MULTIDRUG RESISTANCE GENE

Abstract: A novel protein associated with multidrug resistance in living cells and capable of conferring multidrug resistance on a cell is disclosed and nucleic acids encoding the novel isoforms are disclosed. Transformant cell lines which express the nucleic acid encoding the novel protein are also disclosed. Further, diagnostic and treatment methods using the novel protein, nucleic acids and cell lines are also disclosed.

Description: MULΉDRUG RESISTANCE GENE Cross reference to related applications. This application is a CLP of application 08/029,340 filed March 8, 1993, which is a CIP of application 07/966,923 filed October 27, 1992. BACKGROUND OF THE INVENTION It is well known that many types of cancer regress initially in response to currently available drugs. However, if the disease should recur, as it does with variable frequency, it is often refractory to further treatment with either the agent originally used for treatment or agents to which the patient has not been previously exposed. Currently there is little that can be done for patients whose tumors display this foπn of multidrug resistance. One mechanism by which cancer cells can simultaneously develop resistance to an array of structurally diverse drugs has been elucidated over the last 15 years with the characterization of P-glycoprotein. P-glycoprotein is a member of a superfamily of membrane proteins that serve to transport a variety of molecules, ranging from ions to proteins, across cell membranes. This superfamily is known as the ATP-binding cassette (ABC) superfamily of membrane transport proteins. For a review see C. F. Higgins, Ann. Rev. Cell Biol. 8, 67 (1992). For example, in addition to P-glycoprotein which transports chemotherapeutic drugs, this family includes the cystic fibrosis transmembrane conductance regulator, which controls chloride ion fluxes, as well as insect proteins that mediate resistance to antimalarial drugs. P-glycoprotein is believed to confer resistance to multiple anticancer drugs by acting as an energy dependent efflux pump that limits the intracelluiar accumulation of a wide range of cytotoxic agents and other xenobiotics. Compounds that are excluded from mammalian cells by P-glycoprotein are frequently natural product-type drugs but other large heterocyclic molecules are also "substrates" for this efflux pump. The discovery of P-glycoprotein and its occurrence in a variety of tumor types has stimulated the search for compounds that are capable of blocking its function and consequently, of reversing resistance. These investigations have resulted in identification of a large number of so-called chemosensitizers or reversing agents. Some of these compounds act by inhibiting the pumping action of P-glycoprotein RECTIFIED SHEET (RULE 9ti while the mechanism of action of others is still undetermined. A select group of these agents are currently under intensive clinical investigation and they show considerable promise as adjuncts to conventional chemotherapy. Chemosensitizers which can reverse P-glycoprotein-mediated multidrug resistance include verapamil and cyclosporin A. Unfortunately, overexpression of P-glycoprotein does not explain the high frequency of multidrug resistance in some of the more prevalent forms of cancer, such as lung cancer. In the Western world, lung cancer accounts for approximately 30% of total cancer deaths. There are four major histological categories of lung tumors: epidermoid or squamous cell adenocarcinomas, large cell carcinomas, adenocarcinomas and small cell carcinomas. The first three categories, known collectively as non-small cell lung cancers, differ from the last in their initial response to chemotherapy and radiotherapy. Non-small cell lung cancers are relatively resistant to both forms of treatment from the outset. In contrast, small cell lung cancer, which accounts for 20% of all lung tumors, exhibits a high initial response rate (80-90% in limited disease) to chemotherapy. However, almost all patients relapse with a multidrug resistant form of the disease and two year survival rates are less than 10%. Although the drug resistance profile displayed in relapsed small cell lung cancer patients is similar to that conferred by P-glycoprotein, P-glycoprotein appears not to be involved. In addition, limited studies in cell culture and in patients indicate that multidrug resistance in small cell lung cancer does not respond to chemosensitizers, such as verapamil and cyclosporin A, that show promise with other types of drug resistant tumors. Survival rates in lung cancer have not improved significantly in forty years and, because of its common occurrence, there is clearly a great need for improved therapy for this disease. SUMMARY OF THE INVENTION The present invention is based, at least in part, on the discovery of a nucleic acid which encodes a protein which can confer multidrug resistance on a drug sensitive mammalian cell when expressed in the cell and which is overexpressed in certain multidrug resistant cancer cell lines. The nucleic acid of the invention was isolated from a multidrug resistant cancer cell line which does not overexpress P- glycoprotein and whose resistance is not substantially reversed by chemosensitizers which inhibit P-glycoprotein. The nucleic acid and encoded protein of the present invention represent molecules which can be targeted therapeutically in multidrug resistant tumors expressing the nucleic acid and protein. The present invention provides an isolated nucleic acid having a nucleotide sequence which encodes a protein associated with multidrug resistance which is overexpressed in multidrug resistant cells independently of overexpression of P-glycoprotein. The protein has been named multidrug resistance-associated protein (referred to as MRP). The protein of the invention differs in amino acid sequence from P-glycoprotein. The isolated nucleic acid, when expressed in a cell which is not multidrug resistant, can confer on the cell multidrug resistance. In a preferred embodiment, an isolated nucleic acid is provided having a sequence which codes for a protein associated with multidrug resistance having an amino acid sequence which has substantial sequence homology with the amino acid sequence shown in SEQ ID NO:2. Most preferably the isolated nucleic acid has a sequence having substantial sequence homology with the nucleotide sequence shown in SEQ ID NO:l. The invention further provides an isolated nucleic acid which is antisense to a nucleic acid having substantial sequence homology with the nucleotide sequence shown in SEQ ID NO:l. The invention further provides a recombinant expression vector adapted for transformation of a host cell comprising the nucleic acid of the invention operatively linked to a regulatory sequence. The invention also provides a recombinant expression vector adapted for transformation of a host cell comprising a DNA molecule operatively linked to a regulatory sequence to allow expression of an RNA molecule which is antisense to a nucleotide sequence of SEQ ID NO: 1. The invention also provides a method of preparing a protein capable of conferring multidrug resistance utilizing the nucleic acid of the invention. The method comprises culturing a transformant host cell including a recombinant expression vector comprising a nucleic acid of the invention and an regulatory sequence operatively linked to nucleic acid in a suitable medium until a multidrug resistance protein is formed and thereafter isolating the protein. The invention further provides an isolated protein having the biological activity of MRP, which can confer multidrug resistance on a drug sensitive cell when the protein is expressed in the cell, said resistance not being reversed by chemosensitizers of P-glycoprotein. The isolated protein of the invention is associated with multidrug resistance in tumor cells and is overexpressed in multidrug resistant cells which may or may not overexpress P-glycoprotein. In a preferred embodiment the protein has an amino acid sequence which has substantial homology with the amino acid sequence shown in SEQ ID NO: 2. The invention further provides an antibody specific for an epitope of a protein of the invention. Preferably the antibody is a monoclonal antibody. The antibody can be coupled to a detectable substance or a substance having toxic or therapeutic activity. The invention also provides a bispecific antibody capable of binding to a tumor cell which expresses a protein of the invention and to a detectable substance, or a substance having toxic or therapeutic activity. Preferably, the toxic substance is a cytotoxic cell and the bispecific antibody is capable of crosslinking the tumor cell and the cytotoxic cell thereby facilitating lysis of the tumor cell. The invention further provides a tetrameric antibody complex of a first monoclonal antibody which is capable of binding to a tumor cell expressing a protein of the invention and a second monoclonal antibody which is capable of binding to a detectable substance or a substance having toxic or therapeutic activity wherein the first and second antibody are from a first animal species, conjugated to form a cyclic tetramer with two monoclonal antibodies of a second animal species directed against the Fc fragment of the antibodies of the first animal species. The antibodies, bispecific antibodies or tetrameric antibody complexes can be incorporated in compositions suitable for administration in a pharmaceutically acceptable carrier. Molecules which bind to a protein of the invention, including the antibodies, bispecific antibodies and tetrameric antibody complexes of the invention, can be used in a method for identifying multidrug resistant tumor cells by labelling the molecule with a detectable substance, contacting the molecule with tumor cells and detecting the detectable substance bound to the tumor cells. A molecule which binds to a protein of the invention can further be used in a method for inhibiting multidrug resistance of a cell by blocking activity of an MRP protein. A molecule which binds to a protein of the invention can further be used to kill a multidrug resistant cell which expresses the protein by contacting the molecule, coupled to a toxic or therapeutic substance, with the multidrug resistant cell. Nucleic acids of the invention can be used in a method for protecting a drug sensitive cell from cytotoxicity due to exposure to a drug by transfecting the cell with a nucleic acid in a form suitable for expression of the protein encoded by the nucleic acid in the cell, thereby conferring drug resistance on the cell. The recombinant molecules of the invention can be used to produce transformant host cells expressing the protein of the invention. The recombinant molecules of the invention can be also used to produce transgenic nonhuman animals and nonhuman knockout animals. The transfected cells, transgenic animals and knockout animals can be used to test substances for their effect on multidrug resistance. A method for identifying a substance which is a chemosensitizer of a therapeutic agent and a method for identifying a cytotoxic substance for multidrug resistant cells, using transformant host cells or animals of the invention, are provided. The invention also relates to a cell line which is multidrug resistant, does not overexpress P-glycoprotein and is substantially resistant to hydrophobic drugs. The cell line may be derived from small cell lung cancer cells, preferably the cell line NCI-H69. Most preferably the multidrug resistant cell line is H69AR (ATCC CRL 11351). A revertant drug sensitive cell line may be obtained from the multidrug resistant cell line by culturing the multidrug resistant cell line in the absence of a drug for a period of time sufficient to produce a revertant drug sensitive cell line. Preferably the revertant drug sensitive cell line is H69PR (ATCC CRL 11350). BRIEF DESCRIPTION OF DRAWINGS Figure IA is a Northern blot of poly(A + )RNA from H69, H69AR and H69PR cells hybridized with a 1.8 kb EcoRI cDNA fragment of the multidrug resistance protein of the invention. Figure IB is a Southern blot analysis of EcoRI - digested genomic DNA from H69, H69AR and H69PR cells hybridized with a 1.8kb EcoRI cDNA fragment of the multidrug resistance protein of the invention. Figure IC is a Northern blot of sensitive and resistant HeLa cell