I . I I I I . : : I . I I : : I : I I I 111:111 151 FDSEKRMWTTVHPGARKMKEKWENDKVVAMSFHYFSMGDCIGWLEDFLMG 201 WEQMLDPTKPPSLA.. PGTTQPKAMATTLSPWSLLIIFLCFILAG 243 : 1:1. • : • I I .1111 : I I I I I Mill: I I I I : I 201 MDSTLEPSAGAPLAMSΞGTTQLRATATTLILCCLLIILPCFILPG 245 In another distinct embodiment of the invention, a ULBP gene designated ULBP-3 was recognized as an EST homologous to the ULBP-1 gene (GenBank accession number AI091180). The EST clone was obtained and sequenced completely, revealing that it encoded a protein sequence, which lacked the first five amino acids of the leader sequence. The nucleotide sequence of the ULBP-3 gene is as follows: AGCCCCGCGA TCCTTCCGCG CCTCGCGATT CTTCCGTACC TGCTATTCGA CTGGTCCGGG ACGGGGCGGG CCGACGCTCA CTCTCTCTGG TATAACTTCA CCATCATTCA TTTGCCCAGA CATGGGCAAC AGTGGTGTGA GGTCCAGAGC CAGGTGGATC AGAAGAATTT TCTCTCCTAT GACTGTGGCA GTGACAAGGT CTTATCTATG GGTCACCTAG AAGAGCAGCT GTATGCCACA GATGCCTGGG GAAAACAACT GGAAATGCTG AGAGAGGTGG GGCAGAGGCT CAGACTGGAA CTGGCTGACA CTGAGCTGGA GGATTTCACA CCCAGTGGAC CCCTCACGCT GCAGGTCAGG ATGTCTTGTG AGTGTGAAGC CGATGGATAC ATCCGTGGAT CTTGGCAGTT CAGCTTCGAT GGACGGAAGT TCCTCCTCTT TGACTCAAAC AACAGAAAGT GGACAGTGGT TCACGCTGGA GCCAGGCGGA TGAAAGAGAA GTGGGAGAAG GATAGCGGAC TGACCACCTT CTTCAAGATG GTCTCAATGA GAGACTGCAA GAGCTGGCTT AGGGACTTCC TGATGCACAG GAAGAAGAGG CTGGAACCCA CAGCACCACC CACCATGGCC CCAGGCTTAG CTCAACCCAA AGCCATAGCC ACCACCCTCA GTCCCTGGAG CTTCCTCATC ATCCTCTGCT TCATCCTCCC TGGCATCTGA GAAGAGTCAT TTAGAGTGAC AGGTGGAAGG TGATATCAAG AAGCCTCTGT TAGCCTGGTC TGGTTCCTGC TCTCCCTTCA GGGAGGCCGC CTGTCTACTC ACCACTGTGC CTTTCTGGAA AGCAGGAGTT CAAGCCTTAG CAAGCCCAGA GGCCCCCAGC AGATGATGAG GACATTGTCG GCTCAACGTC TCAGGCCACT CATTACCTTC GCTCATGATC CCAGCAGCCA [SEQ ID NO: 9] The equivalent amino acid sequence of ULBP-3 is: SPAILPRLAI LPYLLFDWSG TGRADAHSLW YNFTIIHLPR HGQQWCEVQS QVDQKNFLSY DCGSDKVLSM GHLEEQLYAT DAWGKQLEML REVGQRLRLE LADTELEDFT PSGPLTLQVR MSCECEADGY IRGSWQFSFD GRKFLLFDSN NRKWTWHAG ARRMKEKWEK DSGLTTFFKM VSMRDCKSWL RDFLMHRKKR LEPTAPPTMA PGLAQPKAIA TTLSPWSFLI ILCFILPGI [SEQ ID NO: 10] The ULBP-3 is a member of the ULBP family. ULBP-1 and ULBP- 3 are 54% identical, and ULBP-2 and ULBP- 3 are 54% identical, which is evident from the following sequence alignments of the amino acid sequences of ULBP-1 (SEQ ID NO:2), ULBP-2 (SEQ ID NO:4), and ULBP-3 (SEQ ID NO: 10): 1 50 ULBPl MAAAASPAFL LCLPLL . HLL SGWSRAGWVD THCLCYDFII TPKSRPEPQW ULBP2 MAAAAATKIL LCLPL . LL SGWSRAGRAD PHSLCYDITV IPKFRPGPRW ULBP3 SPAIL PRLAILPYLL FDWSGTGRAD AHSLWYNFTI IHLPRHGQQW 51 100 ULBPl CEVQGLVDER PFLHYDCVNH KAKAFASLGK KVNVTKTWEE QTETLRDWD ULBP2 CAVQGQVDEK TFLHYDCGNK TVTPVSPLGK KLNVTTAWKA QNPVLREWD ULBP3 CEVQSQVDQK NFLSYDCGSD KVLSMGHLEE QLYATDAWGK QLEMLREVGQ 101 150 ULBPl FLKGQLLDIQ VENLIPIEPL TLQARMSCEH EAHGHGRGSW QFLFNGQKFL ULBP2 ILTEQLRDIQ LENYTPKEPL T QARMSCEQ KAEGHSSGSW QFSFDGQIFL ULBP3 RLRLELADTE LEDFTPSGPL TLQVRMSCEC EADGYIRGSW QFSFDGRKFL 151 200 ULBPl LFDSNNRKWT ALHPGAKKMT EKWEKNRDVT MFFQKISLGD CKMWLEEFLM ULBP2 LFDSEKRMWT TVHPGARKMK EKWENDKWA MSFHYFSMGD CIGWLEDFLM ULBP3 LFDSNNRKWT WHAGARR K EKWEKDSGLT TFFKMVSMRD CKSWLRDFLM 201 248 ULBPl YWEQMLDPT. . KPPS APGT TQPKAMATTL SPWSLLIIFL CFILAGR* ULBP2 GMDSTLEPSA GAPLAMSSGT TQLRATATTL ILCCLLIILP CFILPGI* ULBP3 HRKKRLEPTA .. PPTMAPGL AQPKAIATTL SPWSFLIIL. CFILPGI* ULBPl 1 MAAAASPAFLLCLPLL.HLLSGWSRAGWVDTHC CYDFIITPKSRPEPQW 49 ULBP3 1 S MPAMILPRLIAMILPYLILIFD-WISIGTMGRADMAHSLIWYhNFiTII IHLP-RIHGQQIWI 45 50 CEVQGLVDERPFLHYDCVNHKAKAFASLGKKVNVTKTWEEQTETLRDWD 99 46 CEVQSQVDQKNFLSYDCGSDKVLSMGHLEEQ YATDAWGKQLEMLREVGQ 95 100 FLKGQLLDIQVENLIPIEPLTLQARMSCEHEAHGHGRGSWQFLFNGQKFL 149 96 RLhRLE:L|ADITE:L:EDF:TPISGPMLTMLQVRMMSMCECEIAMDGYIRMGSWMQFSFMDGMRKFILI 145 . . . . . 150 LFDSNNRIOtfTALHPGAKKMTEKWEKNRDVTMFFQKISLGDCKMWLEEFLM 199 146 LFDSNNRKWTWHAGARRMKEKWEKDSGLTTFFKMVSMRDCKSWLRDFLM 195 200 YWEQMLDPTKPPSLAPGTTQPKAMATTLSPWSLLIIFLCFILAGR* 245 196 HRKKRLEPTAPPTMAPGLAQPKAIATTLSPWSFLII.LCFILPGI* 240 . . . . . ULBP2 1 MAAAAATKILLCLPLL.LLLSGWSRAGRADPHSLCYDITVIPKFRPGPRW 49 • II IM II Ml -1111.111 IMM I I M ULBP3 1 SPAILPRLAILPYLLFDWSGTGRADAHSLWYNFTIIHLPRHGQQW 45 50 CAVQGQVDEKTFLHYDCGNKTVTPVSPLGKKLNVTTAWKAQNPVLREWD 99 46 CEVQSQVDQKNFLSYDCGSDKVLSMGHLEEQLYATDAWGKQLEMLREVGQ 95 100 ILTEQLRDIQLENYTPKEPLTLQARMSCEQKAEGHSSGSWQFSFDGQIFL 149 I M I MhMI MM MM I MIMMIIM! 96 RLRLELADTELEDFTPSGPLTLQVRMSCECEADGYIRGSWQFSFDGRKFL 145 150 LFDSEKRMWTTVHPGARKMKEKWENDKWAMSFHYFSMGDCIGWLEDFLM 199 llll-M II MMMMIMM :• I M M Ml Ml 146 LFDSNNRKWTWHAGARRMKEKWEKDSGLTTFFKMVSMRDCKSWLRDFLM 195 200 GMDSTLEPSAGAPLAMSSGTTQLRATATTLILCCLLIILPCFILPGI* 247 196 HRKKRLEPT.. APPTMAPGLAQPKAIATTLSPWSFLIIL . CFILPGI* 240 A " I " indicates identity of amino acids, a " . " indicates weak conservation of amino acids, and a " : " indicates high conservation of amino acids, as determined by alignment of the sequences using the GCG GAP program with the pep scoring matrix, gap creation penal ty=30, gap extension penalty=l. ULBP-1. ULBP-2. and ULBP-3 polypeptides are distantly related to a number of MHC class 1 proteins, particularly non-classical class 1 molecules or class 1 molecules of non-human species. ULBP-1. ULBP-2, and ULBP-3 polypeptides contain regions homologous to the αl and α2 domains of MHC class I proteins. However, they have a different structure from other class 1 molecules in that they lack the alpha-3 domain, which is needed for binding beta-2- microglobulin. The αl domain is approximately from amino acids 30-117 for ULBP-1 and ULBP-2. and approximately from amino acids 26-113 for ULBP-3. The α2 domain is approximately from amino acids 118-200 for ULBP-1 and ULBP-2. and approximately from amino acids 114-196 for ULBP-3 In addition, the ULBP-1 and the ULBP-2 genes map to chromosome 6q20-23. which is distinct from any other MHC class 1 and related genes. It will be understood that ULBP DNA and proteins and polypeptides encoded by the DNA are not limited to ULBP-1, ULBP-2, and ULBP-3. More particularly, the term "ULBP DNA" refers to a genus of polynucleotides having the same sequence as SEQ ID NO:l, SEQ ID NO:3, and SEQ ID NO:9, as well as those polynucleotides having a high degree of similarity (at least 90% homology) with such DNA sequences. Similarly, the term "ULBP polypeptide" refers to a genus of polypeptides and proteins having the same amino acid sequences as SEQ ID NO:2, SEQ ID NO:4, or SEQ ID NO: 10 as well as polypeptides and proteins having a high degree of similarity (at least 90% identity) with such amino acid sequences and which proteins are biologically active. ULBP polypeptides also include biologically active gene products of polynucleotides of SEQ ID NO: l, SEQ ID NO:3, and SEQ ID NO:9. When used herein, the term "ULBP" is intended to encompass ULBP DNA, as well as ULBP polypeptides. The discovery of the nucleic acids of the invention enables the construction of expression vectors comprising nucleic acid sequences encoding polypeptides; host cells transfected or transformed with the expression vectors; isolated and purified biologically active polypeptides and fragments thereof; the use of the nucleic acids or oligonucleotides thereof as probes to identify nucleic acid encoding proteins having ULBP activity, the use of the nucleic acids or oligonucleotides thereof to identify human chromosome number 6, the use of the nucleic acids or oligonucleotides thereof to map genes on human chromosome number 6, the use of the nucleic acid or oligonucleotides thereof to identify genes associated with certain diseases, syndromes or other human conditions associated with human chromosome number 6 (including Retinitis pigmentosa-25 (6ql4-q21); Diabetes mellitus, insulin-dependent, 15 (6q21); Progressive pseudorheumatoid arthropathy of childhood (6q22); Muscular dystrophy, congenital merosin-deficient (6q22-q23); and Cardiomyopathy, dilated, IF (6q23)), and the use of single-stranded sense or antisense oligonucleotides from the nucleic acids to inhibit expression of polynucleotide encoded by ULBP genes. NUCLEIC ACID MOLECULES In a particular embodiment, the invention relates to certain isolated nucleotide sequences that are free from contaminating endogenous material. A "nucleotide sequence" refers to a polynucleotide molecule in the form of a separate fragment or as a component of a larger nucleic acid construct. The nucleic acid molecule has been derived from DNA or RNA isolated at least once in substantially pure form and in a quantity or concentration enabling identification, manipulation, and recovery of its component nucleotide sequences by standard biochemical methods (such as those outlined in Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd sed., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY ( 1989)). Such sequences are preferably provided and/or constructed in the form of an open reading frame uninterrupted by internal non-translated sequences, or introns, that are typically present in eukaryotic genes. Sequences of non-translated DNA can be present 5' or 3' from an open reading frame, where the same do not interfere with manipulation or expression of the coding region. Nucleic acid molecules of the invention include DNA in both single-stranded and double-stranded form, as well as the RNA complement thereof. DNA includes, for example, cDNA, genomic DNA, chemically synthesized DNA, DNA amplified by PCR, and combinations thereof. Genomic DNA may be isolated by conventional techniques, e.g., using the cDNA of SEQ ID NO:l, SEQ ID NO:3, or SEQ ID NO:9, or a suitable fragment thereof, as a probe. The DNA molecules of the invention include full length genes as well as polynucleotides and fragments thereof. The full length gene may include the N-terminal signal peptide. Other embodiments include DNA encoding a soluble form, e.g., encoding the extracellular domain of the protein, either with or without the signal peptide and/or the GPI linkage. The nucleic acids of the invention are preferentially derived from human sources, but the invention includes those derived from non-human species, as well. Preferred Sequences A particularly preferred nucleotide sequence of the invention is SEQ ID NO: l, SEQ ID NO:3, or SEQ ID NO:9, as set forth above. The sequence of amino acids encoded by the DNA of SEQ ID NO:l is shown in SEQ ID NO:2. The sequence of amino acids encoded by the DNA of SEQ ID NO:3 is shown in SEQ ID NO:4. The sequence of amino acids encoded by the DNA of SEQ ID NO:9 is shown in SEQ ID NO: 10. These sequences identify the ULBP-1, ULBP-2. and ULBP-3 polynucleotides as a member of the ULBP family. Additional Sequences Due to the known degeneracy of the genetic code, wherein more than one codon can encode the same amino acid, a DNA sequence can vary from that shown in SEQ ID NO: 1 , SEQ ID NO:3, or SEQ ID NO:9, and still encode a polypeptide having the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, or SEQ ID NO: 10. Such variant DNA sequences can result from silent mutations (e.g., occurring during PCR amplification), or can be the product of deliberate mutagenesis of a native sequence. The invention thus provides isolated DNA sequences encoding polypeptides of the invention, selected from: (a) DNA comprising the nucleotide sequence of SEQ ID NO: l , SEQ ID NO:3, or SEQ ID NO:9; (b) DNA encoding the polypeptides of SEQ ID NO:2. SEQ ID NO:4, or SEQ ID NO: 10.; (c) DNA capable of hybridization to a DNA of (a) or (b) under conditions of moderate stringency and which encodes polypeptides of the invention; (d) DNA capable of hybridization to a DNA of (a) or (b) under conditions of high stringency and which encodes polypeptides of the invention, and (e) DNA which is degenerate as a result of the genetic code to a DNA defined in (a), (b), (c), or (d) and which encode polypeptides of the invention. Of course, polypeptides encoded by such DNA sequences are encompassed by the invention. The invention thus provides equivalent isolated DNA sequences encoding biologically active ULBP polypeptides selected from: (a) DNA derived from the coding region of a native mammalian ULBP gene; (b) DNA selected from the group consisting of nucleotide sequences 1-735 of SEQ ID NO:l, nucleotide sequences 1-741 of SEQ ID NO:3, and nucleotide sequences 1-717 of SEQ ID NO:9 (c) DNA capable of hybridization to a DNA of (a) or (b) under conditions of moderate stringency and which encodes biologically active ULBP polypeptides; and (d) DNA that is degenerate as a result of the genetic code to a DNA defined in (a), (b) or (c), and which encodes biologically active ULBP polypeptides. ULBP polypeptides encoded by such DNA equivalent sequences are encompassed by the invention. ULBP polypeptides encoded by DNA derived from other mammalian species, wherein the DNA will hybridize to the complement of the DNA of SEQ ID NO: l , SEQ ID NO:3. or SEQ ID NO:9. are also encompassed. As used herein, conditions of moderate stringency can be readily determined by those having ordinary skill in the art based on, for example, the length of the DNA. The basic conditions are set forth by Sambrook et al. Molecular Cloning: A Laboratory Manual, 2 ed. Vol. 1 , pp. 1.101-104. Cold Spring Harbor Laboratory Press, (1989), and include use of a prewashing solution for the nitrocellulose filters 5X SSC, 0.5% SDS. 1.0 mM EDTA (pH 8.0), hybridization conditions of about 50% formamide, 6X SSC at about 42 ° C (or other similar hybridization solution, such as Stark's solution, in about 50% formamide at about 42 ° C), and washing conditions of about 60°C, 0.5X SSC, 0.1%o SDS. Conditions of high stringency can also be readily determined by the skilled artisan based on, for example, the length of the DNA. Generally, such conditions are defined as hybridization conditions as above, and with washing at approximately 68 ° C, 0.2X SSC, 0.1 % SDS. The skilled artisan will recognize that the temperature and wash solution salt concentration can be adjusted as necessary according to factors such as the length of the probe. Also included as an embodiment of the invention is DNA encoding polypeptide fragments and polypeptides comprising inactivated N-glycosylation site(s), inactivated protease processing site(s), or conservative amino acid substitution(s), as described below. In another embodiment, the nucleic acid molecules of the invention also comprise nucleotide sequences that are at least 80% identical to a native sequence. Also contemplated are embodiments in which a nucleic acid molecule comprises a sequence that is at least 90% identical, at least 95% identical, at least 98% identical, at least 99% identical, or at least 99.9% identical to a native sequence. The percent identity may be determined by visual inspection and mathematical calculation. Alternatively, the percent identity of two nucleic acid sequences can be determined by comparing sequence information using the GAP computer program, version 6.0 described by Devereux et al. (Nucl. Acids Res. 12:387. 1984) and available from the University of Wisconsin Genetics Computer Group (UWGCG). The preferred default parameters for the GAP program include: (1 ) a unary comparison matrix (containing a value of 1 for identities and 0 for non- identities) for nucieotides, and the weighted comparison matrix of Gribskov and Burgess. Nucl. Acids Res. 1 :6145, 1986. as described by Schwartz and Dayhoff, eds.. Atlas of Protein Sequence and Structure. National Biomedical Research Foundation, pp. 353-358, 1979; (2) a penalty of 3.0 for each gap and an additional 0.10 penalty for each symbol in each gap; and (3) no penalty for end gaps. Other programs used by one skilled in the art of sequence comparison may also be used. The invention also provides isolated nucleic acids useful in the production of polypeptides. Such polypeptides may be prepared by any of a number of conventional techniques. A DNA sequence encoding a ULBP polypeptide, or desired fragment thereof may be subcloned into an expression vector for production of the polypeptide or fragment. The DNA sequence advantageously is fused to a sequence encoding a suitable leader or signal peptide. Alternatively, the desired fragment may be chemically synthesized using known techniques. DNA fragments also may be produced by restriction endonuclease digestion of a full length cloned DNA sequence, and isolated by electrophoresis on agarose gels. If necessary, oligonucleotides that reconstruct the 5' or 3' terminus to a desired point may be ligated to a DNA fragment generated by restriction enzyme digestion. Such oligonucleotides may additionally contain a restriction endonuclease cleavage site upstream of the desired coding sequence, and position an initiation codon (ATG) at the N-terminus of the coding sequence. The well-known polymerase chain reaction (PCR) procedure also may be employed to isolate and amplify a DNA sequence encoding a desired protein fragment. Oligonucleotides that define the desired termini of the DNA fragment are employed as 5' and 3' primers. The oligonucleotides may additionally contain recognition sites for restriction endonucleases, to facilitate insertion of the amplified DNA fragment into an expression vector. PCR techniques are described in Saiki et al., Science 239:487 (1988); Recombinant DNA Methodology, Wu et al., eds., Academic Press, Inc., San Diego (1989), pp. 189-196; and PCR Protocols: A Guide to Methods and Applications, Innis et al., eds., Academic Press, Inc. (1990). POLYPEPTIDES AND FRAGMENTS THEREOF The invention encompasses polypeptides and fragments thereof in various forms, including those that are naturally occurring or produced through various techniques such as procedures involving recombinant DNA technology. For example, DNAs encoding ULBP polypeptides can be derived from SEQ ID NO:l, SEQ ID NO:3, or SEQ ID NO:9 by in vitro mutagenesis, which includes site-directed mutagenesis, random mutagenesis, and in vitro nucleic acid synthesis. Such forms include, but are not limited to, derivatives, variants, and oligomers, as well as fusion proteins or fragments thereof. Polypeptides and Fragments Thereof The polypeptides of the invention include full length proteins encoded by the nucleic acid sequences set forth above. Particularly preferred polypeptides comprise the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO: 10, with particularly preferred fragments comprising amino acids from about amino acid 22 to about amino acid 219 of SEQ ID NO:2 and from about amino acid 22 to about amino acid 223 of SEQ ID NO:4. The polypeptides of SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO: 10 includes an N- terminal hydrophobic region that functions as a signal peptide. Computer analysis predicts that the signal peptide corresponds to residues 1 to 21 of SEQ ID NO:2 and SEQ ID NO:4, and residues 1-17 of SEQ ID NO:10. Cleavage of the signal peptide thus would yield a mature protein comprising amino acids 22 through 244 of SEQ ID NO:2, amino acids 22 through 246 of SEQ ID NO:4, and amino acids 17 through 239 of SEQ ID NO: 10. The skilled artisan will recognize that the above-described boundaries of such regions of the polypeptide are approximate, and that the boundaries of the regions (which may be predicted by using computer programs available for that puφose) may differ from those described above. The polypeptides of SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO: 10 includes an C- terminal hydrophobic region that functions as a GPI linkage. The predicted GPI linkage approximately corresponds to the last 20 residues of SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO: 10. Removal of the GPI linkage thus would yield a protein comprising amino acids 1 through about 224 of SEQ ID NO:2, 1 through about 226 of SEQ ID NO:4, and 1 through