poly (A + )RNA hybridized with a 1.8 kb EcoRI cDNA fragment of the multidrug resistance protein of the invention. Figure 2 is a cluster analysis of the relative similarity of the multidrug resistance protein of the invention to selected members of the ATP-binding cassette transporter superfamily that contain hydrophobic transmembrane regions followed by nucleotide binding folds. Figures 3A & B are the complete amino acid sequence of the multidrug re¬ sistance protein of the invention aligned with the complete amino acid sequence of ItPgpA (Lei/PgpA). Figure 3C is a diagram showing the alignment of the extended nucleotide binding regions of the multidrug resistance protein of the invention, human CFTR and leishmania ItPgpA and human P-glycoprotein (Hum/Mdrl). Figure 4 is a Northern blot of total RNA from normal tissues hybridized with a 0.9 kb EcoRI cDNA fragment of the multidrug resistance protein of the invention. Figure 5 is an ISCN-derived idiogram of the human karyotype showing silver grain distribution following in situ hybridization of a 1.8 kb EcoRI cDNA fragment of the multidrug resistance protein of the invention to metaphase chromosomes. Figure 6 is a graph depicting the relative cytotoxicity of doxorubicin on MRP-transfected HeLa cell populations (T2, T5), a clone of the T5 population (T5-5), untransfected HeLa cells and HeLa cells transfected with the parental expression vector (Cl). Figure 7A is a Northern blot of poly(A)+ RNA from transfected and control HeLa cells hybridized with a 4 kb MRP cDNA fragment which hybridizes with endogenous MRP mRNA (e) and expression vector-derived MRP mRNA (v). Hybridization with a GAPDH cDNA demonstrates the relative amounts of poly(A) + RNA in each lane. Figure 7B is a Northern blot of poly(A)+ RNA from transfected HeLa cells and control cells hybridized with a DNA fragment from the pRc/CMV vector which hybridizes only to expression vector-derived MRP mRNA (v). Hybridization with a GAPDH cDNA demonstrates the relative amounts of poly(A)+ RNA in each lane. Figure 7C is a Northern blot (MRP mRNA) and immunoblots (MRP protein) depicting the relative levels of expression vector-derived MRP mRNA and protein in transfected HeLa cells and endogenous MRP mRNA and protein in the H69AR cell line. RECTIFIED SHEET (RULE 91) 6 ISA/EP Figure 8A is a Northern blot of poly(A) + RNA from transfected HeLa ceils and control cells hybridized with cDNA probes for topoisomerase II (Topo II) alpha and beta mRNAs. Hybridization with a GAPDH cDNA demonstrates the relative amounts of poly(A)+ RNA in each lane. Figure 8B is a Northern blot of poly(A)+ RNA from transfected HeLa cells and control cells hybridized with a cDNA probe for annexin II mRNA. Hybridization with a GAPDH cDNA demonstrates the relative amounts of poly(A) + RNA in each lane. Figure 9 is a graph depicting the relative cytotoxicity of VP-16, vincristine and cisplatin on MRP-transfected HeLa cell populations (T2, T5), a clone of the T5 population (T5-5), untransfected HeLa cells and HeLa cells transfected with the parental expression vector (Cl). DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Multidrug resistant mammalian cell lines have been derived from a number of tumor types and have provided in vitro models for the study of acquired resistance. Although selected by a single natural product-type drug, these cell lines are cross-resistant to a wide range of chemically unrelated xenobiotics with multiple subcellular targets. Typically, these cells are resistant to anthracyclines [e.g. doxorubicin (DOX), epipodophyllotoxins (e.g. VP-16) and the Vinca alkaloids (e.g. vinblastine)] but not to antimetabolites such as 5-fluorouracil, or to platinum-containing drugs. Multidrug resistant cells also frequently exhibit a collateral sensitivity to certain hydrophobic drugs including local anesthetics and steroid hormones. The most commonly reported alteration in multidrug resistant tumor cells has been the increased expression of the 170 kDa plasma membrane glycoprotein, P-glycoprotein (P-gp), which is encoded by the MDR1 gene. Studies carried out in several laboratories with clinical samples and cell lines representing many tumor types have lead to the conclusion that P-gp, while clinically relevant in some malignancies, is unlikely to be important in others. Overexpression of P-gp is an infrequent occurrence in both small cell lung cancer (SCLC) and non small cell lung cancer (NSCLC). One of the most widely used cell lines in experimental studies of SCLC is NCI-H69 (H69) (Gazdar et al., Cancer Res. 40, 3502-3507 (1980)) (ATCC HTB 119). This cell line was treated repeatedly with an anthracycline, such as daunorubicin or epirubicin and preferably DOX, and step-wise selected to a final concentration of 0.8 μM, to produce a multidrug resistant cell line, designated as H69AR. A description of the procedures which can be used to produce a multidrug resistant cell line such as H69AR is found in Cole, Cancer Chemother Pharmacol. 17, 259-263 (1986) and in Mirski et al., Cancer Research 47, 2594-2598 (1987). The H69AR cell line (ATCC CRL 11351 ) is about 50-fold resistant to DOX as compared to the parental H69 cell line. H69AR is also cross-resistant to a wide variety of natural product-type drugs. On the other hand, drugs such as carboplatin, 5-fluorouracil and bleomycin are equally toxic to both sensitive H69 and resistant H69AR cells. Although the cross-resistance pattern of H69AR cells is typical of resistance associated with increased levels of P-gp, these cells are different in that they display little or no collateral sensitivity to hydrophobic drugs such as steroids or local anaesthetics. Another distinguishing feature of H69AR of potential clinical relevance that distinguishes it from P-gp overexpressing cell lines is the limited ability of verapamil, cyclosporin A and other chemosensitizing agents that interact with P-gp, to reverse DOX resistance in these cells. The absence of P-gp overexpression supports the suggestion that H69AR provides a clinically relevant model of drug resistance in lung cancer. A revertant drug sensitive cell line H69PR (Cole et al., Br J. Cancer 65, 498-502, 1992) (ATCC CRL 11350) was isolated by culturing the H69AR cell line in the absence of drugs such as DOX for a sufficient time to produce a revertant cell line. Preferably the cell line H69PR is cultured in the absence of drugs for at least 3 months and up to about 48 months, most preferably 42 months. The cell lines of the invention may be used to assay for a substance that affects a multidrug resistant tumor cell. Cells from a cell line of the invention may be incubated with a test substance which is suspected of affecting multidrug resistance. The effect of the substance can be determined by analyzing the drug resistance pattern of the cells and comparing the results to a control. As discussed above, the multidrug resistant cell line of the invention is resistant to anthracylines, epipodophyllotoxins, Vinca alkaloids and other natural-product type drugs. Thus, it is possible to screen for an agonist or antagonist substance of multidrug resistance or an antagonist that inhibits the effects of an agonist. In an embodiment of the invention, a substance that is suspected of being cytotoxic to a multidrug resistant tumor cell can be identified. Therefore, it is possible using the above described method to identify substances which may be useful in the treatment of multidrug resistant tumors. As described in the Examples, the H69AR cell line has been used to identify a cDNA encoding a novel protein associated with multidrug resistance designated MRP. The DNA sequence and deduced amino acid sequence of MRP are shown in SEQ ID NO:l and SEQ ID NO. 2, respectively. MRP mRNA is overexpressed in certain multidrug resistant tumor cell lines, including H69AR. Furthermore, expression of MRP protein in a drug sensitive mammalian cell line confers multidrug resistance on the cell line. A protein described herein as "having biological activity of MRP" can confer on a mammalian cell multidrug resistance to anthracyclines, epipodophyllotoxins and Vinka alkaloids when the protein is expressed in the mammalian cell, and this resistance is not substantially reversed by chemosensitizers which reverse P-glycoprotein-mediated multidrug resistance, such as verapamil or cyclosporin A. The terms "drug resistant" or "drug resistance" as used herein to describe a property of a cell refer to the ability of the cell to withstand without cytotoxicity increased concentrations of a drug as compared to an appropriate control cell. An appropriate control cell for a cell which has been made drug resistant by continued exposure to a drug is the parental cell from which the drug resistant cell was derived. An appropriate control cell for a cell which has been made drug resistant by expression in the cell of a protein which confers drug resistance on the cell is the same cell without the protein expressed. Appropriate control cells for naturally occurring tumor cells in vivo made drug resistant by continued exposure to a drug are the same tumor cells at the time of initial exposure to the drug. The invention provides isolated nucleic acids encoding proteins having biological acitivity of MRP. In a preferred embodiment, the nucleic acid is a cDNA comprising a nucleotide sequence shown in SEQ ID NO: 1. The invention further provides antisense nucleic acids of nucleic acids encoding proteins having biological acitivity of MRP. The invention further provides recombinant expression vectors comprising the nucleic acids and antisense nucleic acids of the invention and transformant host cells containing recombinant nucleic acids of the invention. The invention provides isolated proteins having biological acitivity of MRP and a method for preparing such proteins. In a preferred embodiment, the isolated protein having biological activity of MRP comprises an amino acid sequence shown in SEQ ID NO: 2. The protein comprising the amino acid sequence of SEQ ID NO: 2 is a member of the ABC superfamily of membrane transport proteins. The invention further provides antibodies specific for the isolated proteins of the invention and compositions suitable for administration comprising such antibodies. The invention further provides transgenic and knockout nonhuman animals produced using the nucleic acids of the invention. The invention provides a method for identifying multidrug resistant cell using the nucleic acids and antibodies of the invention. The invention further provides methods for inhibiting multidrug resistance of a multidrug resistant cell and for killing a multidrug resistant cell using the nucleic acids and antibodies of the invention. The invention further provides methods for identifying substances which are chemosensitizers of therapeutic agents or cytotoxic to drug resistant cells using the transformant host cells and animals of the invention. Furthermore, the invention provides diagnostic kits for identifying drug resistant tumor cells. These and other aspects of this invention are described in detail in the following subsections. I. Isolated Nucleic Acids The invention provides isolated nucleic acids encoding proteins having biological acitivity activity of MRP. The term "isolated" refers to a nucleic acid substantially free of cellular material or culture medium when produced by recombinant DNA techniques, or chemical precursors or other chemicals when chemically synthesized. An "isolated" nucleic acid is also free of sequences which naturally flank the nucleic acid (i.e., sequences located at the 5' and 3' ends of the nucleic acid) in the organism from which the nucleic acid is derived. The term "nucleic acid" is intended to include DNA and RNA and can be either double stranded or single stranded. In a preferred embodiment, the nucleic acid is a cDNA comprising a nucleotide sequence shown in SEQ ID NO: 1. In another embodiment, the nucleic acid is a cDNA comprising the coding region of the nucleotide sequence shown in SEQ ID NO: 1. In another embodiment, the nucleic acid encodes a protein comprising an amino acid sequence shown in SEQ ID NO: 2. It will be appreciated that the invention includes nucleic acids having substantial sequence homology with the nucleotide sequence shown in SEQ ID NO: 1 or encoding proteins having substantial homology to the amino acid sequence shown in SEQ ID NO: 2. Homology refers to sequence similarity between sequences and can be determined by comparing a position in each sequence which may be aligned for purposes of comparison. When a position in the compared sequence is occupied by the same nucleotide base or amino acid, then the molecules are homologous at that position. A degree of homology between sequences is a function of the number of matching or homologous positions shared by the sequences. The term "sequences having substantial sequence homology" means those nucleotide and amino acid sequences which have slight or inconsequential sequence variations from the sequences disclosed in SEQ ID NO:l and SEQ ID NO: 2, i.e. the homologous nucleic acids function in substantially the same manner to produce substantially the same polypeptides as the actual sequences. The variations may be attributable to local mutations or structural modifications. It is expected that substitutions or alterations can be made in various regions of the nucleotide or amino acid sequence without affecting protein function, particularly if they lie outside the regions predicted to be of functional significance. Analysis of the protein encoded by SEQ ID NO: 1, comprising the amino acid sequence of SEQ ID NO: 2, reveals 12 hydrophobic stretches predicted to be membrane-spanning regions and of functional importance. These amino acid residues correspond to positions 99-115, 137-153, 175-191, 365-381, 444-460, 466-482, 555-571, 591-607, 969-985, 1028-1044, 1102-1118 and 1205-1221 of SEQ ID NO: 2. Nucleotide substitutions that result in amino acid sequence changes within these regions, especially those that reduce the hybrophobic nature of these regions, are not likely to be translated into a functional protein. Analysis of the protein encoded by SEQ ID NO: 1, comprising the amino acid sequence of SEQ ID NO: 2, reveals two regions having the structural characteristics of nucleotide binding folds (NBFs) typical of ATP-binding cassette domains (ABC domains). See Hyde, S.C. et al, Nature 346, 362-365 (1990). Elements comprising part of the structure of these NBFs are conserved in other members of the ABC superfamily of membrane transport proteins and the domains have been shown to bind nucleotides and to be functionally important. For example see Higgins, C.F., Ann. Rev. Cell Biol. 8, 67-113 (1992). In the protein comprising the amino acid sequence shown in SEQ ID NO: 2, the two NBFs are located between about amino acid residues 661-810 and 1310-1469 of SEQ ID NO:2. Nucleotide and corresponding amino acid substitutions which decrease the degree of homology of these regions compared to other members of the ABC superfamily of membrane transport proteins are likely not to be tolerated in a functional protein. Alternatively, nucleotide and corresponding amino acid substitutions which maintain the structure of an NBF are likely to be tolerated. For example, it has been demonstrated that nucleotides encoding an NBF of one member of the ABC superfamily of membrane transport proteins can be substituted for the homologous domain of another member while maintaining function of the protein. See Buschman, F. and Gros, P. Mol. Cell. Biol. 11, 595-603 (1991). Accordingly, the invention provides for a nucleic acid encoding a protein comprising an amino acid sequence represented by V-W-X-Y-Z, wherein V are amino acid residues corresponding to amino acid residues from about 1 to 660 of SEQ ID NO: 2, W are amino acid residues of an NBF substantially homologous with amino acid residues from about 661 to 810 of SEQ ID NO: 2, X are amino acid residues corresponding to amino acid residues from about 811 to 1309 of SEQ ID NO: 2, Y are amino acid residues of an NBF substantially homologous with amino acid residues from about 1310 to 1469 of SEQ ID NO: 2 and Z are amino acid residues corresponding to amino acid residues from about 1470 to 1531 of SEQ ID NO: 2. The term "from about" is intended to mean that the junction between two regions of the protein (e.g. between V and W) may vary by a few amino acids from those specifically indicated. It is anticipated that, outside of the regions specified above, a nucleic acid encoding a protein comprising an amino acid sequence which is about 50% similar with the amino acid sequence shown SEQ ID NO:2 will provide functional proteins. Alternatively, proteins comprising an amino acid sequence which is 60%, 70%, 80% or 90% homologous with the amino acid sequence shown SEQ ID NO:2 may provide proteins having MRP activity. The invention encompasses a nucleic acid encoding a protein having biological activity of MRP which is at least 50% homologous with the amino acid sequence of SEQ ID NO: 2. It will further be appreciated that variant forms of the nucleic acids of the invention which arise by alternative splicing of an mRNA corresponding to a cDNA of the invention are encompassed by the invention. Hybridization of a cDNA of the invention, containing all or part of SEQ ID NO: 1, to cellular RNA identifies an mRNA of approximately 6.5kb with an extended open reading frame of 1531 amino acids. Several cDNA clones have been isolated that contain internal deletions which maintain the original reading frame, suggesting that they may be produced by alternative splicing. The existence of mRNA species containing these deletions was confirmed by reverse PCR of RNA from both multidrug resistant and sensitive cells. In most cases, the variant mRNAs represent minor components of 10% or less. However, some comprise more than 20% of total MRP mRNA. Alternative splice forms have been identified which remove nucleotides 657 to 783 of SEQ ID NO: 1 (amino acids 155-196 inclusive of SEQ ID NO: 2), 1845 to 1992 (amino acids 551-599 inclusive), 2287 to 2463 (amino acids 698-756 inclusive), 2287 to 2628 (amino acids 698-811 inclusive) and 4230 to about 4818 (amino acids 1346 to 1531 inclusive). Two of the more common variants lack segments of the NH2 proximal NBF. Both begin at the same site (amino acid 698) and they affect regions of the cassette that are very near and COOH proximal to the common exon 9 splicing variant of the cystic fibrosis transmembrane conductance regulator (CFTR) mRNA. See Chu, C-S. et al., EMBOJournal 10, 1355-1363 (1991). The shorter of the two (amino acids 698-756) eliminates a phenylalanine at a position corresponding to F508 of CFTR. The longer one (amino acids 698-811) removes the active transport family signature that includes the conserved LSGGQ sequence and the Walker B motif. Another of the more common variants (amino acids 1346-1531) lacks a region specifying a segment of the protein close to the COOH terminus, similar to the location affected by alternative splicing of exon 23 of CFTR mRNA. See Yoshimura. K., et al. J. Biol. Chem. 268, 686-690 (1993). In addition, two other deletions have been identified, one of which eliminates two of the transmembrane domains in the NH2 proximal half of the molecule (amino acids 551-599), and another which removes a potential secretory signal cleavage site located between amino acids 189/190 (amino acids 155-196). Another aspect of the invention provides a nucleic acid which hybridizes under high or low stringency conditions to a nucleic acid which encodes a protein having all or a portion of an amino acid sequence shown SEQ ID NO:2. Appropriate stringency conditions which promote DNA hybridization, for example, 6.0 x sodium chloride/sodium citrate (SSC) at about 45°C, followed by a wash of 2.0 x SSC at 50°C are known to those skilled in the art or can be found in Current Protocols in Molecular Biology, John Wiley & Sons, N.Y. (1989), 6.3.1-6.3.6. For example, the salt concentration in the wash step can be selected from a low stringency of about 2.0 x SSC at 50°C to a high stringency of about 0.2 x SSC at 50°C. In addition, the temperature in the wash step can be increased from low stringency conditions at room temperature, about 22°C to high stringency conditions, at about 65°C. Isolated nucleic acids encoding a protein having the biological activity of MRP, as described herein, and having a sequence which differs from a nucleotide sequence shown in SEQ ID NO:l due to degeneracy in the genetic code are also within the scope of the invention. Such nucleic acids encode functionally equivalent proteins (e.g., a protein having MRP activity) but differ in sequence from the sequence of SEQ ID NO: 1 due to degeneracy in the genetic code. For example, a number of amino acids are designated by more than one triplet. Codons that specify the same amino acid, or synonyms (for example, CAU and CAC are synonyms for histidine) may occur due to degeneracy in the genetic code. As one example, DNA sequence polymorphisms within the nucleotide sequence of an MRP protein (especially those within the third base of a codon) may result in "silent" mutations in the DNA which do not affect the amino acid encoded. However, it is expected that DNA sequence polymorphisms that do lead to changes in the amino acid sequences of an MRP protein will exist within a population. It will be appreciated by one skilled in the art that these variations in one or more nucleotides (up to about 3-4% of the nucleotides) of the nucleic acids encoding proteins having the biological activity of MRP may exist among individuals within a population due to natural allelic variation. Any and all such nucleotide variations and resulting amino acid polymorphisms are within the scope of the invention. Furthermore, there may be one or more isoforms or related, cross-reacting family members of MRP described herein. Such isoforms or family members are defined as proteins related in biological activity and amino acid sequence to MRP, but encoded by genes at different loci. An isolated nucleic acid of the present invention encoding a protein having the biological activity of MRP can be isolated from a multidrug resistant cell line which displays resistance to such drugs as anthracyclines, epipodophyllotoxins and Vinka alkaloids that is not substantially reversed by chemosensitizers which reverse P-glycoprotein-mediated multidrug resistance, such as verapamil or cyclosporin A. One example of such a cell line is H69AR. Other suitable cell lines can be produced by stepwise selection of a non-resistant cell line in the presence of increasing concentrations of a drug for which resistance is to be acquired over a period of several months to years. For example, a cell line is cultured in the presence of an anthracycline, preferably doxorubicin, for about 14 months. Multidrug resistance is then assessed by exposing the selected cell line to other drugs, e.g. an epipodo-phyllotoxin such as VP-16 and a Vinca alkaloid such as vincristine, and determining the cytotoxicity of the drug for the cell line. The ability of chemosensitizers which reverse P-glycoprotein-mediated multidrug resistance, such as verapamil and cyclosporin A, to reverse the multidrug resistance is then assessed by exposing the selected cell line to these agents in the presence of the resistant drugs. A detailed description of the procedures which can be used to produce appropriate multidrug resistant cell line such as H69AR is found in Cole, Cancer Chemother Pharmacol. 