about 219 of SEQ ID NO: 10. Removal of both the signal peptide and the GPI linkage thus would yield a protein comprising amino acids from about 22 through about 224 of SEQ ID NO:2, from about 22 through about 226 of SEQ ID NO:4, and from about 17 through about 219 of SEQ ID NO: 10. The skilled artisan will recognize that the above-described boundaries of such regions of the polypeptide are approximate, and may differ from those described above. The polypeptides of the invention may be membrane bound or they may be secreted and thus soluble. Soluble polypeptides are capable of being secreted from the cells in which they are expressed. In general, soluble polypeptides may be identified (and distinguished from non- soluble membrane-bound counteφarts) by separating intact cells which express the desired polypeptide from the culture medium, e.g., by centrifugation, and assaying the medium (supernatant) for the presence of the desired polypeptide. The presence of polypeptide in the medium indicates that the polypeptide was secreted from the cells and thus is a soluble form of the protein. In one embodiment, the soluble polypeptides and fragments thereof comprise all or part of the extracellular domain, but lack the GPI linkage that would cause retention of the polypeptide on a cell membrane. A soluble polypeptide may include a portion of the GPI linkage, as long as the polypeptide is secreted from the cell in which it is produced. Soluble polypeptides thus include, but are not limited to, polypeptides comprising amino acids having an N-terminus at amino acid 20-30 and a C-terminus at amino acids 200- 227 of SEQ ID NO:2, an N-terminus at amino acid 20-30 and a C-terminus at amino acids 200- 227 of SEQ ID NO:4, and an N-terminus at amino acid 16-26 and a C-terminus at amino acids 196-223 of SEQ ID NO: 10. In general, the use of soluble forms is advantageous for certain applications. Purification of the polypeptides from recombinant host cells is facilitated, since the soluble polypeptides are secreted from the cells. Further, soluble polypeptides are generally more suitable for intravenous administration. The invention also provides polypeptides and fragments of the extracellular domain that retain a desired biological activity. Particular embodiments are directed to polypeptide fragments that retain the ability to bind ULBP counter-structures, such as UL16. Such a fragment may be a soluble polypeptide. as described above. In another embodiment, the polypeptides and fragments advantageously include regions that are conserved in the ULBP family as described above. Also provided herein are polypeptide fragments comprising at least 20, or at least 30, contiguous amino acids of the sequence of SEQ ID NO:2, SEQ ID NO:4, or SEQ ID NO: 10. Polypeptide fragments also may be employed as immunogens, in generating antibodies. Variants Naturally occurring variants as well as derived variants of the polypeptides and fragments are provided herein. An "ULBP variant" as referred to herein means a polypeptide substantially homologous to native ULBP polypeptide. but which has an amino acid sequence different from that of native ULBP polypeptide (human, murine or other mammalian species) because of one or more deletions, insertions, or substitutions. The variant has an amino acid sequence that preferably is at least 80% identical to a native ULBP polypeptide amino acid sequence, most preferably at least 90% identical. The percent identity may be determined, for example, by comparing sequence information using the GAP computer program, version 6.0 described by Devereux et al. (Nucl. Acids Res. 12:387, 1984) and available from the University of Wisconsin Genetics Computer Group (UWGCG). The GAP program utilizes the alignment method of Needleman and Wunsch (J. Mol. Biol. 48:443, 1970), as revised by Smith and Waterman (Adv. Appl. Math 2:482. 1981). The preferred default parameters for the GAP program include: (1) a unary comparison matrix (containing a value of 1 for identities and 0 for non-identities) for nucieotides, and the weighted comparison matrix of Gribskov and Burgess, Nucl. Acids Res. 14:6745, 1986, as described by Schwartz and Dayhoff, eds.. Atlas of Protein Sequence and Structure, National Biomedical Research Foundation, pp. 353-358, 1979; (2) a penalty of 3.0 for each gap and an additional 0.10 penalty for each symbol in each gap; and (3) no penalty for end gaps. Variants also include embodiments in which a polypeptide or fragment comprises an amino acid sequence that is at least 90% identical, at least 95% identical, at least 98% identical, at least 99% identical, or at least 99.9% identical to the preferred polypeptide or fragment thereof. Percent identity may be determined as above. Alternatively, the percent identity of two protein sequences can be determined by comparing sequence information using the GAP computer program, based on the algorithm of Needleman and Wunsch (j. Mol. Bio. 48:443, 1970) and available from the University of Wisconsin Genetics Computer Group (UWGCG). The preferred default parameters for the GAP program include: (1) a scoring matrix, blosum62, as described by Henikoff and Henikoff (Proc. Natl. Acad. Sci. USA 89:10915, 1992); (2) a gap weight of 12; (3) a gap length weight of 4; and (4) no penalty for end gaps. Other programs used by one skilled in the art of sequence comparison may also be used. The variants of the invention include, for example, those that result from alternate mRNA splicing events or from proteolytic cleavage. Alternate splicing of mRNA may, for example, yield a truncated but biologically active protein, such as a naturally occurring soluble form of the protein. Variations attributable to proteolysis include, for example, differences in the N- or C-termini upon expression in different types of host cells, due to proteolytic removal of one or more terminal amino acids from the protein (generally from 1-5 terminal amino acids). Proteins in which differences in amino acid sequence are attributable to genetic polymoφhism (allelic variation among individuals producing the protein) are also contemplated herein. As stated above, the invention provides isolated and purified, or homogeneous, ULBP polypeptides, both recombinant and non-recombinant. Variants and derivatives of native ULBP proteins that retain the desired biological activity can be obtained by mutations of nucleotide sequences coding for native ULBP polypeptides. Alterations of the native amino acid sequence can be accomplished by any of a number of conventional methods. Mutations can be introduced at particular loci by synthesizing oligonucleotides containing a mutant sequence flanked by restriction sites enabling ligation to fragments of the native sequence. Following ligation, the resulting reconstructed sequence encodes an analog having the desired amino acid insertion, substitution, or deletion. Alternatively, oligonucleotide-directed site-specific mutagenesis procedures can be employed to provide an altered gene, wherein predetermined codons can be altered by substitution, deletion, or insertion. Exemplary methods of making the alterations set forth above are disclosed by Walder et al. (Gene 42:133, 1986); Bauer et al. (Gene 37:73, 1985); Craik (BioTechniques, January 1985, 12-19); Smith et al. (Genetic Engineering: Principles and Methods, Plenum Press, 1981); Kunkel (Proc. Natl. Acad. Sci. USA 82:488, 1985); Kunkel et al. (Methods in Enzymol. 154:367, 1987); and U.S. Patent Nos. 4,518,584 and 4,737,462, all of which are incoφorated by reference. ULBP polypeptides can be modified to create ULBP derivatives by forming covalent or aggregative conjugates with other chemical moieties, such as glycosyl groups, polyethylene glycol (PEG) groups, lipids, phosphate, acetyl groups and the like. Covalent derivatives of ULBP polypeptides can be prepared by linking the chemical moieties to functional groups on ULBP amino acid side chains or at the N-terminus or C-terminus of a ULBP polypeptide or the extracellular domain thereof. Other derivatives of ULBP polypeptides within the scope of this invention include covalent or aggregative conjugates of ULBP polypeptides or their fragments with other proteins or polypeptides, such as by synthesis in recombinant culture as N-terminal or C-terminal fusions. For example, the conjugate can comprise a signal or leader polypeptide sequence (e.g. the α-factor leader of Saccharomvces) at the N-terminus of a ULBP polypeptide. The signal or leader peptide co-translationally or post-translationally directs transfer of the conjugate from its site of synthesis to a site inside or outside of the cell membrane or cell wall. Conjugates comprising diagnostic (detectable) or therapeutic agents attached thereto are contemplated herein, as discussed in more detail below. Other derivatives include covalent or aggregative conjugates of the polypeptides with other proteins or polypeptides, such as by synthesis in recombinant culture as N-terminal or C- terminal fusions. Examples of fusion proteins are discussed below in connection with oligomers. Further, fusion proteins can comprise peptides added to facilitate purification and identification. Such peptides include, for example, poly-His or the antigenic identification peptides described in U.S. Patent No. 5,011,912 and in Hopp et al., Bio/Technology 6: 1204, 1988. One such peptide is the FLAG ® peptide, Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys, which is highly antigenic and provides an epitope reversibly bound by a specific monoclonal antibody, enabling rapid assay and facile purification of expressed recombinant protein. A murine hybridoma designated 4E1 1 produces a monoclonal antibody that binds the FLAG ® peptide in the presence of certain divalent metal cations, as described in U.S. Patent 5,01 1,912, hereby incoφorated by reference. The 4E11 hybridoma cell line has been deposited with the American Type Culture Collection under accession no. HB 9259. Monoclonal antibodies that bind the FLAG ® peptide are available from Eastman Kodak Co., Scientific Imaging Systems Division, New Haven, Connecticut. Among the variant polypeptides provided herein are variants of native polypeptides that retain the native biological activity or the substantial equivalent thereof. One example is a variant that binds with essentially the same binding affinity as does the native form. Binding affinity can be measured by conventional procedures, e.g., as described in U.S. Patent No. 5,512,457 and as set forth below. Variants include polypeptides that are substantially homologous to the native form, but which have an amino acid sequence different from that of the native form because of one or more deletions, insertions or substitutions. Particular embodiments include, but are not limited to, polypeptides that comprise from one to ten deletions, insertions or substitutions of amino acid residues, when compared to a native sequence. A given amino acid may be replaced, for example, by a residue having similar physiochemical characteristics. Examples of such conservative substitutions include substitution of one aliphatic residue for another, such as He. Val. Leu, or Ala for one another; substitutions of one polar residue for another, such as between Lys and Arg, Glu and Asp, or Gin and Asn; or substitutions of one aromatic residue for another, such as Phe, Tφ, or Tyr for one another. Other conservative substitutions, e.g., involving substitutions of entire regions having similar hydrophobicity characteristics, are well known. Similarly, the DNAs of the invention include variants that differ from a native DNA sequence because of one or more deletions, insertions or substitutions, but that encode a biologically active polypeptide. The invention further includes polypeptides of the invention with or without associated native-pattern glycosylation. Polypeptides expressed in yeast or mammalian expression systems (e.g., COS-1 or COS-7 cells) can be similar to or significantly different from a native polypeptide in molecular weight and glycosylation pattern, depending upon the choice of expression system. Expression of polypeptides of the invention in bacterial expression systems, such as E. coli, provides non-glycosylated molecules. Further, a given preparation may include multiple differentially glycosylated species of the protein. Glycosyl groups can be removed through conventional methods, in particular those utilizing glycopeptidase. In general, glycosylated polypeptides of the invention can be incubated with a molar excess of glycopeptidase (Boehringer Mannheim). Correspondingly, similar DNA constructs that encode various additions or substitutions of amino acid residues or sequences, or deletions of terminal or internal residues or sequences are encompassed by the invention. For example. N-glycosylation sites in the polypeptide extracellular domain can be modified to preclude glycosylation, allowing expression of a reduced carbohydrate analog in mammalian and yeast expression systems. N-glycosylation sites in eukaryotic polypeptides are characterized by an amino acid triplet Asn-X-Y, wherein X is any amino acid except Pro and Y is Ser or Thr. Appropriate substitutions, additions, or deletions to the nucleotide sequence encoding these triplets will result in prevention of attachment of carbohydrate residues at the Asn side chain. Alteration of a single nucleotide, chosen so that Asn is replaced by a different amino acid, for example, is sufficient to inactivate an N-glycosylation site. Alternatively, the Ser or Thr can by replaced with another amino acid, such as Ala. Known procedures for inactivating N-glycosylation sites in proteins include those described in U.S. Patent 5,071.972 and EP 276.846. hereby incoφorated by reference. In another example of variants, sequences encoding Cys residues that are not essential for biological activity can be altered to cause the Cys residues to be deleted or replaced with other amino acids, preventing formation of incorrect intramolecular disulfide bridges upon folding or renaturation. Other variants are prepared by modification of adjacent dibasic amino acid residues, to enhance expression in yeast systems in which KEX2 protease activity is present. EP 212,914 discloses the use of site-specific mutagenesis to inactivate KEX2 protease processing sites in a protein. KEX2 protease processing sites are inactivated by deleting, adding or substituting residues to alter Arg- Arg, Arg-Lys, and Lys-Arg pairs to eliminate the occurrence of these adjacent basic residues. Lys-Lys pairings are considerably less susceptible to KEX2 cleavage, and conversion of Arg-Lys or Lys-Arg to Lys-Lys represents a conservative and preferred approach to inactivating KEX2 sites. Oligomers Encompassed by the invention are oligomers or fusion proteins that contain ULBP polypeptides. Such oligomers may be in the form of covalently-linked or non-covalently- linked multimers, including dimers, trimers, or higher oligomers. As noted above, preferred polypeptides are soluble and thus these oligomers may comprise soluble polypeptides. In one aspect of the invention, the oligomers maintain the binding ability of the polypeptide components and provide therefor, bivalent, trivalent, etc., binding sites. One embodiment of the invention is directed to oligomers comprising multiple polypeptides joined via covalent or non-covalent interactions between peptide moieties fused to the polypeptides. Such peptides may be peptide linkers (spacers), or peptides that have the property of promoting oligomerization. Leucine zippers and certain polypeptides derived from antibodies are among the peptides that can promote oligomerization of the polypeptides attached thereto, as described in more detail below. Immunoglobulin-based Oligomers As one alternative, an oligomer is prepared using polypeptides derived from immunoglobulins. Preparation of fusion proteins comprising certain heterologous polypeptides fused to various portions of antibody-derived polypeptides (including the Fc domain) has been described, e.g., by Ashkenazi et al. (PNAS USA 88:10535, 1991); Byrn et al. (Nature 344:677, 1990); and Hollenbaugh and Aruffo ("Construction of Immunoglobulin Fusion Proteins", in Current Protocols in Immunology, Suppl. 4, pages 10.19.1 - 10.19.11, 1992). One embodiment of the present invention is directed to a dimer comprising two fusion proteins created by fusing a polypeptide of the invention to an Fc polypeptide derived from an antibody. A gene fusion encoding the polypeptide/Fc fusion protein is inserted into an appropriate expression vector. Polypeptide/Fc fusion proteins are expressed in host cells transformed with the recombinant expression vector, and allowed to assemble much like antibody molecules, whereupon interchain disulfide bonds form between the Fc moieties to yield divalent molecules. The term "Fc polypeptide" as used herein includes native and mutein forms of polypeptides made up of the Fc region of an antibody comprising any or all of the CH domains of the Fc region. Truncated forms of such polypeptides containing the hinge region that promotes dimerization are also included. Preferred polypeptides comprise an Fc polypeptide derived from a human IgGl antibody. One suitable Fc polypeptide, described in PCT application WO 93/10151 (hereby incoφorated by reference), is a single chain polypeptide extending from the N-terminal hinge region to the native C-terminus of the Fc region of a human IgGl antibody. Another useful Fc polypeptide is the Fc mutein described in U.S. Patent 5,457,035 and in Baum et al., (EMBO J. 13:3992-4001, 1994) incoφorated herein by reference. The amino acid sequence of this mutein is identical to that of the native Fc sequence presented in WO 93/10151, except that amino acid 19 has been changed from Leu to Ala, amino acid 20 has been changed from Leu to Glu, and amino acid 22 has been changed from Gly to Ala. The mutein exhibits reduced affinity for Fc receptors. The above-described fusion proteins comprising Fc moieties (and oligomers formed therefrom) offer the advantage of facile purification by affinity chromatography over Protein A or Protein G columns. In other embodiments, the polypeptides of the invention may be substituted for the variable portion of an antibody heavy or light chain. If fusion proteins are made with both heavy and light chains of an antibody, it is possible to form an oligomer with as many as four ULBP extracellular regions. Both ULBP-1 and ULBP-2 polypeptides can be expressed as Fc fusion proteins using an Fc mutein to provide fused polypeptides having the following sequences: ULBPl-Fc FUSION POLYPEPTIDE MAAAASPAFL LCLPLLHLLS GWSRAGWVDT HCLCYDFIIT PKSRPEPQWC EVQGLVDERP FLHYDCVNHK AKAFASLGKK VNVTKTWEEQ TETLRDVVDF LKGQLLDIQV ENLIPIEPLT LQARMSCEHE AHGHGRGSWQ FLFNGQKFLL FDSNNRKWTA LHPGAKKMTE KWEKNRDVTM FFQKISLGDC KMWLEEFLMY WEQMLDPTKP PSLAPGTTQP RSCDKTHTCP PCPAPEAEGA PSVFLFPPKP KDTLMISRTP EVTCVVVDVS HEDPEVKFNW YVDGVEVHNA KTKPREEQYN STYRVVSVLT VLHQDWLNGK EYKCKVSNKA LPAPIEKTIS KAKGQPREPQ VYTLPPSRDE LTKNQVSLTC LVKGFYPSDI AVEWESNGQP ENNYKTTPPV LDSDGSFFLY SKLTVDKSRW QQGNVFSCSV MHEALHNHYT QKSLSLSPGK [SEQ ID NO: 5] ; ULBP2-Fc FUSION POLYPEPTIDE MAAAAATKIL LCLPLLLLLS GWSRAGRADP HSLCYDITVI PKFRPGPRWC AVQGQVDEKT FLHYDCGNKT VTPVSPLGKK LNVTTAWKAQ NPVLREVVDI LTEQLRDIQL ENYTPKEPLT LQARMSCEQK AEGHSSGSWQ FSFDGQIFLL FDSEKRMWTT VHPGARKMKE KWENDKVVAM SFHYFSMGDC IGWLEDFLMG MDSTLEPSAG APLAMSSGTT QLRRSCDKTH TCPPCPAPEA EGAPSVFLFP PKPKDTLMIS RTPEVTCVVV DVSHEDPEVK FNWYVDGVEV HNAKTKPREE QYNSTYRVVS VLTVLHQDWL NGKEYKCKVS NKALPAPIEK TISKAKGQPR EPQVYTLPPS RDELTKNQVS