17, 259-263 (1986) and in Mirski et al., Cancer Research 47, 2594-2598 (1987). An appropriate multidrug resistant cell line (e.g. a multidrug resistant cell line which displays resistance to anthracyclines, epipodophyllotoxins and Vinca alkaloids that is not substantially reversed by verapamil or cyclosporin A) is used to isolate a nucleic acid of the invention by preparing a cDNA library from this cell line by standard techniques and screening this library with cDNA produced from total mRNA isolated from the multidrug resistant cell line and its drug sensitive parental cell line. For example, a cDNA library constructed from total mRNA from H69AR cells is prepared. The library is plated and two sets of replica filters are prepared by standard methods. One set of filters is then screened with cDNA prepared from H69AR mRNA and the other set of filters is screened with a comparable amount of cDNA prepared from H69 mRNA. The cDNA used for screening the library is labelled, typically with a radioactive label. Following visualization of the hybridization results by standard procedures, cDNA clones displaying increased hybridization with H60AR cDNA when compared to H69 cDNA can be selected from the library. These cDNAs are derived from mRNAs overexpressed in H69AR cells when compared with H69 cells. For descriptions of differential cDNA library screening see King, C.R., et al. J. Bio. Chem 254, 6781 (1979); Van der Bliek, A.M., et al., Mol. Cell. Biol. 6, 1671 (1986). Determination of whether a cDNA so isolated has the biological activity of MRP can be accomplished by expressing the cDNA in a nonresistant mammalian cell, by standard techniques, and assessing whether expression in the cell of the protein encoded by the cDNA confers on the cell multidrug resistance to anthracyclines, epipodophyllotoxins and Vinca alkaloids that is not substantially reversed by verapamil or cyclosporin A. A cDNA having the biological activity of MRP so isolated can be sequenced by standard techniques, such as dideoxynucleotide chain termination or Maxam-Gilbert chemical sequencing, to determine the nucleic acid sequence and the predicted amino acid sequence of the encoded protein. An isolated nucleic acid of the invention which is DNA can also be isolated by preparing a labelled nucleic acid probe encompassing all or part of the nucleotide sequence shown in SEQ ID NO: 1 and using this labelled nucleic acid probe to screen an appropriate DNA library (e.g. a cDNA or genomic DNA library). For instance, a cDNA library made from a multi-drug resistant cell line as described above can be used to isolate a cDNA encoding a protein having MRP activity by screening the library with the labelled probe using standard techniques. Preferably, an H69AR cDNA library is used. Alternatively, a genomic DNA library can be similarly screened to isolate a genomic clone encompassing a gene encoding a protein having MRP activity. As demonstrated in Example 4, a human MRP gene has been mapped to chromosome 16. Therefore, a chromosome 16 library rather than a total genomic DNA library can also be used to isolate a human MRP gene. Nucleic acids isolated by screening of a cDNA or genomic DNA library can be sequenced by standard techniques. An isolated nucleic acid of the invention which is DNA can also be isolated by selectively amplifying a nucleic acid encoding a protein having MRP activity using the polymerase chain reaction (PCR) method and genomic DNA or mRNA. To prepare cDNA from mRNA, total cellular mRNA can be isolated, for instance from a multidrug resistant cell line, by a variety of techniques, e.g., by using the guanidinium-thiocyanate extraction procedure of Chirgwin et al., Biochemistry, 18, 5294-5299 (1979). cDNA is then synthesized from the mRNA using reverse transcriptase. Moloney MLV reverse transcriptase available from Gibco/BRL, Bethesda, MD, or AMV reverse transcriptase available from Seikagaku America, Inc., St. Petersburg, FL, are preferably employed. It is possible to design synthetic oligonucleotide primers from the nucleotide sequence shown in SEQ ID NO:l for use in PCR. A nucleic acid can be amplified from cDNA or genomic DNA using these oligonucleotide primers and standard PCR amplification techniques. The nucleic acid so amplified can be cloned into an appropriate vector and characterized by DNA sequence analysis. A isolated nucleic acid of the invention which is RNA can be be isolated by cloning a cDNA of the invention into an appropriate vector which allows for transcription of the cDNA to produce an RNA molecule which encodes a protein having MRP activity. For example, a cDNA can be cloned downstream of a bacteriophage promoter, e.g. a T7 promoter, in a vector and the cDNA can be transcribed in vitro with T7 polymerase. A resultant RNA can be isolated by standard techniques. A nucleic acid of the invention, for instance an oligonucleotide, can also be chemically synthesized using standard techniques. Various methods of chemically synthesizing polydeoxynucleotides are known, including solid-phase synthesis which, like peptide synthesis, has been fully automated in commercially available DNA synthesizers (See e.g., Itakura et al. U.S. Patent No. 4,598,049; Caruthers et al. U.S. Patent No. 4,458,066; and Itakura U.S. Patent Nos. 4,401,796 and 4,373,071). Analysis of the nucleotide sequence of SEQ ID NO: 1 using currently available computer software designed for the purpose, such as PC/Gene - IntelliGenetics Inc., Calif., permits the identification of the initiation codon and untranslated sequences of an MRP. The cDNA coding strand, depicted as SEQ ID NO: 1, contains a 4593 nucleotide open reading frame encoding 1531 amino acids, as well as 195 5' untranslated nucleotides and 223 3' untranslated nucleotides. The intron-exon structure and the transcription regulatory sequences of the gene encoding the MRP cDNA can be identified by using a nucleic acid of the invention to probe a genomic DNA clone library. Regulatory elements, such as promoter and enhancers necessary for expression of the gene encoding the MRP in various tissues, can be identified using conventional techniques. The function of the elements can be confirmed by using them to express a reporter gene such as the bacterial gene lacZ which is operatively linked to the fragments. Such a construct can be introduced into cultured cells using standard procedures or into non-human transgenic animal models. In addition to identifying regulatory elements in DNA, such constructs can also be used to identify nuclear proteins interacting with said elements, using techniques known in the art. A number of unique restriction sites for restriction enzymes are present in the nucleic acid comprising the nucleotide sequence shown in SEQ ID NO:l These restriction sites provide access to nucleotide fragments which code for polypeptides unique to the protein encoded by SEQ ID NO:l (i.e. a protein of the invention). The isolated nucleic acids of the invention or oligonucleotide fragments of the isolated nucleic acids, allow those skilled in the art to construct nucleotide probes for use in the detection of nucleotide sequences in biological materials, such as tumor cell samples. A nucleotide probe can be labelled with a radioactive element which provides for an adequate signal as a means for detection and has sufficient half-life to be useful for detection, such as 32 P, 3 H, 14 C or the like. Other materials which can be used to label the probe include antigens that are recognized by a specific labelled antibody, fluorescent compounds, enzymes, antibodies specific for a labelled antigen, and chemiluminescent compounds. An appropriate label can be selected having regard to the rate of hybridization and binding of the probe to the nucleotide to be detected and the amount of nucleotide available for hybridization. II. Antisense Nucleic Acids The invention also relates to an antisense nucleic acid, or oligonucleotide fragment thereof, of a nucleic acid of the invention. An antisense nucleic acid can comprise a nucleotide sequence which is complementary to a coding strand of a nucleic acid, e.g. complementary to an mRNA sequence, constructed according to the rules of Watson and Crick base pairing, and can hydrogen bond to the coding strand of the nucleic acid. The antisense sequence complementary to a sequence of an mRNA can be complementary to a sequence found in the coding region of the mRNA or can be complementary to a 5' or 3' untranslated region of the mRNA. Furthermore, an antisense nucleic acid can be complementary in sequence to a regulatory region of the gene encoding the mRNA, for instance a transcription initiation sequence or regulatory element. Preferably, an antisense nucleic acid complementary to a region preceding or spanning the initiation codon or in the 3' untranslated region of an mRNA is used. An antisense nucleic acid can be designed based upon the nucleotide sequence shown in SEQ ID NO: 1. A nucleic acid is designed which has a sequence compementary to a sequence of the coding or untranslated region of the shown nucleic acid. Alternatively, an antisense nucleic acid can be designed based upon sequences of an MRP gene, identified by screening a genomic library as described above. For example, the sequence of an important regulatory element can be determined as described above and a sequence which is antisense to the regulatory element can be designed. The antisense nucleic acids and oligonucleotides of the invention can be constructed using chemical synthesis and enzymatic ligation reactions using procedures known in the art. The antisense nucleic acid or oligonucleotide can be chemically synthesized using naturally occurring nucleotides or variously modified nucleotides designed to increase the biological stability of the molecules or to increase the physical stability of the duplex formed between the antisense and sense nucleic acids e.g. phosphorothioate derivatives and acridine substituted nucleotides can be used. Alternatively, the antisense nucleic acids and oligonucleotides can be produced biologically using an expression vector into which a nucleic acid has been subcloned in an antisense orientation (i.e. nucleic acid transcribed from the inserted nucleic acid will be of an antisense orientation to a target nucleic acid of interest). The antisense expression vector is introduced into cells in the form of a recombinant plasmid, phagemid or attenuated virus in which antisense nucleic acids are produced under the control of a high efficiency regulatory region, the activity of which can be determined by the cell type into which the vector is introduced. For a discussion of the regulation of gene expression using antisense genes see Weintraub, H. et al., Antisense RNA as a molecular tool for genetic analysis, Reviews - Trends in Genetics, Vol. 1(1) 1986. HI. Recombinant Expression Vectors The nucleic acids of the present invention which encode proteins having MRP activity can be incorporated in a known manner into a recombinant expression vector which ensures good expression of the encoded protein or part thereof. The recombinant expression vectors are "suitable for transformation of a host cell", which means that the recombinant expression vectors contain a nucleic acid or an oligonucleotide fragment thereof of the invention and a regulatory sequence, selected on the basis of the host cells to be used for expression, which is operatively linked to the nucleic acid or oligonucleotide fragment. Operatively linked is intended to mean that the nucleic acid is linked to a regulatory sequence in a manner which allows expression of the nucleic acid. Regulatory sequences are art-recognized and are selected to direct expression of the desired protein in an appropriate host cell. Accordingly, the term regulatory sequence includes promoters, enhancers and other expression control elements. Such regulatory sequences are known to those skilled in the art or one described in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, CA (1990) can be used. It should be understood that the design of the expression vector may depend on such factors as the choice of the host cell to be transfected and/or the type of protein desired to be expressed. Such expression vectors can be used to transfect cells to thereby produce proteins or peptides encoded by nucleic acids as described herein. The recombinant expression vectors of the invention can be designed for expression of encoded proteins in prokaryotic or eukaryotic cells. For example, proteins can be expressed in bacterial cells such as E. coli, insect cells (using baculovirus), yeast cells or mammalian cells. Other suitable host cells can be found in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, CA (1990). Expression in prokaryotes is most often carried out in E. coli with vectors containing constitutive or inducible promotors directing the expression of either fusion or non-fusion proteins. Fusion vectors add a number of amino acids usually to the amino terminus of the expressed target gene. Such fusion vectors typically serve three purposes: 1) to increase expression of recombinant protein; 2) to increase the solubility of the target recombinant protein; and 3) to aid in the purification of the target recombinant protein by acting as a ligand in affinity purification. Often, in fusion expression vectors, a proteolytic cleavage site is introduced at the junction of the fusion moiety and the target recombinant protein to enable separation of the target recombinant protein from the fusion moiety subsequent to purification of the fusion protein. Such enzymes, and their cognate recognition sequences, include Factor Xa, thrombin and enterokinase. Typical fusion expression vectors include pGEX (Amrad Corp., Melbourne, Australia), pMAL (New England Biolabs, Beverly, MA) and pRrT5 (Pharmacia, Piscataway, NJ) which fuse glutathione S-tranferase, maltose E binding protein, or protein A, respectively, to the target recombinant protein. Inducible non-fusion expression vectors include pTrc (Amann et al., (1988) Gene 69:301-315) and pET lid (Studier et al., Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, California (1990) 60-89). While target gene expression relies on host RNA polymerase transcription from the hybrid trp-lac fusion promoter in pTrc, expression of target genes inserted into pET lid relies on transcription from the T7 gnlO-lac 0 fusion promoter mediated by coexpressed viral RNA polymerase (T7 gnl). This viral polymerase is supplied by host strains BL21(DE3) or HMS174(DE3) from a resident λ prophage harboring a T7 gnl under the transcriptional control of the lacUV 5 promoter. One strategy to maximize recombinant protein expression in E. coli is to express the protein in a host bacteria with an impaired capacity to proteolytically cleave the recombinant protein (Gottesman, S., Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, California (1990) 119-128). Another strategy is to alter the nucleic acid sequence of the nucleic acid to be inserted into an expression vector (e.g. a nucleic acid encoding an MRP protein) so that the individual codons for each amino acid would be those preferentially utilized in highly expressed E. coli proteins (Wada et al., (1992) Nuc. Acids Res. 20:2111-2118). Such alteration of nucleic acid sequences of the invention could be carried out by standard DNA synthesis techniques. Examples of vectors for expression in yeast S.cerivisae include pYepSecl (Baldari. et al., (1987) Embo J. 6:229-234), pMFa (Kurjan and Herskowitz, (1982) Cell 30:933-943), pJRY88 (Schultz et al., (1987) Gene 54:113-123), and pYES2 (Invitrogen Corporation, San Diego, CA). Baculovirus vectors available for expression of proteins in cultured insect cells (SF 9 cells) include the pAc series (Smith et al., (1983) Mol. Cell Biol. 3:2156-2165) and the pVL series (Lucklow, V.A., and Summers, M.D., (1989) Virology 170:31-39). Expression of an MRP protein is mammalian cells is accomplished using a mammalian expression vector. Examples of mammalian expression vectors include pCDMδ (Seed, B., (1987) Nature 329:840) and pMT2PC (Kaufman et al. (1987), EMBOJ. 6:187-195). When used in mammalian cells, the expression vector's control functions are often provided by viral material. For example, commonly used promoters are derived from polyoma, Adenovirus 2, cytomegalovirus and most frequently, Simian Virus 40. Preferably, the pRc/CMV expression vector (Invitrogen) is used. In the pRc/CMV vector, nucleic acid introduced into the vector to be expressed is under the control of the enhancer/promoter sequence from the immediate early gene of human cytomegalovirus. Additionally, a gene conferring neomycin resistance is encoded by the vector. In one embodiment, the recombinant expression vector is capable of directing expression of the nucleic acid preferentially in a particular cell type. This means that the expression vector's control functions are provided by regulatory sequences which allow for preferential expression of a nucleic acid contained in the vector in a particular cell type, thereby allowing for tissue or cell-type specific expression of an encoded protein. For example, a nucleic acid encoding a protein with MRP activity can be preferentially expressed in cardiac muscle cells using promoter and enhancer sequences from a gene which is expressed preferentially in cardiac muscle cells, such as a cardiac myosin gene or a cardiac actin gene. The recombinant expression vector of the invention can be a plasmid. The recombinant expression vector of the invention further can be a virus, or portion thereof, which allows for expression of a nucleic acid introduced into the viral nucleic acid. For example, replication defective retroviruses, adenoviruses and adeno-associated viruses can be used. The invention further provides a recombinant expression vector comprising a DNA molecule of the invention cloned into the expression vector in an antisense orientation. That is, the DNA molecule is operatively linked to a regulatory sequence in a manner which allows for expression, by transcription of the DNA molecule, of an RNA molecule which is antisense to the nucleotide sequence of SEQ ID NO: 1. Regulatory sequences operatively linked to the antisense nucleic acid can be chosen which direct the continuous expression of the antisense RNA molecule in a variety of cell types, for instance a viral promoter and/or enhancer, or regulatory sequences can be chosen which direct tissue or cell type specific expression of antisense RNA, as described above. IV. Transformant Host Cells The recombinant expression vectors of the invention can be used to make a transformant host cell including the recombinant expression vector. The term "transformant host cell" is intended to include prokaryotic and eukaryotic cell which have been transformed or transfected with a recombinant expression vector of the invention. The terms "transformed with", "transfected with", "transformation" and "transfection" are intended to encompass introduction of nucleic acid (e.g. a vector) into a cell by one of many possible techniques known in the art. Prokaryotic cells can be transformed with nucleic acid by, for example, electroporation or calcium-chloride mediated transformation. Nucleic acid can be introduced into mammalian cells via conventional techniques such as calcium phosphate or calcium chloride co-precipitation, DEAE-dextran-mediated transfection, lipofectin, electroporation or microinjection. Suitable methods for transforming and transfecting host cells can be found in Sambrook et al. (Molecular Cloning: A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory press (1989)), and other laboratory textbooks. The number of host cells transformed with a recombinant expression vector of the invention by techniques such as those described above will depend upon the type of recombinant expression vector used and the type of transformation technique used. Plasmid vectors introduced into mammalian cells are integrated into host cell DNA at only a low frequency. In order to identify these integrants, a gene that contains a selectable marker (i.e., resistance to antibiotics) is generally introduced into the host cells along with the gene of interest. Preferred selectable markers include those which confer resistance to certain drugs, such as G418 and hygromycin. Selectable markers can be introduced on a separate plasmid from the nucleic acid of interest or, preferably, are introduced on a the same plasmid. Host cells transformed with a one or more recombinant expression vectors containing a nucleic acid of the invention and a gene for a selectable marker can be identified by selecting for cells using the selectable marker. For example, if the selectable marker encoded a gene conferring neomycin resistance (such as pRc/CMV), transformant cells can be selected with G418. Cells that have incorporated the selectable marker gene will survive, while the other cells die. As demonstrated in Examples 5 and 6, the nucleic acids of the invention can confer multidrug resistance to drugs including anthracyclines, epipodophyllotoxins and Vinka alkaloids on a drug sensitive cell when transfected into the cell. Thus, these drugs can be used as selecting agents when preparing a transformant host cell rather than using an independent selectable marker (such as neomycin resistance). Therefore, the nucleoc acids of the invention are useful not only for conferring multidrug resistance on a cell but also as selectable markers for cells into which the nucelic acid has been introduced. See for example Pastan et al. U.S. Patent No. 5,166,059; Croop et al. U.S. Patent No. 5,198,344. Cells are selected by exposure to one or more drugs for which resistance is conferred by the nucleic acid. An MRP- encoding nucleic acid in a recombinant expression vector can be introduced into a cell together with a second nucleic acid comprising a gene of interest, either in the same vector or in separate vectors, and transformant cells can be selected based upon their acquired drug resistance. Drug resistant cells which are selected will contain the MRP-encoding nucleic acid often cointegrated with the gene of interest. Furthermore, by increasing stepwise the concentration of drug used in selecting the cells, it is possible to obtain transformant cells with a higher number of copies of the introduced nucleic acid, including both the MRP-encoding nucleic acid and a gene of interest. Therefore, the nucleic acids of the invention are also useful as amplifiable markers. The nucleic acids of the invention encode proteins "having biological activity of MRP". The biological activity of MRP is defined as the ability of the protein, when expressed in a drug sensitive mammalian cell, to confer on the cell multidrug resistance to such drugs as anthracyclines, epipodophyllotoxins and Vinca alkaloids that is not substantially reversed by chemosensitizers which reverse P-glycoprotein-mediated multidrug resistance, such as verapamil or cyclosporin A. An isolated nucleic acid of the invention can be tested for MRP activity by incorporating the nucleic acid into a recombinant expression vector of the invention, transforming a drug sensitive mammalian cell with the recombinant expression vector to make a transformant host cell of the invention as described above and testing the multidrug resistance of the transformant host cell. The multidrug resistance of the transformant host cell is tested by determining the cytotoxicity of the drugs to be tested (i.e. anthracyclines, epipodophyllotoxins and Vinca alkaloids) for the transformed cell as compared to the untransformed cell, and the ability of other drugs (i.e. verapamil and cyclosporin A) to reverse multidrug resistance. For example, in a preferred embodiment, the transformant host cell is a HeLa cell, and the multidrug resistance of transfected HeLa cells is compared to that of untransfected HeLa cells or preferably to HeLa cells transfected with the parental expression vector lacking the nucleic acid encoding a protein having MRP activity. V. Isolated Proteins The invention provides isolated proteins having biological activity of MRP. The term "isolated" refers to a protein substantially free of cellular material or culture medium when produced by recombinant DNA techniques, or chemical precursors or other chemicals when chemically synthesized. In a preferred embodiment the protein having biological activity of MRP comprises an amino acid sequence shown in SEQ ID NO: 2. Alternatively, the protein can be encoded by a nucleic acid comprising the nucleotide sequence of SEQ ID NO: 1. Proteins having biological activity of MRP which have substantial sequence homology to the amino acid sequence of SEQ ID NO: 2, as defined above, are also encompassed by the invention. Furthermore, proteins having biological activity of MRP that are encoded by nucleic acids which hybridize under high or low stringency conditions, as defined above, to a nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1 are encompassed by the invention. Additionally, immunogenic portions of MRP proteins are within the scope of the invention. As demonstrated in Example 7, two immunogenic portions of a protein comprising an amino acid sequence shown in SEQ ID NO: 2 correspond to amino acid residues 932-943 shown in SEQ ID NO: 2 (residues AELQKAEAKKEE) and amino acid residues 1427-1441 (residues GENLSVGQRQLVCLA). Preferred immunogenic portions correspond to regions of the protein not conserved in other ABC superfamily members, i.e. outside of the two NBF domains (amino acid residues 661-810 and 1310-1469), and include regions between the 12 membrane spanning regions. An immunogenic portion will be of at least about eight amino acids in length. The MRP protein, or isoforms or parts thereof, of the invention can be isolated by expression in a suitable host cell using techniques known in the art. Suitable host cells include prokaryotic or eukaryotic organisms or cell lines, for example, yeast, E. coli and insect cells. The recombinant expression vectors of the invention, described above, can be used to express a protein having MRP activity in a host cell in order to isolate the protein. The invention provides a method of preparing an isolated protein of the invention comprising introducing into a host cell a recombinant nucleic acid encoding the protein, allowing the protein to be expressed in the host cell and isolating the protein. Preferably, the recombinant nucleic acid is a recombinant expression vector. Proteins can be isolated from a host cell expressing the protein according to standard procedures of the art, including ammonium sulfate precipitation, fractionation column chromatography (e.g. ion exchange, gel filtration, electrophoresis, affinity chromatography, etc.) and ultimately, crystallization (see generally, "Enzyme Purification and Related Techniques", Methods in Enzymology, 22, 233-577 (1971)). Alternatively, the protein or parts thereof can be prepared by chemical synthesis using techniques well known in the chemistry of proteins such as solid phase synthesis (Merrifield, 1964, J. Am. Chem. Assoc. 