LTCLVKGFYP SDIAVEWESN GQPENNYKTT PPVLDSDGSF FLYSKLTVDK SRWQQGNVFS CSVMHEALHN HYTQKSLSLS PGK [SEQ ID NO: 6] . ULBP-1 and ULBP-2 are membrane-bound glycoproteins that are held on the cell surface by a GPI linkage. See Yan et al., J. Mol. Biol. 275:25-33 (1998) and Medof et al. FASEB J. 10:574-86 (1996). The approximately 20 carboxy-terminal amino acids of the ULBP polypeptides specify the GPI linkage. The deletion of these amino acids in the ULBP-1 and ULBP-2 Fc fusion proteins results in proteins lacking the GPI linkage. The invention encompasses ULBP polypeptides and fragments lacking the GPI linkage. A preferred ULBP polypeptide fragment comprises at least 6 contiguous amino acids of an amino acid sequence. In other embodiments, a preferred ULBP polypeptide fragment comprises at least 10, at least 20, or at least 100 contiguous amino acids of an amino acid sequence. These polypeptides can be produced in soluble form. Both ULBP-1 and ULBP-2 bind UL16-Fc. In addition, they bind to a number of human cell types, including mitogen-stimulated human T cells and natural killer (NK) cells. It has also been discovered that the ULBP-Fc proteins bind to K299 cells, an anaplastic lymphoma. ULBP 1 -Fc binds better than ULBP2-Fc. Peptide-linker Based Oligomers Alternatively, the oligomer is a fusion protein comprising multiple polypeptides, with or without peptide linkers (spacer peptides). Among the suitable peptide linkers are those described in U.S. Patents 4,751 , 180 and 4,935,233, which are hereby incoφorated by reference. A DNA sequence encoding a desired peptide linker may be inserted between, and in the same reading frame as, the DNA sequences of the invention, using any suitable conventional technique. For example, a chemically synthesized oligonucleotide encoding the linker may be ligated between the sequences. In particular embodiments, a fusion protein comprises from two to four soluble ULBP polypeptides, separated by peptide linkers. Leucine-Zippers Another method for preparing the oligomers of the invention involves use of a leucine zipper. Leucine zipper domains are peptides that promote oligomerization of the proteins in which they are found. Leucine zippers were originally identified in several DNA-binding proteins (Landschulz et al., Science 240: 1759, 1988), and have since been found in a variety of different proteins. Among the known leucine zippers are naturally occurring peptides and derivatives thereof that dimerize or trimerize. The zipper domain (also referred to herein as an oligomerizing, or oligomer-forming, domain) comprises a repetitive heptad repeat, often with four or five leucine residues interspersed with other amino acids. Examples of zipper domains are those found in the yeast transcription factor GCN4 and a heat-stable DNA-binding protein found in rat liver (C/EBP; Landschulz et al., Science 243: 1681 , 1989). Two nuclear transforming proteins, fos andjun, also exhibit zipper domains, as does the gene product of the murine proto-oncogene, c-myc (Landschulz et al., Science 240: 1759, 1988). The products of the nuclear oncogenes/os and jun comprise zipper domains that preferentially form heterodimer (O'Shea et al.. Science 245:646, 1989, Turner and Tjian, Science 243:1689, 1989). The zipper domain is necessary for biological activity (DNA binding) in these proteins. The fusogenic proteins of several different viruses, including paramyxovirus, coronavirus, measles virus and many retroviruses, also possess zipper domains (Buckland and Wild, Nature 338:547,1989; Britton, Nature 353:394, 1991 ; Delwart and Mosialos, AIDS Research and Human Retroviruses 6:703, 1990). The zipper domains in these fusogenic viral proteins are near the transmembrane region of the proteins; it has been suggested that the zipper domains could contribute to the oligomeric structure of the fusogenic proteins. Oligomerization of fusogenic viral proteins is involved in fusion pore formation (Spruce et al, Proc. Natl. Acad. Sci. U.S.A. 88:3523, 1991). Zipper domains have also been recently reported to play a role in oligomerization of heat-shock transcription factors (Rabindran et al., Science 259:230, 1993). Zipper domains fold as short, parallel coiled coils (O'Shea et al., Science 254:539; 1991). The general architecture of the parallel coiled coil has been well characterized, with a "knobs-into-holes" packing as proposed by Crick in 1953 (Ada Crystallogr. 6:689). The dimer formed by a zipper domain is stabilized by the heptad repeat, designated (abcdefg) n according to the notation of McLachlan and Stewart (J. Mol. Biol. 98:293; 1975), in which residues a and d are generally hydrophobic residues, with d being a leucine, which line up on the same face of a helix. Oppositely-charged residues commonly occur at positions g and e. Thus, in a parallel coiled coil formed from two helical zipper domains, the "knobs" formed by the hydrophobic side chains of the first helix are packed into the "holes" formed between the side chains of the second helix. The residues at position d (often leucine) contribute large hydrophobic stabilization energies, and are important for oligomer formation (Krystek: et al., Int. J. Peptide Res. 38:229, 1991). Lovejoy et al. (Science 259: 1288, 1993) recently reported the synthesis of a triple- stranded α-helical bundle in which the helices run up-up-down. Their studies confirmed that hydrophobic stabilization energy provides the main driving force for the formation of coiled coils from helical monomers. These studies also indicate that electrostatic interactions contribute to the stoichiometry and geometry of coiled coils. Further discussion of the structure of leucine zippers is found in Harbury et al. (Science 262: 1401 , 26 November 1993). Examples of leucine zipper domains suitable for producing soluble oligomeric proteins are described in PCT application WO 94/10308, and the leucine zipper derived from lung surfactant protein D (SPD) described in Hoppe et al. (FEBS Letters 344:191 , 1994), hereby incoφorated by reference. The use of a modified leucine zipper that allows for stable trimerization of a heterologous protein fused thereto is described in Fanslow et al. (Semin. Immunol. 6:267-278, 1994). Recombinant fusion proteins comprising a soluble polypeptide fused to a leucine zipper peptide are expressed in suitable host cells, and the soluble oligomer that forms is recovered from the culture supernatant. Certain leucine zipper moieties preferentially form trimers. One example is a leucine zipper derived from lung surfactant protein D (SPD), as described in Hoppe et al. (FEBS Letters 344:191 , 1994) and in U.S. Patent 5,716,805, hereby incoφorated by reference in their entirety. This lung SPD-derived leucine zipper peptide comprises the amino acid sequence Pro Asp Val Ala Ser Leu Arg Gin Gin Val Glu Ala Leu Gin Gly Gin Val Gin His Leu Gin Ala Ala Phe Ser Gin Tyr. Another example of a leucine zipper that promotes trimerization is a peptide comprising the amino acid sequence Arg Met Lys Gin He Glu Asp Lys He Glu Glu He Leu Ser Lys He Tyr His He Glu Asn Glu He Ala Arg He Lys Lys Leu He Gly Glu Arg, as described in U.S. Patent 5,716,805. In one alternative embodiment, an N-terminal Asp residue is added; in another, the peptide lacks the N-terminal Arg residue. Fragments of the foregoing zipper peptides that retain the property of promoting oligomerization may be employed as well. Examples of such fragments include, but are not limited to, peptides lacking one or two of the N-terminal or C-terminal residues presented in the foregoing amino acid sequences. Leucine zippers may be derived from naturally occurring leucine zipper peptides, e.g., via conservative substitution(s) in the native amino acid sequence, wherein the peptide's ability to promote oligomerization is retained. Other peptides derived from naturally occurring trimeric proteins may be employed in preparing trimeric ULBP. Alternatively, synthetic peptides that promote oligomerization may be employed. In particular embodiments, leucine residues in a leucine zipper moiety are replaced by isoleucine residues. Such peptides comprising isoleucine may be referred to as isoleucine zippers, but are encompassed by the term "leucine zippers" as employed herein. Both ULBP-1 and ULBP-2 polypeptides can be expressed as leucine zipper fusion proteins using a leucine zipper motif, having the following sequences: ULBP1-LZ FUSION POLYPEPTIDE 1 METDTLLLWV LLLWVPGSTG WSRAGWVDTH CLCYDFIITP KSRPEPQWCE 51 VQGLVDERPF LHYDCVNHKA KAFASLGKKV NVTKTWEEQT ETLRDVVDFL 101 KGQLLDIQVE NLIPIEPLTL QARMSCEHEA HGHGRGΞWQF LFNGQKFLLF 151 DSNNRKWTAL HPGAKKMTEK WEKNRDVTMF FQKISLGDCK MWLEEFLMYW 201 EQMLDPTKPP SLAPGTTQPR SGSSRMKQIE DKIEEILSKI YHIENEIARI 251 KKLIGERGTS SRGSHHHHHH [SEQ ID NO: 7] Amino acids 1-19 are the signal peptide from the human Ig kappa that replace the ULBP-1 signal peptide. This improves expression somewhat, but the native ULBP-1 signal peptide could also be used, as well as other heterologous signal peptides. Amino acids 20-218 are from ULBP-1 (SEQ ID NO:2), followed by a short spacer, PRSGSS, the leucine zipper (strictly isoleucine zipper) from amino acids 225-257, a spacer, and six histidines to act as a tag for purification. ULBP2-LZ FUSION POLYPEPTIDE 1 MAAAAATKIL LCLPLLLLLS GWSRAGRADP HSLCYDITVI PKFRPGPRWC 51 AVQGQVDEKT FLHYDCGNKT VTPVSPLGKK LNVTTAWKAQ NPVLREWDI 101 LTEQLRDIQL ENYTPKEPLT LQARMSCEQK AEGHSSGSWQ FSFDGQIFLL 151 FDSEKRM TT VHPGARK KE KWENDKWAM SFHYFSMGDC IGWLEDFLMG 201 MDSTLEPSAG APLAMSSGTT QLRGSGSSRM KQIEDKIEEI LSKIYHIENE 251 IARIKKLIGE RGTSSRGSHH HHHH [SEQ ID NO: 8] Amino acids 1-223 are from ULBP-2 polypeptide (SEQ ID NO:4), including its native signal peptide, and the spacers, leucine zipper, and poly His are the same as indicated above. PRODUCTION OF POLYPEPTIDES AN; D FRAGMENTS THEREOF Expression, isolation and purification of the polypeptides and fragments of the invention may be accomplished by any suitable technique, including but not limited to the following: Expression Systems The present invention also provides recombinant cloning and expression vectors containing DNA, as well as host cell containing the recombinant vectors. Expression vectors comprising DNA may be used to prepare the polypeptides or fragments of the invention encoded by the DNA. A method for producing polypeptides comprises culturing host cells transformed with a recombinant expression vector encoding the polypeptide, under conditions that promote expression of the polypeptide, then recovering the expressed polypeptides from the culture. The skilled artisan will recognize that the procedure for purifying the expressed polypeptides will vary according to such factors as the type of host cells employed, and whether the polypeptide is membrane-bound or a soluble form that is secreted from the host cell. Any suitable expression system may be employed. The vectors include a DNA encoding a polypeptide or fragment of the invention, operably linked to suitable transcriptional or translational regulatory nucleotide sequences, such as those derived from a mammalian, microbial, viral, or insect gene. Examples of regulatory sequences include transcriptional promoters, operators, or enhancers, an mRNA ribosomal binding site, and appropriate sequences which control transcription and translation initiation and termination. Nucleotide sequences are operably linked when the regulatory sequence functionally relates to the DNA sequence. Thus, a promoter nucleotide sequence is operably linked to a DNA sequence if the promoter nucleotide sequence controls the transcription of the DNA sequence. An origin of replication that confers the ability to replicate in the desired host cells, and a selection gene by which transformants are identified, are generally incoφorated into the expression vector. In addition, a sequence encoding an appropriate signal peptide (native or heterologous) can be incoφorated into expression vectors. A DNA sequence for a signal peptide (secretory leader) may be fused in frame to the nucleic acid sequence of the invention so that the DNA is initially transcribed, and the mRNA translated, into a fusion protein comprising the signal peptide. A signal peptide that is functional in the intended host cells promotes extracellular secretion of the polypeptide. The signal peptide is cleaved from the polypeptide upon secretion of polypeptide from the cell. The skilled artisan will also recognize that the position(s) at which the signal peptide is cleaved may differ from that predicted by computer program, and may vary according to such factors as the type of host cells employed in expressing a recombinant polypeptide. A protein preparation may include a mixture of protein molecules having different N-terminal amino acids, resulting from cleavage of the signal peptide at more than one site. Suitable host cells for expression of polypeptides include prokaryotes, yeast or higher eukaryotic cells. Mammalian or insect cells are generally preferred for use as host cells. Appropriate cloning and expression vectors for use with bacterial, fungal, yeast, and mammalian cellular hosts are described, for example, in Pouwels et al. Cloning Vectors: A Laboratory Manual, Elsevier, New York, (1985). Cell-free translation systems could also be employed to produce polypeptides using RNAs derived from DNA constructs disclosed herein. Prokaryotic Systems Prokaryotes include gram-negative or gram-positive organisms. Suitable prokaryotic host cells for transformation include, for example, E. coli, Bacillus subtilis, Salmonella typhimurium, and various other species within the genera Pseudomonas, Streptomyces, and Staphylococcus. In a prokaryotic host cell, such as E. coli, a polypeptide may include an N- terminal methionine residue to facilitate expression of the recombinant polypeptide in the prokaryotic host cell. The N-terminal Met may be cleaved from the expressed recombinant polypeptide. Expression vectors for use in prokaryotic host cells generally comprise one or more phenotypic selectable marker genes. A phenotypic selectable marker gene is, for example, a gene encoding a protein that confers antibiotic resistance or that supplies an autotrophic requirement. Examples of useful expression vectors for prokaryotic host cells include those derived from commercially available plasmids such as the cloning vector pBR322 (ATCC 37017). pBR322 contains genes for ampicillin and tetracycline resistance and thus provides simple means for identifying transformed cells. An appropriate promoter and a DNA sequence are inserted into the pBR322 vector. Other commercially available vectors include, for example, pKK223-3 (Pharmacia Fine Chemicals, Uppsala, Sweden) and pGEMl (Promega Biotec, Madison, WI, USA). Promoter sequences commonly used for recombinant prokaryotic host cell expression vectors include β-lactamase (penicillinase), lactose promoter system (Chang et al., Nature 275:615, 1978; and Goeddel et al., Nature 281:544, 1979), tryptophan (tφ) promoter system (Goeddel et al., Nucl. Acids Res. 8:4051, 1980; and EP-A-36776) and tac promoter (Maniatis, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, p. 412, 1982). A particularly useful prokaryotic host cell expression system employs a phage λP L promoter and a cI857ts thermolabile repressor sequence. Plasmid vectors available from the American Type Culture Collection which incoφorate derivatives of the λP L promoter include plasmid pHUB2 (resident in E. coli strain JMB9, ATCC 37092) and pPLc28 (resident in E. coli RR1, ATCC 53082). ULBP DNA may be cloned in-frame into the multiple cloning site of an ordinary bacterial expression vector. Ideally the vector would contain an inducible promoter upstream of the cloning site, such that addition of an inducer leads to high-level production of the recombinant protein at a time of the investigator's choosing. For some proteins, expression levels may be boosted by incoφoration of codons encoding a fusion partner (such as hexahistidine) between the promoter and the gene of interest. The resulting "expression plasmid" may be propagated in a variety of strains of E. coli. For expression of the recombinant protein, the bacterial cells are propagated in growth medium until reaching a pre-determined optical density. Expression of the recombinant protein is then induced, e.g. by addition of IPTG (isopropyl-b-D-thiogalactopyranoside), which activates expression of proteins from plasmids containing a lac operator/promoter. After induction (typically for 1-4 hours), the cells are harvested by pelleting in a centrifuge, e.g. at 5,000 x G for 20 minutes at 4°C. For recovery of the expressed protein, the pelleted cells may be resuspended in ten volumes of 50 mM Tris-HCl (pH 8)/l M NaCl and then passed two or three times through a French press. Most highly expressed recombinant proteins form insoluble aggregates known as inclusion bodies. Inclusion bodies can be purified away from the soluble proteins by pelleting in a centrifuge at 5,000 x G for 20 minutes, 4°C. The inclusion body pellet is washed with 50 mM Tris-HCl (pH 8)/l% Triton X-100 and then dissolved in 50 mM Tris-HCl (pH 8)/8 M urea/ 0.1 M DTTf. Any material that cannot be dissolved is removed by centrifugation (10,000 x G for 20 minutes, 20°C). The protein of interest will, in most cases, be the most abundant protein in the resulting clarified supernatant. This protein may be "refolded" into the active conformation by dialysis against 50 mM Tris-HCl (pH 8)/5 mM CaCl 2 /5 mM Zn(OAc) : /l mM GSSG/0.1 mM GSH. After refolding, purification can be carried out by a variety of chromatographic methods, such as ion exchange or gel filtration. In some protocols, initial purification may be carried out before refolding. As an example, hexahistidine-tagged fusion proteins may be partially purified on immobilized Nickel. While the preceding purification and refolding procedure assumes that the protein is best recovered from inclusion bodies, those skilled in the art of protein purification will appreciate that many recombinant proteins are best purified out of the soluble fraction of cell lysates. In these cases, refolding is often not required, and purification by standard chromatographic methods can be carried out directly. Yeast Systems Alternatively, the polypeptides may be expressed in yeast host cells, preferably from the Saccharomyces genus (e.g., S. cerevisiae). Other genera of yeast, such as Pichia or Kluyveromyces, may also be employed. Yeast vectors will often contain an origin of replication sequence from a 2μ yeast plasmid, an autonomously replicating sequence (ARS), a promoter region, sequences for polyadenylation, sequences for transcription termination, and a selectable marker gene. Suitable promoter sequences for yeast vectors include, among others, promoters for metallothionein, 3-phosphoglycerate kinase (Hitzeman et al., J. Biol. Chem. 255:2073, 1980) or other glycolytic enzymes (Hess et al., J. Adv. Enzyme Reg. 7: 149, 1968; and Holland et al., Biochem. 77:4900, 1978), such as enolase, glyceraldehyde-3-phosphate dehydrogenase, hexokinase, pyruvate decarboxylase, phosphofructokinase, glucose-6- phosphate isomerase, 3-phosphoglycerate mutase, pyruvate kinase, triosephosphate isom erase, phospho-glucose isomerase, and glucokinase. Other suitable vectors and promoters for use in yeast expression are further described in Hitzeman, EPA-73,657. Another alternative is the glucose-repressible ADH2 promoter described by Russell et al. (J. Biol. Chem. 258:2614, 1982) and Beier et al. (Nature 300:124, 1982). Shuttle vectors replicable in both yeast and E. coli may be constructed by inserting DNA sequences from pBR322 for selection and replication in E. coli (Amp r gene and origin of replication) into the above-described yeast vectors. The yeast α-factor leader sequence may be employed to direct secretion of the polypeptide. The α-factor leader sequence is often inserted between the promoter sequence and the structural gene sequence. See, e.g., Kurjan et al., Cell 50:933, 1982 and Bitter et al., Proc. Natl. Acad. Sci. USA 81:5330, 1984. Other leader sequences suitable for facilitating secretion of recombinant polypeptides from yeast hosts are known to those of skill in the art. A leader sequence may be modified near its 3' end to contain one or more restriction sites. This will facilitate fusion of the leader sequence to the structural gene. Yeast transformation protocols are known to those of skill in the art. One such protocol is described by Hinnen et al., Proc. Natl. Acad. Sci. USA 75: 1929. 1978. The Hinnen et al. protocol selects for Tφ * transformants in a selective medium, wherein the selective medium consists of 0.67% yeast nitrogen base, 0.5% casamino acids, 2% glucose, 10 mg/ml adenine and 20 mg/ml uracil. Yeast host cells transformed by vectors containing an ADH2 promoter sequence may be grown for inducing expression in a "rich" medium. An example of a rich medium is one consisting of 1% yeast extract, 2% peptone, and 1% glucose supplemented with 80 mg/ml adenine and 80 mg/ml uracil. Derepression of the ADH2 promoter occurs when glucose is exhausted from the medium. Mammalian or Insect Systems Mammalian or insect host cell culture systems also may be employed to express recombinant polypeptides. Bacculovirus systems for production of heterologous proteins in insect cells are reviewed by Luckow and Summers, Bio/Technology 6:47 (1988). Established cell lines of mammalian origin also may be employed. Examples of suitable mammalian host cell lines include the COS-7 line of monkey kidney cells (ATCC CRL 1651) (Gluzman et al., Cell 25:175, 1981), L cells, C127 cells, 3T3 cells (ATCC CCL 163), Chinese hamster ovary (CHO) cells, HeLa cells, and BHK (ATCC CRL 10) cell lines, and the CN1/EBNA cell line derived from the African green monkey kidney cell line CV1 (ATCC CCL 70) as described by McMahan et al. (EMBO J. 10: 2821, 1991). Established methods for introducing DNA into mammalian cells have been described (Kaufman, R.J., Large Scale Mammalian Cell Culture, 1990, pp. 15-69). Additional protocols using commercially available reagents, such as Lipofectamine lipid reagent (Gibco/BRL) or Lipofectamine-Plus lipid reagent, can be used to transfect cells (Feigner et al., Proc. Natl. Acad. Sci. USA 84:14X3-14X1, 1987). In addition, electroporation can be used to transfect mammalian cells using conventional procedures, such as those in Sambrook et al. (Molecular Cloning: A Laboratory Manual, 2 ed. Vol. 1-3, Cold Spring Harbor Laboratory Press, 1989). Selection of stable transformants can be performed using methods known in the art, such as, for example, resistance to cytotoxic drugs. Kaufman et al., Meth. in Enzymology 755:487-511, 1990, describes several selection schemes, such as dihydrofolate reductase (DHFR) resistance. A suitable host strain for DHFR selection can be CHO strain DX-B 1 1 , which is deficient in DHFR (Urlaub and Chasin, Proc. Natl. Acad. Sci. USA 77:4216-4220, 1980). A plasmid expressing the DHFR cDNA can be introduced into strain DX-B 1 1, and only cells that contain the plasmid can grow in the appropriate selective media. Other examples of selectable markers that can be incoφorated into an expression vector include cDNAs conferring resistance to antibiotics, such as G418 and hygromycin B. Cells harboring the vector can be selected on the basis of resistance to these compounds. Transcriptional and translational control sequences for mammalian host cell expression vectors can be excised from viral genomes. Commonly used promoter sequences and enhancer sequences are derived from polyoma virus, adenovirus 2, simian virus 40 (SV40), and human cytomegalovirus. DNA sequences derived from the SV40 viral genome, for example, SV40 origin, early and late promoter, enhancer, splice, and polyadenylation sites can be used to provide other genetic elements for expression of a structural gene sequence in a mammalian host cell. Viral early and late promoters are particularly useful because both are easily obtained from a viral genome as a fragment, which can also contain a viral origin of replication (Fiers et al., Nature 275: 1 13, 1978; Kaufman, ?tΛ. in Enzymology, 1990). Smaller or larger SV40 fragments can also be used, provided the approximately 250 bp sequence extending from the Hind III site toward the Bgl I site located in the SV40 viral origin of replication site is included. Additional control sequences shown to improve expression of heterologous genes from mammalian expression vectors include such elements as the expression augmenting sequence element (EASE) derived from CHO cells (Morris et al., Animal Cell Technology, 1997, pp. 529-534 and PCT Application WO 97/25420) and the tripartite leader (TPL) and VA gene RNAs from Adenovirus 2 (Gingeras et al., J. Biol. Chem. 257:13475-13491, 1982). The internal ribosome entry site (IRES) sequences of viral origin allows dicistronic mRNAs to be translated efficiently (Oh and Sarnow, Current Opinion in Genetics and Development 5:295- 300, 1993; Ramesh et al., Nucleic Acids Research 24:2691-2100, 1996). Expression of a heterologous cDNA as part of a dicistronic mRNA followed by the gene for a selectable marker (e.g. DHFR) has been shown to improve transfectability of the host and expression of the heterologous cDNA (Kaufman, Meth. in Enzymology, 1990). Exemplary expression vectors that employ dicistronic mRNAs are pTR-DC/GFP described by Mosser et al., Biotechniques 22: 150-161 , 1997, and p2A5I described by Morris et al., Animal Cell Technology, 1997, pp. 529-534. A useful high expression vector, pCAVNOT, has been described by Mosley et al., Cell 59:335-348, 1989. Other expression vectors for use in mammalian host cells can be constructed as disclosed by Okayama and Berg (Mol. Cell. Biol. 5:280, 1983). A useful system for stable high level expression of mammalian cDNAs in C 127 murine mammary epithelial cells can be constructed substantially as described by Cosman et al. (Mol. Immunol. 23:935, 1986). A useful high expression vector, PMLSV N1/N4, described by Cosman et al., Nature 572:768, 1984, has been deposited as ATCC 39890. Additional useful mammalian expression vectors are described in EP-A-0367566, and in WO 91/18982, incoφorated by reference herein. In yet another alternative, the vectors can be derived from retroviruses. Additional useful expression vectors, pFLAG ® and pDC311, can also be used. FLAG ® technology is centered on the fusion of a low molecular weight (lkD), hydrophilic, FLAG ® marker peptide to the N-terminus of a recombinant protein expressed by pFLAG ® expression vectors. pDC311 is another specialized vector used for expressing proteins in CHO cells. pDC311 is characterized by a bicistronic sequence containing the gene of interest and a dihydrofolate reductase (DHFR) gene with an internal ribosome binding site for DHFR translation, an expression augmenting sequence element (EASE), the human CMV promoter, a tripartite leader sequence, and a polyadenylation site. Regarding signal peptides that may be employed, the native signal peptide may be replaced by a heterologous signal peptide or leader sequence, if desired. The choice of signal peptide or leader may depend on factors such as the type of host cells in which the recombinant polypeptide is to be produced. To illustrate, examples of heterologous signal peptides that are functional in mammalian host cells include the signal sequence for interleukin-7 (IL-7) described in United States Patent 4,965,195; the signal sequence for interleukin-2 receptor described in Cosman et al., Nature 312:768 (1984); the interleukin-4 receptor signal peptide described in EP 367,566; the type I interleukin-1 receptor signal peptide described in U.S. Patent 4,968,607; and the type II interleukin-1 receptor signal peptide described in EP 460,846. Purification The invention also includes methods of isolating and purifying the polypeptides and fragments thereof. An isolated and purified ULBP polypeptide according to the invention can be produced by recombinant expression systems as described above or purified from naturally occurring cells. ULBP polypeptide can be substantially purified, as indicated by a single protein band upon analysis by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). One process for producing ULBP comprises culturing a host cell transformed with an expression vector comprising a DNA sequence that encodes ULBP polypeptide under conditions sufficient to promote expression of ULBP. ULBP polypeptide is then recovered from culture medium or cell extracts, depending upon the expression system employed. Isolation and Purification The expression "isolated and purified" as used herein means that ULBP is essentially free of association with other DNA, proteins, or polypeptides, for example, as a purification product of recombinant host cell culture or as a purified product from a non-recombinant source. The term "substantially purified" as used herein refers to a mixture that contains ULBP and is essentially free of association with other DNA, proteins, or polypeptides, but for the presence of known DNA or proteins that can be removed using a specific antibody, and which substantially purified ULBP proteins retain biological activity. The term "purified ULBP" refers to either the "isolated and purified" form of ULBP or the "substantially purified" form of ULBP, as both are described herein. The term "biologically active" as it refers to ULBP protein, means that the ULBP protein is capable of associating with UL16 or being co-immunoprecipitated with UL16 using an antibody to the UL16. Similarly, the association of a ULBP-2 protein with UL16 demonstrates that the ULBP-2 protein is biologically active. In one preferred embodiment, the purification of recombinant polypeptides or fragments can be accomplished using fusions of polypeptides or fragments of the invention to another polypeptide to aid in the purification of polypeptides or fragments of the invention. Such fusion partners can include the poly-His or other antigenic identification peptides described above as well as the Fc moieties described previously. With respect to any type of host cell, as is known to the skilled artisan, procedures for purifying a recombinant polypeptide or fragment will vary according to such factors as the type of host cells employed and whether or not the recombinant polypeptide or fragment is secreted into the culture medium. In general, the recombinant polypeptide or fragment can be isolated from the host cells if not secreted, or from the medium or supernatant if soluble and secreted, followed by one or more concentration, salting-out, ion exchange, hydrophobic interaction, affinity purification or size exclusion chromatography steps. As to specific ways to accomplish these steps, the culture medium first can be concentrated using a commercially available protein concentration filter, for example, an Amicon or Millipore Pellicon ultrafiltration unit. Following the concentration step, the concentrate can be applied to a purification matrix such as a gel filtration medium. Alternatively, an anion exchange resin can be employed, for example, a matrix or substrate having pendant diethylaminoethyl (DEAE) groups. The matrices can be acrylamide, agarose, dextran, cellulose or other types commonly employed in protein purification. Alternatively, a cation exchange step can be employed. Suitable cation exchangers include various insoluble matrices comprising sulfopropyl or carboxymethyl groups. In addition, a chromato focusing step can be employed. Alternatively, a hydrophobic interaction chromatography step can be employed. Suitable matrices can be phenyl or octyl moieties bound to resins. In addition, affinity chromatography with a matrix which selectively binds the recombinant protein can be employed. Examples of such resins employed are lectin columns, dye columns, and metal- chelating columns. Finally, one or more reversed-phase high performance liquid chromatography (RP-HPLC) steps employing hydrophobic RP-HPLC media, (e.g., silica gel or polymer resin having pendant methyl, octyl, octyldecyl or other aliphatic groups) can be employed to further purify the polypeptides. Some or all of the foregoing purification steps, in various combinations, are well known and can be employed to provide an isolated and purified recombinant protein. Recombinant protein produced in bacterial culture is usually isolated by initial disruption of the host cells, centrifugation, extraction from cell pellets if an insoluble polypeptide, or from the supernatant fluid if a soluble polypeptide, followed by one or more concentration, salting-out, ion exchange, affinity purification or size exclusion chromatography steps. Finally, RP-HPLC can be employed for final purification steps. Microbial cells can be disrupted by any convenient method, including freeze-thaw cycling, sonication, mechanical disruption, or use of cell lysing agents. Transformed yeast host cells are preferably employed to express ULBP as a secreted polypeptide in order to simplify purification. Secreted recombinant polypeptide from a yeast host cell fermentation can be purified by methods analogous to those disclosed by Urdal et al. (J. Chromatog. 296:X1X, 1984). Urdal et al. describe two sequential, reversed-phase HPLC steps for purification of recombinant human IL-2 on a preparative HPLC column. In addition to recombinantly producing ULBP, ULBP can be isolated and purified from cell lines and in particular from Namalwa human B cell lymphoma cells. It is also possible to utilize an affinity column comprising a polypeptide-binding protein of the invention, such as a monoclonal antibody generated against polypeptides of the invention, to affinity-purify expressed polypeptides. These polypeptides can be removed from an affinity column using conventional techniques, e.g., in a high salt elution buffer and then dialyzed into a lower salt buffer for use or by changing pH or other components depending on the affinity matrix utilized, or be competitively removed using the naturally occurring substrate of the affinity moiety, such as a polypeptide derived from the invention. In this aspect of the invention, polypeptide-binding proteins, such as the anti- polypeptide antibodies of the invention or other proteins that may interact with the polypeptide of the invention, can be bound to a solid phase support such as a column chromatography matrix or a similar substrate suitable for identifying, separating, or purifying cells that express polypeptides of the invention on their surface. Adherence of polypeptide-binding proteins of the invention to a solid phase contacting surface can be accomplished by any means, for example, magnetic microspheres can be coated with these polypeptide-binding proteins and held in the incubation vessel through a magnetic field. Suspensions of cell mixtures are contacted with the solid phase that has such polypeptide-binding proteins thereon. Cells having polypeptides of the invention on their surface bind to the fixed polypeptide-binding protein and unbound cells then are washed away. This affinity-binding method is useful for purifying, screening, or separating such polypeptide-expressing cells from solution. Methods of releasing positively selected cells from the solid phase are known in the art and encompass, for example, the use of enzymes. Such enzymes are preferably non-toxic and non-injurious to the cells and are preferably directed to cleaving the cell-surface binding partner. Alternatively, mixtures of cells suspected of containing polypeptide-expressing cells of the invention first can be incubated with a biotinylated polypeptide-binding protein of the invention. Incubation periods are typically at least one hour in duration to ensure sufficient binding to polypeptides of the invention. The resulting mixture then is passed through a column packed with avidin-coated beads, whereby the high affinity of biotin for avidin provides the binding of the polypeptide-binding cells to the beads. Use of avidin-coated beads is known in the art. See Berenson, et al. J. Cell. Biochem., 10D:239 (1986). Wash of unbound material and the release of the bound cells is performed using conventional methods. In the methods described above, suitable ULBP-binding polypeptides are anti-ULBP antibodies and other proteins that are capable of high-affinity binding of ULBP. A preferred ULBP-binding protein is an anti-ULBP monoclonal antibody. The desired degree of purity depends on the intended use of the protein. A relatively high degree of purity is desired when the polypeptide is to be administered in vivo, for example. In such a case, the polypeptides are purified such that no protein bands corresponding to other proteins are detectable upon analysis by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). It will be recognized by one skilled in the pertinent field that multiple bands corresponding to the polypeptide may be visualized by SDS-PAGE, due to differential glycosylation, differential post-translational processing, and the like. Most preferably, the polypeptide of the invention is purified to substantial homogeneity, as indicated by a single protein band upon analysis by SDS-PAGE. The protein band may be visualized by silver staining, Coomassie blue staining, or (if the protein is radiolabeled) by autoradiography. Assays The purified polypeptides of the invention (including proteins, polypeptides, fragments, variants, oligomers, and other forms) may be tested for the ability to bind UL-16 or a ULBP counter-structure molecule in any suitable assay, such as a conventional binding assay. To illustrate, the polypeptide may be labeled with a detectable reagent (e.g., a radionuclide, chromophore, enzyme that catalyzes a colorimetric or fluorometric reaction, and the like). The labeled polypeptide is contacted with cells expressing a ULBP counter-structure molecule. The cells then are washed to remove unbound labeled polypeptide, and the presence of cell- bound label is determined by a suitable technique, chosen according to the nature of the label. One example of a binding assay procedure is as follows. A recombinant expression vector containing UL16 cDNA is constructed. DNA and amino acid sequence information for UL16 is presented in (Kaye et al., J. Virol. 