85:2149-2154) or synthesis in homogeneous solution (Houbenweyl, 1987, Methods of Organic Chemistry, ed. E. Wansch, Vol. 15 I and II, Thieme, Stuttgart). VI. Antibodies The proteins of the invention, or portions thereof, can be used to prepare antibodies specific for the proteins. Antibodies can be prepared which bind a distinct epitope in an unconserved region of the protein. An unconserved region of the protein is one which does not have substantial sequence homology to other proteins, for example other members of the ABC superfamily of membrane transport proteins. For example, unconserved regions encompassing sequences between the twelve membrane spanning regions, excluding the NBF domains, can be used. Alternatively, a region from one of the two NBF domains can be used to prepare an antibody to a conserved region of an MRP protein. An antibody to a conserved region may be capable of reacting with other members of the ABC family of membrane transport proteins. Conventional methods can be used to prepare the antibodies. For example, by using a peptide of an MRP protein, polyclonal antisera or monoclonal antibodies can be made using standard methods. As demonstrated in Example 7, a mammal, (e.g., a mouse, hamster, or rabbit) can be immunized with an immunogenic form of the peptide which elicits an antibody response in the mammal. Techniques for conferring immunogenicity on a peptide include conjugation to carriers or other techniques well known in the art. For example, the peptide can be administered in the presence of adjuvant. The progress of immunization can be monitored by detection of antibody titers in plasma or serum. Standard ELISA or other immunoassay can be used with the immunogen as antigen to assess the levels of antibodies. Following immunization, antisera can be obtained and, if desired, polyclonal antibodies isolated from the sera. To produce monoclonal antibodies, antibody producing cells (lymphocytes) can be harvested from an immunized animal and fused with myeloma cells by standard somatic cell fusion procedures thus immortalizing these cells and yielding hybridoma cells. Such techniques are well known in the art. For example, the hybridoma technique originally developed by Kohler and Milstein (Nature 256, 495-497 (1975)) as well as other techniques such as the human B-cell hybridoma technique (Kozbor et al., Immunol. Today 4, 72 (1983)), the EBV-hybridoma technique to produce human monoclonal antibodies (Cole et al. Monoclonal Antibodies in Cancer Therapy (1985) Allen R. Bliss, Inc., pages 77-96), and screening of combinatorial antibody libraries (Huse et al., Science 246, 1275 (1989)). Hybridoma cells can be screened immunochemically for production of antibodies specifically reactive with the peptide and monoclonal antibodies isolated. The term antibody as used herein is intended to include fragments thereof which are also specifically reactive with a protein, or peptide thereof, having the biological activity of MRP. Antibodies can be fragmented using conventional techniques and the fragments screened for utility in the same manner as described above for whole antibodies. For example, F(ab')2 fragments can be generated by treating antibody with pepsin. The resulting F(ab')2 fragment can be treated to reduce disulfide bridges to produce Fab' fragments. When antibodies produced in non-human subjects are used therapeutically in humans, they are recognized to varying degrees as foreign and an immune response may be generated in the patient. One approach for minimizing or eliminating this problem, which is preferable to general immunosuppression, is to produce chimeric antibody derivatives, i.e., antibody molecules that combine a non-human animal variable region and a human constant region. Chimeric antibody molecules can include, for example, the antigen binding domain from an antibody of a mouse, rat, or other species, with human constant regions. A variety of approaches for making chimeric antibodies have been described and can be used to make chimeric antibodies containing the immunoglobulin variable region which recognizes the gene product of the novel B lymphocyte antigens of the invention. See, for example, Morrison et al., Proc. Natl. Acad. Sci. U.S.A. 81, 6851 (1985); Takeda et al., Nature 314, 452 (1985), Cabilly et al., U.S. Patent No. 4,816,567; Boss et al., U.S. Patent No. 4,816,397; Tanaguchi et al, European Patent Publication EP171496; European Patent Publication 0173494, United Kingdom Patent GB 2177096B. It is expected that such chimeric antibodies would be less immunogenic in a human subject than the corresponding non-chimeric antibody. For human therapeutic purposes the monoclonal or chimeric antibodies specifically reactive with a protein, or peptide thereof, having the biological activity of a MRP as described herein can be further humanized by producing human constant region chimeras, in which parts of the variable regions, especially the conserved framework regions of the antigen-binding domain, are of human origin and only the hypervariable regions are of non-human origin. Such altered immunoglobulin molecules may be made by any of several techniques known in the art, (e.g., Teng et al., Proc. Natl. Acad. Sci. U.S.A., 80, 7308-7312 (1983); Kozbor et al., Immunology Today, 4, 7279 (1983); Olsson et al., Meth. Enzymol., 92, 3-16 (1982)), and are preferably made according to the teachings of PCT Publication WO92/06193 or EP 0239400. Humanized antibodies can be commercially produced by, for example, Scotgen Limited, 2 Holly Road, Twickenham, Middlesex, Great Britain. Humanized antibodies which have reduced immunogenicity are preferred for immunotherapy in human subjects. Immunotherapy with a humanized antibody will likely reduce the necessity for any concomitant immunosuppression and may result in increased long term effectiveness for the treatment of chronic disease situations or situations requiring repeated antibody treatments. Another method of generating specific antibodies, or antibody fragments, reactive against protein, or peptide thereof, having the biological activity of a MRP is to screen expression libraries encoding immunoglobulin genes, or portions thereof, expressed in bacteria with peptides produced from the nucleic acid molecules of the present invention. For example, complete Fab fragments, VH regions and FV regions can be expressed in bacteria using phage expression libraries. See for example Ward et al., Nature 341, 544-546: (1989); Huse et al, Science 246, 1275-1281 (1989); and McCafferty et al. Nature 348, 552-554 (1990). Screening such libraries with, for example, a B7-2 peptide can identify imunoglobulin fragments reactive with B7-2. Alternatively, the SCID-hu mouse developed by Genpharm can be used to produce antibodies, or fragments thereof. The polyclonal, monoclonal or chimeric monoclonal antibodies can be used to detect the proteins of the invention, portions thereof or closely related isoforms in various biological materials, for example they can be used in an ELISA, radioimmunoassay or histochemical tests. Thus, the antibodies can be used to quantify the amount of an MRP protein of the invention, portions thereof or closely related isoforms in a sample in order to diagnose multidrug resistance, and to determine the role of MRP proteins in particular cellular events or pathological states, particularly its role in multidrug resistance. Using methods described hereinbefore, polyclonal, monoclonal antibodies, or chimeric monoclonal antibodies can be raised to nonconserved regions of MRP and used to distinguish MRP from closely related isoforms and other proteins that share a common conserved epitope. The polyclonal or monoclonal antibodies can be coupled to a detectable substance. The term "coupled" is used to mean that the detectable substance is physically linked to the antibody. Suitable detectable substances include various enzymes, prosthetic groups, fluorescent materials, luminescent materials and radioactive materials. Examples of suitable enzymes include horseradish peroxidase, alkaline phosphatase, β-galactosidase, or acetylcholinesterase; examples of suitable prosthetic group complexes include streptavidin/biotin and avidin/biotin; examples of suitable fluorescent materials include umbelliferone, fluorescein, fluorescein isothiocyanate, rhodamine, dichlorotriazinylamine fluorescein, dansyl chloride or phycoerythrin; an example of a luminescent material includes luminol; and examples of suitable radioactive material include 125 I, 131 1, 35 S or 3 H. The present invention allows the skilled artisan to prepare bispecific antibodies and tetrameric antibody complexes. Bispecific antibodies can be prepared by forming hybrid hybridomas. The hybrid hybridomas can be prepared using the procedures known in the art such as those disclosed in Staerz & Bevan, (PNAS (USA) 83: 1453, 1986 and Immunology Today, 7:241, 1986). In general, a hybrid hybridoma is formed by fusing a first cell line which produces a first monoclonal antibody which is capable of binding to a tumor cell expressing a protein of the invention and a second cell line which produces a second monoclonal antibody which is capable of binding to a detectable substance, or a substance having toxic or therapeutic activity. The bispecific antibodies can also be constructed by chemical means using procedures such as those described by Staerz et al., (Nature, 314:628, 1985) and Perez et al., (Nature 316:354, 1985). Bispecific monoclonal antibodies containing a variable region of an antibody, preferably a human antibody, specific for an MRP protein of the invention or portion thereof, a variable region of an antibody which is capable of binding to a detectable substance, or a substance having toxic or therapeutic activity and the constant regions of human immunoglobulins such as human IgGl, IgG2, IgG3 and IgG4 can also be constructed as described above. Bispecific chimeric monoclonal antibodies can also be constructed as described above. A tetrameric antibody complex can be prepared by preparing a first monoclonal antibody which is capable of binding to a tumor cell expressing a protein of the invention and a second monoclonal antibody which is capable of binding to a detectable substance or a substance having toxic or therapeutic activity. The first and second antibody are from a first animal species. The first and second antibody are reacted with an about equimolar amount of antibodies of a second animal species or Fab fragments thereof, which are directed against the Fc-fragments of the antibodies of the first animal species. The tetrameric complex formed is then isolated. (See U.S. Patent No. 4,868,109 to Lansdorp for a description of methods for preparing tetrameric antibody complexes). Examples of detectable substances are enzymes, such as horseradish peroxidase, alkaline phosphatase, glucose oxidase and galactosidase. Examples of substances having toxic activity are cytotoxic cells such as macrophages, neutrophils, eosinophils, NK cells, LAK cells, and large granular lymphocytes or substances which are toxic to tumor cells such as radionuclides, and toxins such as diptheria toxin and ricin or attenuated derivatives thereof. It will be appreciated that the antibody can be directed against the Fc receptor on cytotoxic cells. Examples of substances having therapeutic activity are chemotherapeutic agents such as carboplatin and methotrexate. Preferably, the chemotherapeutic agent is not a drug to which a protein having MRP activity confers resistance. The antibodies, bispecific antibodies and tetrameric antibody complexes of the invention directed against a substance having toxic or therapeutic activity coupled with the substance having toxic or therapeutic activity can be used to treat multidrug resistant tumors. Accordingly, the invention provides a composition comprising antibodies, bispecific antibodies or tetrameric antibody complexes in a pharmaceutically acceptable carrier. Preferably, the antibodies, bispecific antibodies or tetrameric antibody complexes are coupled to or capable of binding to a substance having toxic or therapeutic activity and to a tumor cell expressing a protein of the invention. The compositions of the invention are administered to subjects in a biologically compatible form suitable for pharmaceutical administration in vivo. By "biologically compatible form suitable for administration in vivo" is meant a form of the antibody to be administered in which any toxic effects are outweighed by the therapeutic effects of the antibody. The term subject is intended to include living organisms in which an immune response can be elicited, e.g., mammals. Examples of subjects include humans, dogs, cats, mice, rats, and transgenic species thereof. Administration of a therapeutically active amount of the therapeutic compositions of the present invention is defined as an amount effective, at dosages and for periods of time necessay to achieve the desired result. For example, a therapeutically active amount of an antibody reactive with an MRP protein of the invention may vary according to factors such as the disease state, age, sex, and weight of the individual, and the ability of antibody to elicit a desired response in the individual. Dosage regima may be adjusted to provide the optimum therapeutic response. For example, several divided doses may be administered daily or the dose may be proportionally reduced as indicated by the exigencies of the therapeutic situation. The active compound (e.g., antibody) may be administered in a convenient manner such as by injection (subcutaneous, intravenous, etc.), oral administration, inhalation, transdermal application, or rectal administration. Depending on the route of administration, the active compound may be coated in a material to protect the compound from the action of enzymes, acids and other natural conditions which may inactivate the compound. An antibody composition can be administered to a subject in an appropriate carrier or diluent, co-administered with enzyme inhibitors or in an appropriate carrier such as liposomes. The term "pharmaceutically acceptable carrier" as used herein is intended to include diluents such as saline and aqueous buffer solutions. To administer an antibody reactive with an MRP protein by other than parenteral administration, it may be necessary to coat the antibody with, or co-administer the antibody with, a material to prevent its inactivation. Enzyme inhibitors include pancreatic trypsin inhibitor, diisopropylfluorophosphate (DEP) and trasylol. Liposomes include water-in-oil-in-water emulsions as well as conventional liposomes (Strejan et al., (1984) J. Neuroimmunol 7:27). The active compound may also be administered parenterally or intraperitoneally. Dispersions can also be prepared in glycerol, liquid polyethylene glycols, and mixtures thereof and in oils. Under ordinary conditions of storage and use, these preparations may contain a preservative to prevent the growth of microorganisms. Pharmaceutical compositions suitable for injectable use include sterile aqueous solutions (where water soluble) or dispersions and sterile powders for the extemporaneous preparation of sterile injectable solutions or dispersion. In all cases, the composition must be sterile and must be fluid to the extent that easy syringability exists. It must be stable under the conditions of manufacture and storage and must be preserved against the contaminating action of microorganisms such as bacteria and fungi. The pharmaceutically acceptable carrier can be a solvent or dispersion medium containing, for example, water, ethanol, polyol (for example, glycerol, propylene glycol, and liquid polyetheylene glycol, and the like), and suitable mixtures thereof. The proper fluidity can be maintained, for example, by the use of a coating such as lecithin, by the maintenance of the required particle size in the case of dispersion and by the use of surfactants. Prevention of the action of microorganisms can be achieved by various antibacterial and antifungal agents, for example, parabens, chlorobutanol, phenol, asorbic acid, thimerosal, and the like. In many cases, it will be preferable to include isotonic agents, for example, sugars, polyalcohols such as manitol, sorbitol, sodium chloride in the composition. Prolonged absoφtion of the injectable compositions can be brought about by including in the composition an agent which delays absorption, for example, aluminum monostearate and gelatin. Sterile injectable solutions can be prepared by incoφorating active compound (e.g., antibody reactive against an MRP protein) in the required amount in an appropriate solvent with one or a combination of ingredients enumerated above, as required, followed by filtered sterilization. Generally, dispersions are prepared by incoφorating the active compound into a sterile vehicle which contains a basic dispersion medium and the required other ingredients from those enumerated above. In the case of sterile powders for the preparation of sterile injectable solutions, the preferred methods of preparation are vacuum drying and freeze-drying which yields a powder of the active ingredient (e.g., antibody) plus any additional desired ingredient from a previously sterile-filtered solution thereof. When the active compound is suitably protected, as described above, the composition may be orally administered, for example, with an inert diluent or an assimilable edible carrier. As used herein "pharmaceutically acceptable carrier" includes any and all solvents, dispersion media, coatings, antibacterial and antifungal agents, isotonic and absoφtion delaying agents, and the like. The use of such media and agents for pharmaceutically active substances is well known in the art. Except insofar as any conventional media or agent is incompatible with the active compound, use thereof in the therapeutic compositions is contemplated. Supplementary active compounds can also be incoφorated into the compositions. It is especially advantageous to formulate parenteral compositions in dosage unit form for ease of administration and uniformity of dosage. Dosage unit form as used herein refers to physically discrete units suited as unitary dosages for the mammalian subjects to be treated; each unit containing a predetermined quantity of active compound calculated to produce the desired therapeutic effect in association with the required pharmaceutical carrier. The specification for the dosage unit forms of the invention are dictated by and directly dependent on (a) the unique characteristics of the active compound and the particular therapeutic effect to be achieved, and (b) the limitations inherent in the art of compounding such an active compound for the therapeutic treatment of individuals. VII. Transgenic and Knockout Animals Nucleic acids which encode proteins having biological activity of MRP can be used to generate either transgenic animals or "knock out" animals which, in turn, are useful in the development and screening of therapeutically useful reagents. A transgenic animal (e.g., a mouse) is an animal having cells that contain a transgene, which transgene was introduced into the animal or an ancestor of the animal at a prenatal, e.g., an embryonic stage. A transgene is a DNA which is integrated into the genome of a cell from which a transgenic animal develops. In one embodiment, a human MRP cDNA, comprising the nucleotide sequence shown in SEQ ID NO: l,or an appropriate sequence thereof, can be used to clone a murine MRP gene in accordance with established techniques and the genomic nucleic acid used to generate transgenic animals that contain cells which express MRP protein. Methods for generating transgenic animals, particularly animals such as mice, have become conventional in the art and are described, for example, in U.S. Patent Nos. 4,736,866 and 4,870,009. In a preferred embodiment, plasmids containing recombinant molecules of the invention are microinjected into mouse embryos. In particular, the plasmids are microinjected into the male pronuclei of fertilized one-cell mouse eggs; the injected eggs are transferred to pseudo-pregnant foster females; and, the eggs in the foster females are allowed to develop to term. [Hogan, B. et al., (1986) A Laboratory Manual, Cold Spring Harbor, New York, Cold Spring Harbor Laboratory]. Alternatively, an embryonal stem cell line can be transfected with an expression vector containing nucleic acid encoding a protein having MRP activity and cells containing the nucleic acid can be used to form aggregation chimeras with embryos from a suitable recipient mouse strain. The chimeric embryos can then be implanted into a suitable pseudopregnant female mouse of the appropriate strain and the embryo brought to term. Progeny harbouring the transfected DNA in their germ cells can be used to breed uniformly transgenic mice. Typically, particular cells would be targeted for MRP transgene incoφoration by use of tissue specific enhancers operatively linked to the MRP-encoding gene. For example, promoters and/or enhancers which direct expression of a gene to which they are operatively linked preferentially in cardiac muscle cells can be used to create a transgenic animal which expresses an MRP protein preferentially in cardiac muscle tissue. Examples of suitable promoters and enhancers include those which regulate the expression of the genes for cardiac myosin and cardiac actin. Transgenic animals that include a copy of an MRP transgene introduced into the germ line of the animal at an embryonic stage can also be used to examine the effect of increased MRP expression in various tissues. The pattern and extent of expression of a recombinant molecule of the invention in a transgenic mouse is facilitated by fusing a reporter gene to the recombinant molecule such that both genes are co-transcribed to form a polycistronic mRNA. The reporter gene can be introduced into the recombinant molecule using conventional methods such as those described in Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual. Cold Spring Harbor Laboratory Press. Efficient expression of both cistrons of the polycistronic mRNA encoding the protein of the invention and the reporter protein can be achieved by inclusion of a known internal translational initiation sequence such as that present in poliovirus mRNA. The reporter gene should be under the control of the regulatory sequence of the recombinant molecule of the invention and the pattern and extent of expression of the gene encoding a protein of the invention can accordingly be determined by assaying for the phenotype of the reporter gene. Preferably the reporter gene codes for a phenotype not displayed by the host cell and the phenotype can be assayed quantitatively. Examples of suitable reporter genes include lacZ (β-galactosidase), neo (neomycin phosphotransferase), CAT (chloramphenicol acetyltransferase) dhfr (dihydrofolate reductase), aphlV (hygromycin phosphotransferase), lux (luciferase), uidA (β-glucuronidase). Preferably, the reporter gene is lacZ which codes for β- galactosidase. β-galactosidase can be assayed using the lactose analogue X-gal(5-bromo-4-chloro-3-indolyl-b-D-galactopyranoside) which is broken down by β-galactosidase to a product that is blue in color. (See for example Old R.W. & Primrose S.B., Principles of Gene Manipulation. An Introduction to Genetic Engineering, 4th ed. Oxford University Press at pages 63-66 for a discussion of procedures for screening for recombinants). Although experimental animals used in the preferred embodiment disclosed are mice, the invention should not be