66:6609, 1992). For example, UL16-Fc fuses the extracellular domain of UL16 to the IgG-I Fc (mutein form) as previously described for OX40-Fc (Baum et al., EMBO J. 13:3992-4001, 1994). CVl-EBNA-1 cells in 10 cm 2 dishes are transfected with the recombinant expression vector. CV-l/EBNA-1 cells (ATCC CRL 10478) constitutively express EBV nuclear antigen- 1 driven from the CMV immediate-early enhancer/promoter. CVl-EBNA-1 was derived from the African Green Monkey kidney cell line CV-1 (ATCC CCL 70), as described by McMahan et al. (EMBO J. 10:2821, 1991). The transfected cells are cultured for 24 hours, and the cells in each dish then are split into a 24-well plate. After culturing an additional 48 hours, the transfected cells (about 4 x 10 4 cells/well) are washed with BM-NFDM, which is binding medium (RPMI 1640 containing 25 mg/ml bovine serum albumin, 2 mg/ml sodium azide, 20 mM Hepes pH 7.2) to which 50 mg/ml nonfat dry milk has been added. The cells then are incubated for 1 hour at 37°C with various concentrations of, for example, a soluble polypeptide/Fc fusion protein made as set forth above. Cells then are washed and incubated with a constant saturating concentration of a l25 I-mouse anti-human IgG in binding medium, with gentle agitation for 1 hour at 37°C. After extensive washing, cells are released via trypsinization. The mouse anti-human IgG employed above is directed against the Fc region of human IgG and can be obtained from Jackson Immunoresearch Laboratories, Inc., West Grove, PA. The antibody is radioiodinated using the standard chloramine-T method. The antibody will bind to the Fc portion of any polypeptide/Fc protein that has bound to the cells. In all assays, non-specific binding of 125 I-antibody is assayed in the absence of the Fc fusion protein/Fc, as well as in the presence of the Fc fusion protein and a 200-fold molar excess of unlabeled mouse anti-human IgG antibody. Cell-bound 125 I-antibody is quantified on a Packard Autogamma counter. Affinity calculations (Scatchard, Ann. N. Y. Acad. Sci. 51 :660, 1949) are generated on RS/1 (BBN Software, Boston, MA) run on a Microvax computer. Another type of suitable binding assay is a competitive binding assay. To illustrate, biological activity of a variant may be determined by assaying for the variant's ability to compete with the native protein for binding to UL16 or cells expressing a ULBP counterstructure.. Competitive binding assays can be performed by conventional methodology. Reagents that may be employed in competitive binding assays include radiolabeled UL16 and intact cells expressing ULBP (endogenous or recombinant) on the cell surface. For example, a radiolabeled soluble ULBP fragment can be used to compete with a soluble ULBP variant for binding to cell surface (binding partner). Instead of intact cells, one could substitute a soluble ULlό/Fc fusion protein bound to a solid phase through the interaction of Protein A or Protein G (on the solid phase) with the Fc moiety. Chromatography columns that contain Protein A and Protein G include those available from Pharmacia Biotech, Inc., Piscataway, NJ. Another type of competitive binding assay utilizes radiolabeled soluble UL16 such as a soluble UL16/Fc fusion protein, and intact cells expressing ULBP. Qualitative results can be obtained by competitive autoradiographic plate binding assays, while Scatchard plots (Scatchard, Ann. N Y. Acad. Sci. 51 :660, 1949) may be utilized to generate quantitative results. USE OF ULBP NUCLEIC ACID OR OLIGONUCLEOTIDES In addition to being used to express polypeptides as described above, the nucleic acids of the invention, including DNA, and oligonucleotides thereof can be used: as probes to identify nucleic acid encoding proteins having ULBP activity; to identify human chromosome number 6; to map genes on human chromosome number 6; - to identify genes associated with certain diseases, syndromes, or other conditions associated with human chromosome number 6; as single-stranded sense or antisense oligonucleotides, to inhibit expression of polypeptide encoded by the ULBP gene; to help detect defective genes in an individual; and - for gene therapy. Probes Among the uses of nucleic acids of the invention is the use of fragments as probes or primers. Such fragments generally comprise at least about 17 contiguous nucieotides of a DNA sequence. In other embodiments, a DNA fragment comprises at least 30, or at least 60, contiguous nucieotides of a DNA sequence. Because homologs of SEQ ID NO: l, SEQ ID NO:3, and SEQ ID NO:9, from other mammalian species, are contemplated herein, probes based on the DNA sequence of SEQ ID NO:l . SEQ ID NO:3, and SEQ ID NO:9 may be used to screen cDNA libraries derived from other mammalian species, using conventional cross-species hybridization techniques. Using knowledge of the genetic code in combination with the amino acid sequences set forth above, sets of degenerate oligonucleotides can be prepared. Such oligonucleotides are useful as primers, e.g., in polymerase chain reactions (PCR), whereby DNA fragments are isolated and amplified. Chromosome Mapping ULBP-1 and ULBP-2 polypeptides are distantly related to a number of MHC class 1 proteins, particularly non-classical class 1 molecules or class 1 molecules of non-human species. ULBP-1 and ULBP-2 polypeptides contain regions homologous to the αl and α2 domains of MHC class I proteins. However, they have a different structure from other class 1 molecules in that they lack the alpha-3 domain, which is needed for binding beta-2- microglobulin. In addition, the ULBP-1 and the ULBP-2 genes map to chromosome 6q20-23, which is distinct from any other MHC class 1 and related genes. All or a portion of the nucleic acids of SEQ ID NO: l or SEQ ID NO:3, including oligonucleotides, can be used by those skilled in the art using well-known techniques to identify the human chromosome 6, and the specific locus thereof, that contains the DNA of ULBP family members. Useful techniques include, but are not limited to, using the sequence or portions, including oligonucleotides, as a probe in various well-known techniques such as radiation hybrid mapping (high resolution), in situ hybridization to chromosome spreads (moderate resolution), and Southern blot hybridization to hybrid cell lines containing individual human chromosomes (low resolution). For example, chromosomes can be mapped by radiation hybridization. PCR is performed using the Whitehead Institute/MIT Center for Genome Research Genebridge4 panel of 93 radiation hybrids (http://www-genome.wi.mit.edu/ftp/distribution human_STS_releases/july97/rhmap/genebridge4.html). Primers are used which lie within a putative exon of the gene of interest and which amplify a product from human genomic DNA, but do not amplify hamster genomic DNA. The results of the PCRs are converted into a data vector that is submitted to the Whitehead/MIT Radiation Mapping site on the internet (http://www-seq.wi.mit.edu). The data is scored and the chromosomal assignment and placement relative to known Sequence Tag Site (STS) markers on the radiation hybrid map is provided. The following web site provides additional information about radiation hybrid mapping: http://www-genome.wi.mit.edu/ftp/distribution/human_STS_releases/july97/ 07-97.INTRO.html). Identifying Associated Diseases As set forth below, SEQ ID NO: 1 and SEQ ID NO: 3 have been mapped to the 6q20-23 region of chromosome 6. That region is associated with specific diseases which include but are not limited to Retinitis pigmentosa-25 (6ql4-q21); Diabetes mellitus, insulin-dependent, 15 (6q21); Progressive pseudorheumatoid arthropathy of childhood (6q22); Muscular dystrophy, congenital merosin-deficient (6q22-q23); and Cardiomyopathy, dilated, IF (6q23). Thus, the nucleic acid of SEQ ID NO: l or SEQ ID NO:3, or a fragment thereof, can be used by one skilled in the art using well-known techniques to analyze abnormalities associated with gene mapping to chromosome 6. This enables one to distinguish conditions in which this marker is rearranged or deleted. In addition, nucieotides of SEQ ID NO: l or SEQ ID NO:3 or a fragment thereof can be used as a positional marker to map other genes of unknown location. The DNA may be used in developing treatments for any disorder mediated (directly or indirectly) by defective, or insufficient amounts of, the genes corresponding to the nucleic acids of the invention. Disclosure herein of native nucleotide sequences permits the detection of defective genes, and the replacement thereof with normal genes. Defective genes may be detected in in vitro diagnostic assays, and by comparison of a native nucleotide sequence disclosed herein with that of a gene derived from a person suspected of harboring a defect in this gene. Sense-Antisense Other useful fragments of the nucleic acids include antisense or sense oligonucleotides comprising a single-stranded nucleic acid sequence (either RNA or DNA) capable of binding to target mRNA (sense) or DNA (antisense) sequences. Antisense or sense oligonucleotides, according to the present invention, comprise a fragment of DNA (SEQ ID NO: l, SEQ ID NO:3, or SEQ ID NO:9). Such a fragment generally comprises at least about 14 nucieotides, preferably from about 14 to about 30 nucieotides. The ability to derive an antisense or a sense oligonucleotide, based upon a cDNA sequence encoding a given protein is described in, for example. Stein and Cohen (Cancer Res. 48:2659, 1988) and van der Krol et al. (BioTechniques 6:958, 1988). Binding of antisense or sense oligonucleotides to target nucleic acid sequences results in the formation of duplexes that block or inhibit protein expression by one of several means, including enhanced degradation of the mRNA by RNAseH, inhibition of splicing, premature termination of transcription or translation, or by other means. The antisense oligonucleotides thus may be used to block expression of proteins. Antisense or sense oligonucleotides further comprise oligonucleotides having modified sugar-phosphodiester backbones (or other sugar linkages, such as those described in WO91/06629) and wherein such sugar linkages are resistant to endogenous nucleases. Such oligonucleotides with resistant sugar linkages are stable in vivo (i.e., capable of resisting enzymatic degradation) but retain sequence specificity to be able to bind to target nucleotide sequences. Other examples of sense or antisense oligonucleotides include those oligonucleotides which are covalently linked to organic moieties, such as those described in WO 90/10448, and other moieties that increases affinity of the oligonucleotide for a target nucleic acid sequence, such as poly-(L-lysine). Further still, intercalating agents, such as ellipticine, and alkylating agents or metal complexes may be attached to sense or antisense oligonucleotides to modify binding specificities of the antisense or sense oligonucleotide for the target nucleotide sequence. Antisense or sense oligonucleotides may be introduced into a cell containing the target nucleic acid sequence by any gene transfer method, including, for example, lipofection, CaPO 4 - mediated DNA transfection, electroporation, or by using gene transfer vectors such as Epstein- Barr virus. Sense or antisense oligonucleotides are preferably introduced into a cell containing the target nucleic acid sequence by insertion of the sense or antisense oligonucleotide into a suitable retroviral vector, then contacting the cell with the retrovirus vector containing the inserted sequence, either in vivo or ex vivo. Suitable retroviral vectors include, but are not limited to, the murine retrovirus M-MuLV, N2 (a retrovirus derived from M-MuLV), or the double copy vectors designated DCT5A, DCT5B and DCT5C (see PCT Application US 90/02656). Sense or antisense oligonucleotides also may be introduced into a cell containing the target nucleotide sequence by formation of a conjugate with a ligand binding molecule, as described in WO 91/04753. Suitable ligand binding molecules include, but are not limited to, cell surface receptors, growth factors, other cytokines, or other ligands that bind to cell surface receptors. Preferably, conjugation of the ligand binding molecule does not substantially interfere with the ability of the ligand binding molecule to bind to its corresponding molecule or receptor, or block entry of the sense or antisense oligonucleotide or its conjugated version into the cell. Alternatively, a sense or an antisense oligonucleotide may be introduced into a cell containing the target nucleic acid sequence by formation of an oligonucleotide-lipid complex, as described in WO 90/10448. The sense or antisense oligonucleotide-lipid complex is preferably dissociated within the cell by an endogenous lipase. to a monoclonal antibody targeted to a specific cell type. USE OF ULBP POLYPEPTIDES AND FRAGMENTED POLYPEPTIDES Uses include, but are not limited to, the following: - As markers to detect cancer Enhancement of IFN-γ production and NK cell proliferation Enhancement of CTL activity Purifying proteins and measuring activity thereof Delivery Agents - Therapeutic Agents Rational Drug Design Research Reagents Molecular weight and Isoelectric focusing markers Controls for peptide fragmentation - Identification of unknown proteins Preparation of Antibodies As markers to detect cancer The ULBP DNA and polypeptides can be used as markers to detect cancer [hereinafter "ULBP marker"], particularly B cell lymphoma. Generally, markers of cancer represent alterations in the phenotype of a cell that distinguish malignant cells from normal cells. Markers can either be involved in the genesis of a cancer, (such as mutations in oncogenes or tumor suppressor genes), or can be secondary modifications acquired during the progression of the tumor. See, for example, Vincent T. DeVita, Jr., Samuel Hellman, and Steven A. Rosenberg, Cancer Principles & Practice of Oncology. 259-284 (5th ed. 1997) (describing the utilization of cancer markers), hereby incoφorated by reference in its entirety. In either case, an ULBP marker facilitates the detection, identification, diagnosis, or prognosis of cancer, particularly B cell lymphoma. The ULBP marker can be used to distinguish malignant from benign neoplasms, as well as distinguish infiltrating reactive processes from neoplastic cells. Furthermore, the ULBP marker can facilitate diagnosis of a tumor by identifying the normal progenitor cell from which a tumor arises by assessing features of tissue differentiation displayed by the tumor. Moreover, the evaluation of cells and tissues for the ULBP marker is useful in the subtyping and staging of tumors, providing critical information concerning prognosis. The ULBP marker can also be used to monitor residual or recurrent disease after treatment. Finally, the ULBP marker can diagnose patients having an inherited predisposition to the development of specific cancer types by identifying a genomic rearrangement in normal tissues. In contrast, non-inherited, or sporadic tumors, will be present with genomic rearrangements of the ULBP gene only in the tumor sample. Methods of utilizing markers for cancer detection are known to one of ordinary skill in the art. For instance, genomic rearrangement, a hallmark of cancer, can be detected using ULBP DNA. A genomic rearrangement is defined in the instant application as a deletion, an insertion, a translocation, an inversion, an amplification, a duplication, or a point mutation in or proximate to the ULBP gene, including the promoter, the intronic sequence, and the 3' untranslated region (UTR). Methods of detecting a genomic rearrangement in neoplastic cells utilizing the ULBP DNA are known to one of ordinary skill in the art and include, but are not limited to, Southern hybridization, PCR based assays, fluorescence in situ hybridization (FISH), DNA sequence reactions, allelic specific oligonucleotide hybridization, RNase protection, and assays based on conformational differences, such as single-strand conformational polymoφhism (SSCP), denaturing gradient gel electrophoresis (DGGE), and enzymes specifically detecting mismatches between sequences, for example, MutS and resolvase. (See, DeVita, pages 263-269, outlining the above well-known procedures used in the detection of cancer by markers.) Genomic rearrangements can also be detected using the ULBP DNA attached to glass chips with hybridization of a sample against the chip. The ULBP DNA can be used to detect expression of the ULBP gene. Many genes that may or may not be involved in the genesis and progression of cancer are either overexpressed, underexpressed, or absent relative to the expression patterns observed in the cell type from which the cancer is believed to have originated. In contrast, the expression levels may be comparable to the normal expression patterns; however, the messenger RNA (mRNA) contains mutations, such as stop, point, or alternative splicing mutations, not found in the wild type gene. The detection of these altered expression patterns and mRNA species in a cancer cell can be used as a marker of that particular cancer type. The ULBP nucleotide sequence can be used to detect expression patterns and mRNA species in a cancer cell, particularly B cell lymphoma. Again, one skilled in the art would know how to modify known methods to detect expression patterns and mRNA species of the ULBP gene. One method involves performing a Northern hybridization by isolating either mRNA or total RNA from a cancer cell and probing with the ULBP nucleotide. Detection can also be performed by a reverse-transcriptase polymerase chain reaction (RT-PCR) based assay, or a combination of RT-PCR and Southern hybridization. Furthermore, mutations in the mRNA species can be detected by the above procedures for detecting genomic rearrangements. In situ hybridization (ISH) is yet another well-known method that can be used in the instant invention to detect ULBP expression and mRNA species. (See, DeVita, page 267.) For example, paraffin embedded tissues obtained from a patient can be screened using labeled ULBP nucleotide complementary to the mRNA, allowing for the comparison of expression patterns in neoplastic cells adjacent to normal tissues. Thus, by disclosing the sequences of the ULBPs of the present invention, one of ordinary skill in the art can easily modify known methods of utilizing markers to detect mRNA species in normal and cancer cells. Not only can the ULBP nucieotides be used as markers to detect cancer, but also assays utilizing ULBP polypeptides can be designed to detect cancer. In one embodiment, isolated and purified ULBP-1 can be incubated with lymphoma cells under conditions that allow binding of ULBP-1 to lymphoma cells. Unbound ULBP-1 can be removed using a washing step, and bound ULBP-1 can be detected using an anti-ULBP- 1 monoclonal antibody. (See Meyers et al., Leuk. and Lymph. 18:119-122). In another embodiment, assays utilizing antibodies prepared from ULBP polypeptides can be designed to detect cancer. (See, DeVita, pages 260-262.) For example, antibodies raised against ULBP polypeptides can be used in immunohistologic examinations to distinguish benign from malignant proliferations. These immunohistologic examinations may detect oveφroduction, undeφroduction, or miscompartmentalization of the ULBP protein in cancer cells relative to the normal cells of origin. Evidence of an alteration in the production of the wild-type ULBP protein can be a marker associated with a more malignant or advanced stage of cancer. Similarly, abnormally produced ULBP proteins can be detected by antibodies directed against the ULBP protein in samples taken from bodily fluids. For instance, abnormally produced ULBP protein may fail to remain in the cellular membrane, and instead be secreted by the cell into the patient's blood stream. Blood samples taken from a patient are then analyzed using ULBP antibodies to determine the presence of abnormal ULBP protein in the blood by a flow cytometer. The presence of abnormal ULBP protein in the blood stream may help monitor therapy and detect progression or relapse in patients with cancer, particularly B cell lymphoma. Enhancement of IFN-γ production and NK cell proliferation ULBP polypeptides have been used to enhance IFN-γ production and NK cell proliferation in a dose-dependent manner (Figures 9 and 10). Therefore, ULBP polypeptides can be used in methods to enhance IFN-γ production and NK cell proliferation by incubating ULBP polypeptides with NK cells, for example, as described in Example 5. The skilled artisan recognizes that ULBP polypeptides can be used to enhance IFN-γ production and NK cell proliferation both in vitro and in vivo. The skilled artisan further appreciates that the enhancement of IFN-γ production and NK cell proliferation by ULBP polypeptides can modulate an immune response, for example, against viruses, bacteria, parasites, and tumors. Enhancement of CTL activity ULBP polypeptides have been used to enhance CTL activity in a dose-dependent manner (Figures 11 and 12). Therefore, ULBP polypeptides can be used in methods to enhance CTL activity by incubating ULBP polypeptides with CTLs, for example, as described in Example 6. The skilled artisan recognizes that ULBP polypeptides can be used to enhance CTL activity both in vitro and in vivo. The skilled artisan further appreciates that the enhancement of CTL activity by ULBP polypeptides can modulate an immune response, for example, against viruses, bacteria, and tumors. Purification Reagents Each of the polypeptides of the invention finds use as a protein purification reagent. The polypeptides may be attached to a solid support material and used to purify ULBP counter- structure molecules by affinity chromatography. In particular embodiments, a polypeptide (in any form described herein that is capable of binding ULBP counter-structure molecules) is attached to a solid support by conventional procedures. As one example, chromatography columns containing functional groups that will react with functional groups on amino acid side chains of proteins are available (Pharmacia Biotech, Inc., Piscataway, NJ). In an alternative, a polypeptide/Fc protein (as discussed above) is attached to Protein A- or Protein G-containing chromatography columns through interaction with the Fc moiety. The polypeptide also finds use in purifying or identifying cells that express ULBP counter-structure molecules on the cell surface. Polypeptides are bound to a solid phase such as a column chromatography matrix or a similar suitable substrate. For example, magnetic microspheres can be coated with the polypeptides and held in an incubation vessel through a magnetic field. Suspensions of cell mixtures containing ULBP counter-structure molecule- expressing cells are contacted with the solid phase having the polypeptides thereon. Cells expressing ULBP counter-structure molecules on the cell surface bind to the fixed polypeptides, and unbound cells then are washed away. Alternatively, the polypeptides can be conjugated to a detectable moiety, then incubated with cells to be tested for ULBP counter-structure molecules expression. After incubation, unbound labeled matter is removed and the presence or absence of the detectable moiety on the cells is determined. In a further alternative, mixtures of cells suspected of containing cells expressing ULBP counter-structure molecules are incubated with biotinylated polypeptides. Incubation periods are typically at least one hour in duration to ensure sufficient binding. The resulting mixture then is passed through a column packed with avidin-coated beads, whereby the high affinity of biotin for avidin provides binding of the desired cells to the beads. Procedures for using avidin- SUBSTTTUTE SHEET (RULE 26) coated beads are known (see Berenson, et al. J. Cell. Biochem., 10D:239, 1986). Washing to remove unbound material, and the release of the bound cells, are performed using conventional methods. Measuring Activity Polypeptides also find use in measuring the biological activity of ULBP counter- structure molecules in terms of their binding affinity. The polypeptides thus may be employed by those conducting "quality assurance" studies, e.g., to monitor shelf life and stability of protein under different conditions. For example, the polypeptides may be employed in a binding affinity study to measure the biological activity of a ULBP counter-structure molecule that has been stored at different temperatures, or produced in different cell types. The proteins also may be used to determine whether biological activity is retained after modification of a ULBP counter-structure molecule (e.g., chemical modification, truncation, mutation, etc.). The binding affinity of the modified ULBP counter-structure molecule is compared to that of an unmodified ULBP counter-structure molecules to detect any adverse impact of the modifications on biological activity of ULBP counter-structure molecules. The biological activity of a ULBP counter-structure molecule thus can be ascertained before it is used in a research study, for example. Delivery Agents The polypeptides can be used to deliver diagnostic or therapeutic agents to such cells or cell types found to express ULBP counterestructure molecules on the cell surface in in vitro or in vivo procedures. For example, ULBPl-Fc binds to anaplastic lymphomas. Therefore, ULBP-1 polypeptide can be attached to a toxin to bind to lymphoma cells and specifically kill these cells. The methodology can be similar to the successful use of an anti-CD72 immunotoxin to treat therapy-refractory B-lineage acute lymphoblastic leukemia in SCID mice (Meyers et al., Leuk. and Lymph. 18:1 19-122). In addition, ULBPl-Fc and ULBP2-Fc bind to activated T cells more strongly than resting T cells. Therefore, when attached to toxins or made radioactive, they can be used to kill activated T cells in diseases where T cells are overactive (autoimmunity), such as lupus, multiple sclerosis, rheumatoid arthritis, psoriasis, graft rejection, and graft versus host disease. Since they also bind to NK cells, they can be used to kill NK cells, which would help in treating graft rejection. Detectable (diagnostic) and therapeutic agents that may be attached to a polypeptide include, but are not limited to, toxins, other cytotoxic agents, drugs, radionuclides, chromophores, enzymes that catalyze a colorimetric or fluorometric reaction, and the like, with the particular agent being chosen according to the intended application. Among the toxins are ricin, abrin, diphtheria toxin, Pseudomonas aeruginosa exotoxin A, ribosomal inactivating proteins, mycotoxins such as trichothecenes, and derivatives and fragments (e.g., single chains) thereof. Radionuclides suitable for diagnostic use include, but are not limited to, 123 1, 131 I, 99m Tc, '"In, and 76 Br. Examples of radionuclides suitable for therapeutic use are m I, 211 At, 77 Br, l86 Re, 188 Re, 212 Pb, 212 Bi, 109 Pd, M Cu, and 67 Cu. Such agents may be attached to the polypeptide by any suitable conventional procedure. The polypeptide comprises functional groups on amino acid side chains that can be reacted with functional groups on a desired agent to form covalent bonds, for example. Alternatively, the protein or agent may be derivatized to generate or attach a desired reactive functional group. The derivatization may involve attachment of one of the bifunctional coupling reagents available for attaching various molecules to proteins (Pierce Chemical Company, Rockford, Illinois). A number of techniques for radiolabeling proteins are known. Radionuclide metals may be attached to polypeptides by using a suitable bifunctional chelating agent, for example. Conjugates comprising polypeptides and a suitable diagnostic or therapeutic agent (preferably covalently linked) are thus prepared. The conjugates are administered or otherwise employed in an amount appropriate for the particular application. Therapeutic Agents Polypeptides of the invention may be used in developing treatments for any disorder mediated (directly or indirectly) by defective, or insufficient amounts of the polypeptides. These polypeptides may be administered to a mammal afflicted with such a disorder. The polypeptides may also be employed in inhibiting a biological activity of ULBP counterestructure molecules, in in vitro or in vivo procedures. For example, a purified polypeptide may be used to inhibit binding of ULBP counter-structure molecules to endogenous cell surface molecules. Biological effects that result from the binding of ULBP counter- structure molecules to endogenous receptors thus are inhibited. ULBP polypeptides may be administered to a mammal to treat a ULBP counter- structure molecule-mediated disorder. Such ULBP counter-structure molecules-mediated disorders include conditions caused (directly or indirectly) or exacerbated by ULBP counter- structure molecules. Compositions of the present invention may contain a polypeptide in any form described herein, such as native proteins, variants, derivatives, oligomers, and biologically active fragments. In particular embodiments, the composition comprises a soluble polypeptide or an oligomer comprising soluble ULBP polypeptides. Compositions comprising an effective amount of a polypeptide of the present invention, in combination with other components such as a physiologically acceptable diluent, carrier, or excipient, are provided herein. The polypeptides can be formulated according to known methods used to prepare pharmaceutically useful compositions. They can be combined in admixture, either as the sole active material or with other known active materials suitable for a given indication, with pharmaceutically acceptable diluents (e.g., saline, Tris-HCl, acetate, and phosphate buffered solutions), preservatives (e.g., thimerosal, benzyl alcohol, parabens), emulsifiers, solubilizers, adjuvants and/or carriers. Suitable formulations for pharmaceutical compositions include those described in Remington's Pharmaceutical Sciences, 16th ed. 1980, Mack Publishing Company, Easton, PA. In addition, such compositions can be complexed with polyethylene glycol (PEG), metal ions, or incoφorated into polymeric compounds such as polyacetic acid, polyglycolic acid, hydrogels, dextran, etc., or incoφorated into liposomes, microemulsions, micelles, unilamellar or multilamellar vesicles, erythrocyte ghosts or spheroblasts. Such compositions will influence the physical state, solubility, stability, rate of in vivo release, and rate of in vivo clearance, and are thus chosen according to the intended application. The compositions of the invention can be administered in any suitable manner, e.g., topically, parenterally, or by inhalation. The term "parenteral" includes injection, e.g., by subcutaneous, intravenous, or intramuscular routes, also including localized administration, e.g., at a site of disease or injury. Sustained release from implants is also contemplated. One skilled in the pertinent art will recognize that suitable dosages will vary, depending upon such factors as the nature of the disorder to be treated, the patient's body weight, age, and general condition, and the route of administration. Preliminary doses can be determined according to animal tests, and the scaling of dosages for human administration is performed according to art- accepted practices. Compositions comprising nucleic acids in physiologically acceptable formulations are also contemplated. DNA may be formulated for injection, for example. Rational Drug Design In addition, ULBP polypeptides can also be used for structure-based design of ULBP inhibitors. Such structure-based design is also known as "rational drug design." The ULBP polypeptides can be three dimensionally analyzed by, for example, X-ray crystallography, nuclear magnetic resonance, or homology modeling, all of which are well known methods. The use of ULBP structural information in molecular modeling software systems to assist in inhibitor design and inhibitor-ULBP interaction is also encompassed by the invention. Such computer-assisted modeling and drug design may utilize information such as chemical conformational analysis, electrostatic potential of the molecules, protein folding, etc. For example, most of the design of class-specific inhibitors of metalloproteases has focused on attempts to chelate or bind the catalytic zinc atom. Synthetic inhibitors are usually designed to contain a negatively charged moiety to which is attached a series of other groups designed to fit the specificity pockets of the particular protease. A particular method of the invention comprises analyzing the three dimensional structure of ULBP for likely binding sites of substrates, synthesizing a new molecule that incoφorates a predictive reactive site, and assaying the new molecule as described above. Research reagents Another use of the polypeptide of the present invention is as a research tool for studying the biological effects that result from inhibiting ULBP ULBPcounter-structure interactions on different cell types. Polypeptides also may be employed in in vitro assays for detecting ULBP counter-structure molecules or ULBP polypeptides or the interactions thereof. ULBP may also be used as a reagent to identify (a) the proteins to which it binds, and which are involved in ULBP signaling, and (b) other proteins with which it might interact which would be involved in signal transduction pathways. These other proteins would then be useful tools to search for other inhibitors of signaling. ULBP could be used by coupling recombinant protein to an affinity matrix, or by using it as a bait in the 2-hybrid system. The interaction between ULBP polypeptide and its counter-structure enables screening for small molecules that interfere with the ULBP polypeptide ULBP counter-structure association and inhibit activity of ULBP polypeptide or its counter-structure. For example, the yeast two-hybrid system developed at SUNY (described in U.S. Patent No. 5,283,173 to Fields et al.) may be used to screen for inhibitors of ULBP as follows. ULBP polypeptide and its counter-structure, or portions thereof responsible for their interaction, may be fused to the Gal 4 DNA binding domain and Gal 4 transcriptional activation domain, respectively, and introduced into a strain that depends on Gal 4 activity for growth on plates lacking histidine. Compounds that prevent growth may be screened in order to identify ULBP inhibitors. Alternatively, the screen may be modified so that ULBP polypeptide/ULBP polypeptide counter-structure interaction inhibits growth, so that inhibition of the interaction allows growth to occur. Another, in vitro, approach to screening for ULBP inhibition would be to immobilize one of the components (either ULBP polypeptide or its counter-structure) in wells of a microtiter plate, and to couple an easily detected indicator to the other component. An inhibitor of the interaction is identified by the absence of the detectable indicator from the well. In addition, ULBP polypeptides according to the invention are useful for the structure- based design of an ULBP inhibitor. Such a design would comprise the steps of determining the three-dimensional structure of the ULBP polypeptide, analyzing the three-dimensional structure for the likely binding sites of substrates, synthesizing a molecule that incoφorates a predictive reactive site, and determining the inhibiting activity of the molecule. ULBP DNA, ULBP polypeptides, and antibodies against ULBP polypeptides can be used as reagents in a variety of research protocols. A sample of such research protocols are given in Sambrook et al. Molecular Cloning: A Laboratory Manual, 2 ed. Vol. 1-3, Cold Spring Harbor Laboratory Press, (1989). For example, these reagents can serve as markers for cell specific or tissue specific expression of RNA or proteins. Similarly, these reagents can be used to investigate constituitive and transient expression of ULBP RNA or polypeptides. ULBP DNA can be used to determine the chromosomal location of ULBP DNA and to map genes in relation to this chromosomal location. ULBP DNA can also be used to examine genetic heterogeneity and heredity, through the use of techniques such as genetic fingeφrinting, as well as to identify risks associated with genetic disorders. ULBP DNA can be further used to identify additional genes related to ULBP DNA and to establish evolutionary trees based on the comparison of sequences. ULBP DNA and polypeptides can be used to select for those genes or proteins that are homologous to ULBP DNA or polypeptides through positive screening procedures, such as Southern blotting and immunoblotting, and through negative screening procedures, such as subtraction. Molecular Weight, Isoelectric Point Markers The polypeptides of the present invention can be subjected to fragmentation into smaller peptides by chemical and enzymatic means, and the peptide fragments so produced can be used in the analysis of other proteins or polypeptides. For example, such peptide fragments can be used as peptide molecular weight markers, peptide isoelectric point markers, or in the analysis of the degree of peptide fragmentation. Thus, the invention also includes these polypeptides and peptide fragments, as well as kits to aid in the determination of the apparent molecular weight and isoelectric point of an unknown protein and kits to assess the degree of fragmentation of an unknown protein. Although all methods of fragmentation are encompassed by the invention, chemical fragmentation is a preferred embodiment, and includes the use of cyanogen bromide to cleave under neutral or acidic conditions such that specific cleavage occurs at methionine residues (E. Gross, Methods in Enz. X X :238-255, 1967). This can further include additional steps, such as a carboxymethylation step to convert cysteine residues to an unreactive species. Enzymatic fragmentation is another preferred embodiment, and includes the use of a protease such as Asparaginylendo-peptidase, Arginylendo-peptidase, Achromobacter protease I, Trypsin, Staphlococcus aureus V8 protease, Endoproteinase Asp-N, or Endoproteinase Lys-C under conventional conditions to result in cleavage at specific amino acid residues. Asparaginylendo-peptidase can cleave specifically on the carboxyl side of the asparagine residues present within the polypeptides of the invention. Arginylendo-peptidase can cleave specifically on the carboxyl side of the arginine residues present within these polypeptides. Achromobacter protease I can cleave specifically on the carboxyl side of the lysine residues present within the polypeptides (Sakiyama and Nakat, U.S. Patent No. 5,248,599; T. Masaki et al., Biochim. Biophys. Ada 660:44-50, 1981 ; T. Masaki et al., Biochim. Biophys. Ada (560:51 - SUBSTTTUTE SHEET RULE 26) 55, 1981). Trypsin can cleave specifically on the carboxyl side of the arginine and lysine residues present within polypeptides of the invention. Enzymatic fragmentation may also occur with a protease that cleaves at multiple amino acid residues. For example, Staphlococcus aureus V8 protease can cleave specifically on the carboxyl side of the aspartic and glutamic acid residues present within polypeptides (D. W. Cleveland, J. Biol. Chem. 3: 1102-1106, 1977). Endoproteinase Asp-N can cleave specifically on the amino side of the asparagine residues present within polypeptides. Endoproteinase Lys-C can cleave specifically on the carboxyl side of the lysine residues present within polypeptides of the invention. Other enzymatic and chemical treatments can likewise be used to specifically fragment these polypeptides into a unique set of specific peptides. Of course, the peptides and fragments of the polypeptides of the invention can also be produced by conventional recombinant processes and synthetic processes well known in the art. With regard to recombinant processes, the polypeptides and peptide fragments encompassed by invention can have variable molecular weights, depending upon the host cell in which they are expressed. Glycosylation of polypeptides and peptide fragments of the invention in various cell types can result in variations of the molecular weight of these pieces, depending upon the extent of modification. The size of these pieces can be most heterogeneous with fragments of polypeptide derived from the extracellular portion of the polypeptide. Consistent polypeptides and peptide fragments can be obtained by using polypeptides derived entirely from the transmembrane and cytoplasmic regions, pretreating with N-glycanase to remove glycosylation, or expressing the polypeptides in bacterial hosts. The molecular weight of these polypeptides can also be varied by fusing additional peptide sequences to both the amino and carboxyl terminal ends of polypeptides of the invention. Fusions of additional peptide sequences at the amino and carboxyl terminal ends of polypeptides of the invention can be used to enhance expression of these polypeptides or aid in the purification of the protein. In addition, fusions of additional peptide sequences at the amino and carboxyl terminal ends of polypeptides of the invention will alter some, but usually not all, of the fragmented peptides of the polypeptides generated by enzymatic or chemical treatment. Of course, mutations can be introduced into polypeptides of the invention using routine and known techniques of molecular biology. For example, a mutation can be designed so as to eliminate a site of proteolytic cleavage by a specific enzyme or a site of cleavage by a specific chemically induced fragmentation procedure. The elimination of the site will alter the peptide fingeφrint of polypeptides of the invention upon fragmentation with the specific enzyme or chemical procedure. The polypeptides and the resultant fragmented peptides can be analyzed by methods including sedimentation, electrophoresis, chromatography, and mass spectrometry to determine their molecular weights. Because the unique amino acid sequence of each piece specifies a molecular weight, these pieces can thereafter serve as molecular weight markers using such analysis techniques to assist in the determination of the molecular weight of an unknown protein, polypeptides or fragments thereof. The molecular weight markers of the invention serve particularly well as molecular weight markers for the estimation of the apparent molecular weight of proteins that have similar apparent molecular weights and, consequently, allow increased accuracy in the determination of apparent molecular weight of proteins. When the invention relates to the use of ULBP polypeptide fragments and fragmented peptide molecular weight markers, those markers are preferably at least 10 amino acids in size. More preferably, these fragmented peptide molecular weight markers are between 10 and 100 amino acids in size. Even more preferable are fragmented peptide molecular weight markers between 10 and 50 amino acids in size and especially between 10 and 35 amino acids in size. Most preferable are fragmented peptide molecular weight markers between 10 and 20 amino acids in size. Among the methods for determining molecular weight are sedimentation, gel electrophoresis, chromatography, and mass spectrometry. A particularly preferred embodiment is denaturing polyacrylamide gel electrophoresis (U. K. Laemmli, Nature 227:680-685, 1970). Conventionally, the method uses two separate lanes of a gel containing sodium dodecyl sulfate and a concentration of acrylamide between 6-20%. The ability to simultaneously resolve the marker and the sample under identical conditions allows for increased accuracy. It is understood, of course, that many different techniques can be used for the determination of the molecular weight of an unknown protein using polypeptides of the invention, and that this embodiment in no way limits the scope of the invention. Each unglycosylated polypeptide or fragment thereof has a pi that is intrinsically determined by its unique amino acid sequence (which pi can be estimated by the skilled artisan using any of the computer programs designed to predict pi values currently available, calculated using any well-known amino acid pKa table, or measured empirically). Therefore these polypeptides and fragments thereof can serve as specific markers to assist in the determination of the isoelectric point of an unknown protein, polypeptide, or fragmented peptide using techniques such as isoelectric focusing. These polypeptide or fragmented peptide markers serve particularly well for the estimation of apparent isoelectric points of unknown proteins that have apparent isoelectric points close to that of the polypeptide or fragmented peptide markers of the invention. The technique of isoelectric focusing can be further combined with other techniques such as gel electrophoresis to simultaneously separate a protein on the basis of molecular weight and charge. The ability to simultaneously resolve these polypeptide or fragmented peptide markers and the unknown protein under identical conditions allows for increased accuracy in the determination of the apparent isoelectric point of the unknown protein. This is of particular interest in techniques, such as two dimensional electrophoresis (T.D. Brock and M.T. Madigan, Biology of Microorganisms 76-77 (Prentice Hall, 6d ed. 1991)), where the nature of the procedure dictates that any markers should be resolved simultaneously with the unknown protein. In addition, with such methods, these polypeptides and fragmented peptides thereof can assist in the determination of both the isoelectric point and molecular weight of an unknown protein or fragmented peptide. Polypeptides and fragmented peptides can be visualized using two different methods that allow a discrimination between the unknown protein and the molecular weight markers. In one embodiment, the polypeptide and fragmented peptide molecular weight markers of the invention can be visualized using antibodies generated against these markers and conventional immunoblotting techniques. This detection is performed under conventional conditions that do not result in the detection of the unknown protein. It is understood that it may not be possible to generate antibodies against all polypeptide fragments of the invention, since small peptides may not contain immunogenic epitopes. It is further understood that not all antibodies will work in this assay; however, those antibodies which are able to bind polypeptides and fragments of the invention can be readily determined using conventional techniques. The unknown protein is also visualized by using a conventional staining procedure. The molar excess of unknown protein to polypeptide or fragmented peptide molecular weight markers of the invention is such that the conventional staining procedure predominantly detects the unknown protein. The level of these polypeptide or fragmented peptide molecular weight markers is such as to allow little or no detection of these markers by the conventional staining method. The preferred molar excess of unknown protein to polypeptide molecular weight markers of the invention is between 2 and 100,000 fold. More preferably, the preferred molar excess of unknown protein to these polypeptide molecular weight markers is between 10 and 10,000 fold and especially between 100 and 1,000 fold. It is understood of course that many techniques can be used for the determination and detection of molecular weight and isoelectric point of an unknown protein, polypeptides, and fragmented peptides thereof using these polypeptide molecular weight markers and peptide fragments thereof and that these embodiments in no way limit the scope of the invention. In another embodiment, the analysis of the progressive fragmentation of the polypeptides of the invention into specific peptides (D. W. Cleveland et al., J. Biol. Chem. 252:1102-1106, 1977), such as by altering the time or temperature of the fragmentation reaction, can be used as a control for the extent of cleavage of an unknown protein. For example, cleavage of the same amount of polypeptide and unknown protein under identical conditions can allow for a direct comparison of the extent of fragmentation. Conditions that result in the complete fragmentation of the polypeptide can also result in complete fragmentation of the unknown protein. As to the specific use of the polypeptides and fragmented peptides of the invention as molecular weight markers, the fragmentation of the polypeptide of SEQ ID NO:2, SEQ ID NO:4, or SEQ ID NO: 10 with cyanogen bromide generates a unique set of fragmented peptide molecular weight markers with unique molecular weights. The distribution of methionine residues determines the number of amino acids in each peptide and the unique amino acid composition of each peptide determines its molecular weight. In addition, the preferred purified polypeptide of the invention has a calculated molecular weight of approximately 31,000 Daltons in the absence of glycosylation. Where an intact protein is used, the use of these polypeptide molecular weight markers allows increased accuracy in the determination of apparent molecular weight of proteins that have apparent molecular weights close to 31 ,000 Daltons. Where fragments are used, there is increased accuracy in determining molecular weight over the range of the molecular weights of the fragment. Finally, as to the kits that are encompassed by the invention, the constituents of such kits can be varied, but typically contain the polypeptide and fragmented peptide molecular weight markers. Also, such kits can contain the polypeptides wherein a site necessary for fragmentation has been removed. Furthermore, the kits can contain reagents for the specific cleavage of the polypeptide and the unknown protein by chemical or enzymatic cleavage. Kits can further contain antibodies directed against polypeptides or fragments thereof of the invention. Identification of Unknown Proteins As set forth above, a polypeptide or peptide fingeφrint can be entered into or compared to a database of known proteins to assist in the identification of the unknown protein using mass spectrometry (W.J. Henzel et al., Proc. Natl. Acad. Sci. USA 90:5011-5015, 1993; D. Fenyo et al., Electrophoresis 19:998-1005, 1998). A variety of computer software programs to facilitate these comparisons are accessible via the Internet, such as Protein Prospector (Internet site: prospector.uscf.edu), Multildent (Internet site: www.expasy.ch/sprot/multiident.html), PeptideSearch (Internet site:www.mann.embl-heiedelberg.de...deSearch/FR_PeptideSearch Form.html), and ProFound (Internet site:www. chait-sgi.rockefeller.edu/cgi-bin/ prot-id-frag.html). These programs allow the user to specify the cleavage agent and the molecular weights of the fragmented peptides within a designated tolerance. The programs compare observed molecular weights to predicted peptide molecular weights derived from sequence databases to assist in determining the identity of the unknown protein. In addition, a polypeptide or peptide digest can be sequenced using tandem mass spectrometry (MS/MS) and the resulting sequence searched against databases (J.K. Eng, et al., J. Am. Soc. Mass Spec. 5:976-989 (1994); M. Mann and M. Wilm, Anal. Chem. 66:4390-4399 (1994); J.A. Taylor and R.S. Johnson, Rapid Comm. Mass Sped 1 :1067-1075 (1997)). Searching programs that can be used in this process exist on the Internet, such as Lutefisk 97 (Internet site: www.lsbc.com:70/Lutefisk97.html), and the Protein Prospector, Peptide Search and ProFound programs described above. Therefore, adding the sequence of a gene and its predicted protein sequence and peptide fragments to a sequence database can aid in the identification of unknown proteins using mass spectrometry. Antibodies Antibodies that are immunoreactive with the polypeptides of the invention are provided herein. Such antibodies specifically bind to the polypeptides via the antigen-binding sites of the antibody (as opposed to non-specific binding). Thus, the polypeptides, fragments, variants, fusion proteins, etc., as set forth above may be employed as immunogens in producing antibodies immunoreactive therewith. In another aspect of the invention, ULBP and peptides based on the amino acid sequence of ULBP, can be utilized to prepare antibodies that specifically bind to ULBP. Specific examples of such antibody preparations are described in Examples 3 and 4 herein. The term "antibodies" is meant to include polyclonal antibodies, monoclonal antibodies, fragments thereof, such as F(ab')2 and Fab fragments, as well as any recombinantly produced binding partners. Antibodies are defined to be specifically binding if they bind ULBP polypeptide with a K, of greater than or equal to about 10 7 M 1 . Affinities of binding partners or antibodies can be readily determined using conventional techniques, for example those described by Scatchard et al., Ann. N. Y Acad. Sci., 57:660 (1949). Polyclonal antibodies can be readily generated from a variety of sources, for example, horses, cows, goats, sheep, dogs, chickens, rabbits, mice, or rats, using procedures that are well known in the art. In general, purified ULBP or a peptide based on the amino acid sequence of ULBP polypeptide that is appropriately conjugated is administered to the host animal typically through parenteral injection. The immunogenicity of ULBP polypeptide can be enhanced through the use of an adjuvant, for example, Freund's complete or incomplete adjuvant. Following booster immunizations, small samples of serum are collected and tested for reactivity to ULBP polypeptide. Examples of various assays useful for such determination include those described in Antibodies: A Laboratory Manual, Harlow and Lane (eds.), Cold Spring Harbor Laboratory Press, 1988; as well as procedures, such as countercurrent immuno- electrophoresis (CIEP), radioimmunoassay, radio-immunoprecipitation, enzyme-linked immunosorbent assays (ELISA), dot blot assays, and sandwich assays. See U.S. Patent Nos. 4,376,110 and 4,486,530. Monoclonal antibodies can be readily prepared using well known procedures. See, for example, the procedures described in U.S. Patent Nos. RE 32,011, 4,902,614, 4,543,439, and 4,411,993; Monoclonal Antibodies, Hybridomas: A New Dimension in Biological Analyses, Plenum Press, Kennett, McKearn, and Bechtol (eds.), 1980. Briefly, the host animals, such as mice, are injected intraperitoneally at least once and preferably at least twice at about 3 week intervals with isolated and purified ULBP or conjugated ULBP peptide, optionally in the presence of adjuvant. Mouse sera are then assayed by conventional dot blot technique or antibody capture (ABC) to determine which animal is best to fuse. Approximately two to three weeks later, the mice are given an intravenous boost of ULBP or conjugated ULBP peptide. Mice are later sacrificed and spleen cells fused with commercially available myeloma cells, such as Ag8.653 (ATCC), following established protocols. Briefly, the myeloma cells are washed several times in media and fused to mouse spleen cells at a ratio of about three spleen cells to one myeloma cell. The fusing agent can be any suitable agent used in the art, for example, polyethylene glycol (PEG). Fusion is plated out into plates containing media that allows for the selective growth of the fused cells. The fused cells can then be allowed to grow for approximately eight days. Supematants from resultant hybridomas are collected and added to a plate that is first coated with goat anti-mouse Ig. Following washes, a label, such as 125 I- ULBP, is added to each well followed by incubation. Positive wells can be subsequently detected by autoradiography. Positive clones can be grown in bulk culture and supematants are subsequently purified over a Protein A column (Pharmacia). The monoclonal antibodies of the invention can be produced using alternative techniques, such as those described by Alting-Mees et al., "Monoclonal Antibody Expression Libraries: A Rapid Alternative to Hybridomas", Strategies in Molecular Biology 5: 1-9 (1990), which is incoφorated herein by reference. Similarly, binding partners can be constructed using recombinant DNA techniques to incoφorate the variable regions of a gene that encodes a specific binding antibody. Such a technique is described in Larrick et al., Biotechnology, 7:394 (1989). Antigen-binding fragments of such antibodies, which may be produced by conventional techniques, are also encompassed by the present invention. Examples of such fragments include, but are not limited to, Fab and F(ab') 2 fragments. Antibody fragments and derivatives produced by genetic engineering techniques are also provided. The monoclonal antibodies of the present invention include chimeric antibodies, e.g., humanized versions of murine monoclonal antibodies. Such humanized antibodies may be prepared by known techniques, and offer the advantage of reduced immunogenicity when the antibodies are administered to humans. In one embodiment, a humanized monoclonal antibody comprises the variable region of a murine antibody (or just the antigen binding site thereof) and a constant region derived from a human antibody. Alternatively, a humanized antibody fragment may comprise the antigen binding site of a murine monoclonal antibody and a variable region fragment (lacking the antigen-binding site) derived from a human antibody. Procedures for the production of chimeric and further engineered monoclonal antibodies include those described in Riechmann et al. (Nature 332:323, 1988), Liu et al. (PNAS 84:3439, 1987), Larrick et al. (Bio/Technology 7:934, 1989), and Winter and Harris (TIPS 14Λ39, May, 1993). Procedures to generate antibodies transgenically can be found in GB 2,272,440, US Patent Nos. 5,569,825 and 5,545,806 and related patents claiming priority therefrom, all of which are incoφorated by reference herein. In one embodiment, the antibodies are specific for the polypeptides of the present invention, and do not cross-react with other proteins. Screening procedures by which such antibodies may be identified are well known, and may involve immunoaffinity chromatography, for example. Hybridoma cell lines that produce monoclonal antibodies specific for the polypeptides of the invention are also contemplated herein. Such hybridomas may be produced and identified by conventional techniques. One method for producing such a hybridoma cell line comprises immunizing an animal with a polypeptide; harvesting spleen cells from the immunized animal; fusing said spleen cells to a myeloma cell line, thereby generating hybridoma cells; and identifying a hybridoma cell line that produces a monoclonal antibody that binds the polypeptide. The monoclonal antibodies may be recovered by conventional techniques. Antibodies that are immunoreactive with the polypeptides of the invention are provided herein. Such antibodies specifically bind to the polypeptides via the antigen-binding sites of the antibody (as opposed to non-specific binding). Thus, the polypeptides, fragments, variants, fusion proteins, etc., as set forth above may be employed as "immunogens" in producing antibodies immunoreactive therewith. More specifically, the polypeptides, fragment, variants, fusion proteins, etc. contain antigenic determinants or epitopes that elicit the formation of antibodies. These antigenic determinants or epitopes can be either linear or conformational (discontinuous). Linear epitopes are composed of a single section of amino acids of the polypeptide, while conformational or discontinuous epitopes are composed of amino acids sections from different regions of the polypeptide chain that are brought into close proximity upon protein folding (C. A. Janeway, Jr. and P. Travers, Immuno Biology 3:9 (Garland Publishing Inc., 2nd ed. 1996)). Because folded proteins have complex surfaces, the number of epitopes available is quite numerous; however, due to the conformation of the protein and steric hinderances, the number of antibodies that actually bind to the epitopes is less than the number of available epitopes (C. A. Janeway, Jr. and P. Travers, Immuno Biology 2:14 (Garland Publishing Inc., 2nd ed. 1996)). Epitopes may be identified by any of the methods known in the art. Thus, one aspect of the present invention relates to the antigenic epitopes of the polypeptides of the invention. Such epitopes are useful for raising antibodies, in particular monoclonal antibodies, as described in more detail below. Additionally, epitopes from the polypeptides of the invention can be used as research reagents, in assays, and to purify specific binding antibodies from substances such as polyclonal sera or supematants from cultured hybridomas. Such epitopes or variants thereof can be produced using techniques well known in the art such as solid-phase synthesis, chemical or enzymatic cleavage of a polypeptide, or using recombinant DNA technology. As to the antibodies that can be elicited by the epitopes of the polypeptides of the invention, whether the epitopes have been isolated or remain part of the polypeptides, both polyclonal and monoclonal antibodies may be prepared by conventional techniques. See, for example, Monoclonal Antibodies, Hybridomas: A New Dimension in Biological Analyses, Kennet et al. (eds.), Plenum Press, New York (1980); and Antibodies: A Laboratory Manual, Harlow and Land (eds.), Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, (1988). Uses Thereof The antibodies of the invention can be used in assays to detect the presence of the polypeptides or fragments of the invention, either in vitro or in vivo. The antibodies also may be employed in purifying polypeptides or fragments of the invention by immunoaffinity chromatography. Those antibodies that additionally can block binding of the polypeptides of the invention to ULBP counter-structure molecules may be used to inhibit a biological activity that results from such binding. Such blocking antibodies may be identified using any suitable assay procedure, such as by testing antibodies for the ability to inhibit binding of ULBP polypeptides to certain cells expressing ULBP counter-structure molecules. Examples of such cells are NK cells and cytotoxic T lymphocytes. Alternatively, blocking antibodies may be identified in assays for the ability to inhibit a biological effect that results from binding of ULBP counter- structure molecules to target cells. Antibodies may be assayed for the ability to inhibit ULBP counter-structure molecules-mediated lysis of cells, for example. Such an antibody may be employed in an in vitro procedure, or administered in vivo to inhibit a biological activity mediated by the entity that generated the antibody. Disorders caused or exacerbated (directly or indirectly) by the interaction of ULBP counter-structure molecules with cell surface (binding partner) receptor thus may be treated. A therapeutic method involves in vivo administration of a blocking antibody to a mammal in an amount effective in inhibiting a ULBP counter-structure molecule-mediated biological activity. Monoclonal antibodies are generally preferred for use in such therapeutic methods. In one embodiment, an antigen-binding antibody fragment is employed. Antibodies may be screened for agonistic (i.e., ligand-mimicking) properties. Such antibodies, upon binding to cell surface ULBP polypeptides, induce biological effects (e.g., transduction of biological signals) similar to the biological effects induced when ULBP counter-structure molecules binds to cell surface ULBP polypeptides. Compositions comprising an antibody that is directed against ULBP polypeptides, and a physiologically acceptable diluent, excipient, or carrier, are provided herein. Suitable components of such compositions are as described above for compositions containing ULBP polypeptides. Also provided herein are conjugates comprising a detectable (e.g., diagnostic) or therapeutic agent, attached to the antibody. Examples of such agents are presented above. The conjugates find use in in vitro or in vivo procedures. The references cited herein are incoφorated by reference herein in their entirety. The embodiments within the specification and the following Examples provide an illustration of embodiments of the invention and should not be construed to limit the scope of the invention. The skilled artisan recognizes many other embodiments are encompassed by the claimed invention. In the following Examples, all methods described are conventional unless otherwise specified. EXAMPLE 1 Recombinant Expression of Human ULBP-1 A human ULBPl-Fc construct was generated in the following manner. ULBP-1 DNA encoding the extracellular domain of human ULBP-1 polypeptide(residues 1-218) was generated by PCR using an upstream primer containing an Xhol site (5*-GCAACTCGAGAGCTCCAGGTCTACAATGGCAG-3') and a downstream primer containing a Bglll site (5'-GATGAGATCTGGGTTGGGTTGTGCCTGGGGCCAG-3'). The human ULBP-1 DNA fragment was cloned in frame into the pDC409 vector containing a 5' Sail site and the human immunoglobulin Fc region mutein (described in EMBO J. 13:3992, 1994) preceded by a Bglll site. The human ULBPl-Fc fusion polypeptide was produced and purified as described in Fanslow et al., J. Immunol. 149:655, 1992, which is incoφorated by reference. The human ULBP1-LZ fusion polypeptide was produced using the expression vectors, transfection, and cell culture procedures used for the ULBPl-Fc polypeptide. For purification, the poly His tag is used to bind the recombinant protein to Nickel-NTA resin (manufactured by Qiagen, http://www.qiagen.com/) according to the manufacturer's instructions. The resin is washed with 30 column volumes of 20 mM NaPO4 pH 7.4 + 300 mM NaCl + 5 mM Imidazole. The recombinant protein is then eluted using increasing concentrations of Imidazole. Initially a gradient of 5-20mM Imidazole 20 mM NaPO4 pH 7.4 + 300 mM NaCl is used, followed by 20mM Imidazole 20 mM NaPO4 pH 7.4 + 300 mM NaCl, followed by a gradient of 20-lOOmM Imidazole 20 mM NaPO4 pH 7.4 + 300 mM NaCl, followed by lOOmM Imidazole 20 mM NaPO4 pH 7.4 + 300 mM NaCl, followed by 500mM Imidazole 20 mM NaPO4 pH 7.4 + 300 mM NaCl. Fractions are collected and analysed by SDS-PAGE to identify those containing the recombinant protein. EXAMPLE 2 Recombinant Expression of Human ULBP-2 A human ULBP2-Fc construct was generated in the following manner. ULBP2-DNA encoding the extracellular domain of human ULBP-2 polypeptide(residues 1-223) was generated by PCR using an upstream primer containing an Xlwl site (5'-GATTCTCGAGTCCTTAATGGCAGCAGCC-3') and a downstream primer containing a BamΗI site (5'-ACAAGGATCCCCTGAGTTGGGTTGTGCC-3'). The human ULBP-1 DNA fragment was cloned in frame into the pDC409 vector containing a 5' Sail site and the human immunoglobulin Fc region mutein (described in EMBO J. 13:3992, 1994) preceded by a BgH site. The human ULBP2-Fc fusion polypeptide was produced and purified as described in Fanslow et al., J. Immunol. 149:655, 1992, which is incoφorated by reference. The human ULBP2-LZ fusion polypeptide was produced and purified as described above for the ULBP1-LZ fusion polypeptide. EXAMPLE 3 Preparation of Antibodies Against ULBP-1 Monoclonal antibodies were generated against ULBP-1. Balb/c mice were immunized with 10 μg of ULBPl-Fc (1 mg/ml stock, batch #1) in Titermax adjuvant (CytRx Coφ., Norcross, GA). The animals were boosted 2 weeks later with 10 μg of the same protein in Freunds Complete Adjuvant (Sigma). Twelve weeks after second immunization, one mouse was boosted intravenously with 10 g of ULBPl-Fc (Batch #3) in saline. Four days later, the mouse was sacrificed and spleen and lymph nodes were fused with NS1 myelomas with 50% PEG/DMSO solution. The fused cells were placed into 96 well plates (Costar) with HAT (Sigma) selective media. Hybridoma supematants were screened for antibody production by antibody capture assay (ABC). Briefly, 96 well plates (Nunc) were coated overnight with goat- anti-mouse IgG Fc specific (Pierce), then washed 4 times with PBS. Supematants were incubated 1 hour at room temperature then washed 5 times with PBS. ULBPl-Fc was added to each well and incubated 1 hour at room temperature followed by 5 washes of PBS. Goat-anti- human Ig Fc fragment specific (Jackson) was added and incubated for 1 hour followed by 5 washes with PBS. The plates were developed with TMB substrate (KPL) and read on a plate reader at 650 nm. Positive wells were subsequently screened against irrelevant Fc protein to remove antibodies reactive to human Fc. They were also screened by flow cytometry for binding to ULBP-1 transfected CV-1 cells. Positive hybridomas were cloned by titration and limiting dilution methods and grown in bulk culture. Supematants were purified over a Protein A column (Bio-Rad) and stored at -20 °C. EXAMPLE 4 Preparation of Antibodies Against ULBP-2 Monoclonal antibodies were generated against ULBP-2. Balb/c mice were immunized with 10 μg of ULBP2-Fc in Titermax adjuvant (CytRX Coφ., Norcross, GA). The animals were boosted 2 weeks later with 10 μg of the same protein in Freunds Complete Adjuvant (Sigma). Eight weeks after second immunization, one mouse was boosted intravenously with 6 μg of ULBP2-Fc in saline. Four days later the mouse was sacrificed and spleen and lymph nodes were fused with NS1 myelomas with 50% PEG/DMSO solution. The fused cells were plated into 96 well plates (Costar) with HAT (Sigma) selective media. Hybridoma supematants were screened for antibody production by antibody capture assay (ABC). Briefly, 96 well plates (Nunc) were coated overnight with goat-anti-mouse IgG Fc specific (Pierce), then washed 4 times with PBS. Supematants were incubated 1 hour at room temperature then washed 5 times with PBS. ULBP2-Fc was added to each well and incubated 1 hour at room temperature followed by 5 washes of PBS. Goat-anti-human Ig Fc fragment specific (Jackson) was added and incubated for 1 hour followed by 5 washed with PBS. The plates were developed with TMB substrate (KPL) and read on a plate reader at 650 nm. Positive wells were subsequently screened against irrelevant Fc protein to remove antibodies reactive to human Fc. They were also screened by flow cytometry for binding to Con-A stimulated T cells and by immunoprecipitation of ULBP-2 transfected CV-1 cells. Positive hybridomas were cloned by titration and limiting dilution methods and grown in bulk culture. Supematants were purified over a Protein A column (Bio-Rad) and stored at -20°C. Purified antibody was isotyped by ELISA capture assay (Mouse MonoAb-ID Kit, Zymed). EXAMPLE 5 ULBP-mediated Enhancement of IFN-γ production and NK cell proliferation NK cells were generated after short term culture of peripheral blood lymphocytes as previously described (B. Perussia et al., Nat. Immun. Cell Growth Rgulat. 6: 171-188, 1987). Further enrichment was obtained by negative selection with employment of the the magnetic cell separator and magnetic beads (Miltenyi Biotec, Auburn, CA), according to the manufacturer's protocol. This procedure resulted in a population of >95% CD3-. CD16+, CD56+ NK cells as assessed by FACS analysis performed after staining of the cells with the respective PE-conjugated monoclonal antibodies (Pharmingen). Purified NK cells were stimulated with 50ng/ml of IL-15 (Immunex Coφ.) for 20 hours in RPMI1640 medium containing 10%FCS, supplemented with antibiotics and glutamine, in a 37°C humidified incubator with 5% CO 2 at a concentration of 2xl0 6 cells/ml. After three washes with PBS, cells were cultured in RPMI1640 medium containing 10%FCS, supplemented with antibiotics and glutamine, in triplicate in a 96-well plate, at 100,000 cell well in 250μl volume, in the presence of rIL-12 (R&D Systems Inc., Minneapolis, MN) at a concentration of lng/ml, alone or with the concentration of ULBPl -LZ or ULBP2-LZ indicated in Figures 9 and 10. After 48 hours of culture, cell- free supematants were analyzed for IFN-γ concentration using an IFN-γ specific ELISA, with employment of B133.1 monoclonal and P3 polyclonal anti-human IFN-γ antibiodies. Results are shown in Figure 9. To evaluate cell proliferation, after 48 hours of culture, NK cells were pulsed for the last 16 hours with 30μl of medium containing 40μCi/ml of [methyl- 3 H]Thymidine, harvested, and counted in a beta counter (Packard). Results are shown in Figure 10. EXAMPLE 6 ULBP-mediated Enhancement of CTL Activity Peripheral blood mononuclear cells (PBMC) were isolated from heparinized blood by centrifugation over Ficoll-Hypaque and washed three times in culture medium (RPMI 1640 supplemented with 10% heat-inactivated fetal bovine serum, 50U/ml penicillin, 50μg/ml streptomycin and 2mM glutamine). One million irradiated (3500 rads.) effector cells were cocultured with one million responder cells and ULBP1LZ or ULBP2LZ proteins at the concentrations indicated in Figures 1 1 and 12. Cultures were conducted in a total volume of 2mls culture medium at 37°C in a humidified atmosphere of 5% CO 2 for 7 days. At this time, cultured cells were washed and put into a 4hr chromium-release assay to assess cytolytic activity against 51 Cr-labeled day 3 PHA blasts as described previously (M. R.Alderson, H. M. Sassenfeld and M. B. Widmer. J. Exp. Med. 172:577-587, 1990). Results are shown in Figures 1 1 and 12, and are expressed as % specific 51 Cr release from wells containing serially-diluted responder cells and a fixed number of labeled target cells.

Claims: What is claimed is: 1. An isolated nucleic acid molecule comprising the DNA sequence of SEQ ID NO: 1 , SEQ ID NO:3, or SEQ ID NO:9. 2. An isolated nucleic acid molecule encoding an amino acid sequence comprising the sequence of SEQ ID NO:2, SEQ ID NO:4, or SEQ ID NO: 10. 3. An isolated nucleic acid molecule that hybridizes to either strand of a denatured, double-stranded DNA comprising the nucleic acid sequence of any one of claims 1 or 2 under conditions of moderate stringency in 50% formamide and 6XSSC, at 42°C with washing conditions of 60 °C, 0.5XSSC, 0.1% SDS. 4. The isolated nucleic acid molecule as claimed in claim 3, wherein said isolated nucleic acid molecule is derived by in vitro mutagenesis from SEQ ID NO:l, SEQ ID NO:3, or SEQ ID NO:9. 5. An isolated nucleic acid molecule degenerate from SEQ ID NO:l, SEQ ID NO:3, or SEQ ID NO:9 as a result of the genetic code. 6. An isolated nucleic acid molecule, which is human ULBP-1 , ULBP-2, or ULBP-3 DNA, an allelic variant of human ULBP-1 , ULBP-2, or ULBP-3 DNA, or a species homolog of ULBP-1 , ULBP-2, or ULBP-3 DNA. 7. A recombinant vector that directs the expression of a nucleic acid molecule selected from the group consisting of the nucleic acid molecules of claims 1, 2, 5, and 6. 8. A recombinant vector that directs the expression of a nucleic acid molecule of claim 3. 9. A recombinant vector that directs the expression of a nucleic acid molecule of claim 10. An isolated polypeptide encoded by a nucleic acid molecule selected from the group consisting of the nucleic acid molecules of claims 1, 2, 5, and 6. 11. An isolated polypeptide according to claim 10 having a molecular weight of approximately 31 kD as determined by SDS-PAGE. 12. An isolated polypeptide according to claim 10 in non-glycosylated form. 13. An isolated polypeptide encoded by a nucleic acid molecule of claim 3. 14. An isolated polypeptide according to claim 13 in non-glycosylated form. 15. An isolated polypeptide encoded by a nucleic acid molecule of claim 4. 16. An isolated polypeptide according to claim 15 in non-glycosylated form. 17. Isolated antibodies that bind to a polypeptide of claim 10. 18. Isolated antibodies according to claim 17, wherein the antibodies are monoclonal antibodies. 19. Isolated antibodies that bind to a polypeptide of claim 13. 20. Isolated antibodies according to claim 19, wherein the antibodies are monoclonal antibodies. 21. Isolated antibodies that bind to a polypeptide of claim 15. 22. Isolated antibodies according to claim 21, wherein the antibodies are monoclonal antibodies. 23. A host cell transfected or transduced with the vector of claim 7. 24. A method for the production of ULBP- 1 , ULBP-2, or ULBP-3 polypeptide comprising culturing a host cell of claim 23 under conditions promoting expression, and recovering the polypeptide from the culture medium. 25. The method of claim 24, wherein the host cell is selected from the group consisting of bacterial cells, yeast cells, plant cells, and animal cells. 26. A host cell transfected or transduced with the vector of claim 8. 27. A method for the production of ULBP-1, ULBP-2, or ULBP-3 polypeptide comprising culturing a host cell of claim 26 under conditions promoting expression, and recovering the polypeptide from the culture medium. 28. The method of claim 27, wherein the host cell is selected from the group consisting of bacterial cells, yeast cells, plant cells, and animal cells. 29. A host cell transfected or transduced with the vector of claim 9. 30. A method for the production of ULBP-1, ULBP-2, or ULBP-3 polypeptide comprising culturing a host cell of claim 29 under conditions promoting expression, and recovering the polypeptide from the culture medium. 31. The method of claim 30, wherein the host cell is selected from the group consisting of bacterial cells, yeast cells, plant cells, and animal cells. 32. A method of detecting lymphoma cells comprising: (A) providing a biological sample; (B) incubating said sample with antibodies that specifically bind to ULBP-1, ULBP-2, or ULBP-3; and (C) detecting the complexes formed. 33. A method of killing lymphoma cells comprising: (A) providing lymphoma cells; (B) providing a ULBP polypeptide-toxin conjugate; and (C) incubating said cells with said conjugate under conditions to kill said cells with said toxin. 34. A method of increasing IFN-γ production in vivo comprising: (A) providing natural killer cells; (B) providing a ULBP polypeptide; (C) incubating said natural killer cells with said ULBP polypeptide. 35. A method of increasing natural killer cell proliferation in vivo comprising: (A) providing natural killer cells; (B) providing a ULBP polypeptide; (C) incubating said natural killer cells with said ULBP polypeptide. 36. A method of directly or indirectly increasing cytotoxic T lymphocyte activity in vivo comprising: (A) providing cytotoxic T lymphocytes; (B) providing a ULBP polypeptide; (C) incubating said cytotoxic T lymphocytes with said ULBP polypeptide. 37. A ULBP polypeptide comprising a fragment of SEQ ID NO:2, SEQ ID NO:4, or SEQ ID NO: 10, wherein said fragment is capable of binding to UL-16. 38. A ULBP polypeptide comprising a fragment of SEQ ID NO:2, SEQ ID NO:4, or SEQ ID NO: 10, wherein said fragment lacks the GPI linkage.

Inventors: COSMAN DAVID JOHN, FANSLOW WILLIAM CRISTIAN, MULLBERG JURGEN H, COSMAN, DAVID, JOHN, FANSLOW, WILLIAM, CRISTIAN, MULLBERG, JURGEN, H.
Applicants: COSMAN DAVID JOHN, FANSLOW WILLIAM CRISTIAN, IMMUNEX CORP, MULLBERG JURGEN H, COSMAN, DAVID, JOHN, FANSLOW, WILLIAM, CRISTIAN, IMMUNEX CORPORATION, MULLBERG, JURGEN, H.
Application year: 1998
Main classification: C12N15
IPC codes: A61K38, C07K14, C07K16, C12N1, C12N5, C12N15, C12P21, C12Q1