limited thereto. It can be desirable to use other species such as rats, hamsters and rabbits. The transgenic animals of the invention can be used to investigate the molecular basis of multidrug resistance. The transgenic animals of the invention can also be used to test substances for the ability to prevent, slow or reverse the development of multidrug resistance. A transgenic animal can be treated with the substance in parallel with an untreated control transgenic animal. Cells from the transgenic animals of the invention can be cultured using standard tissue culture techniques. In particular, cells carrying the recombinant molecule of the invention can be cultured and used to test substances for the ability to prevent, slow or reverse multidrug resistance. Additionally, the non-human homologues of genes encoding proteins having MRP activity can be used to construct an MRP "knock out" animal which has a defective or altered MRP gene. For example, a human MRP cDNA, comprising the nucleotide sequence shown in SEQ ID NO: 1, or an appropriate sequence thereof, can be used to clone a murine MRP gene in accordance with established techniques. A portion of the genomic MRP DNA (e.g., an exon) can be deleted or replaced with another gene, such as a gene encoding a selectable marker which can be used to monitor integration. The altered MRP DNA can then be transfected into an embryonal stem cell line. The altered MRP DNA will homologously recombine with the endogenous MRP gene in certain cells and clones containing the altered gene can be selected. Cells containing the altered gene are injected into a blastocyst of an animal, such as a mouse, to form aggregation chimeras as described for transgenic animals. Chimeric embryos are implanted as described above. Transmission of the altered gene into the germline of a resultant animal can be confirmed using standard techniques and the animal can be used to breed animals having an altered MRP gene in every cell. Accordingly, a knockout animal can be made which cannot express a functional MRP protein. Such a knockout animal can be used, for example, to test the effectiveness of a chemotherapeutic agent in the absence of an MRP multidrug resistance protein. VIII. Uses of the Invention The isolated nucleic acids of the invention are useful as molecular probes for use diagnostically to determine multidrug resistance of a tumor. As demonstrated in Example 1, multidrug resistance of certain tumor cell lines is associated with increased expression of cellular mRNA corresponding to the nucleotide sequence of SEQ ID NO: 1. Accordingly, the nucleic acids of the invention can be labelled with a detectable marker, such as a radioactive, fluorescent or biotinylated marker, and used in conventional dot blot, Northern hybridization or in situ hybridization procedures to probe mRNA molecules of total cellular or poly(A)+ RNAs from a biological sample, for instance cells of a tumor biopsy. The nucleic acid probes can be used to detect genes, preferably in human cells, that encode proteins related to or analogous to the MRP of the invention. Preferably, nucleic acid comprising the nucleotide sequence of the invention, or a segment thereof, can be used as a probe to identify DNA fragments comprising genes or parts of genes that are co-amplified with the gene of the invention and which reside within the same amplification unit, or amplicon, at the chromosomal location 16pl3.1. More specifically a nucleic acid of the invention can be used as a probe to screen human genomic DNA libraries constructed in cosmid or yeast artificial chromosome vectors, using procedures standard in the art, to define a contiguous segment of DNA that comprises the amplification unit detected in a multidrug resistant cell line such as H69AR. In this manner additional genes can be identified which also confer or contribute to the multidrug resistance phenotype of H69AR and other human cell lines yet to be examined but which are known to include the HeLa cell line J2c and HT1080 DR4 cell line. The antisense nucleic acids of the invention are useful for inhibiting expression of nucleic acids (e.g. mRNAs) encoding proteins having MRP activity, thereby decreasing expression of proteins having MRP activity. Since increased expression of proteins having MRP activity is associated with and can confer multidrug resistance on a cell, decreasing expression of such proteins can inhibit or reverse multidrug resistance of a cell into which the antisense nucleic acid has been introduced. Antisense nucleic acids can be introduced into a multidrug resistant cell in culture to inhibit MRP expression. One or more antisense nucleic acids, such as oligonucleotides, can be added to cells in culture media, typically at 200 μg/ml. A cultured multidrug resistant cell in which MRP expression is inhibited is useful for testing the efficacy of potential therapeutic agents. For example, MRP expression could be inhibited in a tumor cell line which expresses both MRP and P-glycoprotein to determine the contribution of MRP to an observed resistance or sensitivity of the cell to a particular therapeutic agent. The antisense nucleic acids of the invention, or oligonucleotides thereof, can also be used in gene therapy to correct or prevent multidrug resistance in a subject. For example, antisense sequences can be used to render multidrug resistant malignant cells sensitive to chemotherapeutic agents. Administration of antisense nucleic acids to a subject may be most effective when the antisense nucleic acid is contained in a recombinant expression vector which allows for continuous production of antisense RNA. Recombinant molecules comprising an antisense nucleic acid or oligonucleotides thereof, can be directly introduced into tissues, including lung tissue in vivo, using delivery vehicles such as liposomes, retroviral vectors, adenoviral vectors and DNA virus vectors. A delivery vehicle can be chosen which can be targeted to a cell of interest in the subject (e.g. a multidrug resistant tumor cell). Antisense nucleic acids can also be introduced into isolated cells, such as those of the hematopoietic system, ex vivo using viral vectors or physical techniques such as microinjection and electroporation or chemical methods such as coprecipitation and incoφoration of DNA into liposomes and such cells can be returned to the donor. Recombinant molecules can also be delivered in the form of an aerosol or by lavage. In the treatment of lung malignancies, antisense sequences can be directly delivered to lung tissue by an aerosol or by lavage. Accordingly, the invention provides a method for inhibiting multidrug resistance of a multidrug resistant cell by introducing into the multidrug resistant cell a nucleic acid which is antisense to a nucleic acid which encodes the protein shown in SEQ ID NO: 2. The isolated nucleic acids and antisense nucleic acids of the invention can be used to construct recombinant expression vectors as described previously. These recombinant expression vectors are then useful for making transformant host cells containing the recombinant expression vectors, for expressing proteins encoded by the nucleic acids of the invention, and for isolating proteins of the invention as described previously. The isolated nucleic acids and antisense nucleic acids of the invention can also be used to construct transgenic and knockout animals as described previously. As demonstrated in Examples 5 and 6, a recombinant expression vector containing a nucleic acid of the invention can be used to transfect a drug sensitive cell line to produce a protein in the cell which can confer multidrug resistance on the transfected cell line. Thus, the recombinant expression vectors of the invention are useful for conferring multidrug resistance on a drug sensitive cell. Accordingly, the invention provides a method for protecting a drug sensitive cell from cytotoxicity due to exposure to a drug by transfecting the cell with nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1 to confer drug resistance on the cell. In preferred embodiments, the drug sensitive cell is a cardiac muscle cell or a hematopoietic stem cell. The ability to confer drug resistance on a cell has important clinical applications. A major dose-limiting factor for chemotherapeutic agents is their cytotoxicity for normal cells in a patient as well as tumor cells. In patients with multi-drug resistant tumors, increasing the dosage of chemotherapeutic agents is prohibited by the toxicity of these agents for normal cells. In the case of anthracyclines, cardiotoxicity of the drugs can be a major clinical limitation. For chemotherapeutic drugs which target rapidly dividing cells, toxicity to hematopoietic cells can be a major clinical limitation. Additionally, neurotoxicity can occur. Protecting nonresistant nontumor cells from the effects of chemotherapeutic agents, by conferring on the cell multidrug resistance, thus has major clinical importance. The transformant host cells of the invention, and recombinant expression vectors used to make them, are useful for testing potential therapeutic agents for their effectiveness against multidrug resistant cells. These agents include agents which are themselves cytotoxic for multidrug resistant cells or which are chemosensitizers of other therapeutic agents. As used herein, the term "chemosensitizer" refers to a substance which can increase the efficacy of a therapeutic agent against a multidrug resistant cell and/or decrease the resistance of a multidrug resistant cell for a therapeutic agent. A method is provided for identifying a chemosensitizer of a therapeutic agent. The method involves incubating the therapeutic agent with a cell transfected with a nucleic acid which confers resistance to the therapeutic agent on the cell, both with and without a substance to be tested, determining resistance of the cell to the therapeutic agent when incubated with and without the substance to be tested and identifying a substance which is a chemosensitizer of the therapeutic agent by the ability of the substance to decrease the resistance of the cell to the therapeutic agent when incubated with the substance as compared to the resistance of the cell to the therapeutic agent when incubated without the substance. In a preferred embodiment, the nucleic acid is a recombinant expression vector containing nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1. Preferably, the cell into which the nucleic acid is transfected is drug sensitive prior to transfection so that the effects of a potential chemosensitizer are assessed in the presence of a single, isolated multidrug resistance-conferring protein. The cell used to test potential chemosensitizing substances can be a cell in culture, e.g. a transformant host cell of the invention, and the therapeutic agent and substance to be tested are incubated in culture with the cell. Alternatively, the cell can be a multidrug resistant cell in a transgenic animal, transgenic for a nucleic acid of the invention, and the therapeutic agent and substance to be tested are administered to the transgenic animal. Furthermore, the cell can be a cell in culture isolated from a multidrug resistant transgenic animal of the invention. The resistance of the cell for the therapeutic agent in the presence and absence of the potential therapeutic agent is assessed by determining the concentration of the therapeutic agent which is cytotoxic for the cell either in the presence or in the absence of the substance being tested. The invention provides a method for identifying a substance which is directly cytotoxic to a multidrug resistant cell involving incubating a substance to be tested with a cell transfected with a nucleic acid which confers multidrug resistance on the cell and determining the cytotoxicity of the substance for the cell. In a preferred embodiment, the nucleic acid is a recombinant expression vector containing nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1. Preferably, the cell into which the nucleic acid is transfected is drug sensitive prior to transfection so that the effects of a potential chemosensitizer are assessed in the presence of a single, isolated multidrug resistance-conferring protein. The cell used to test potential cytotoxic substances can be a cell in culture, e.g. a transformant host cell of the invention, and the substance to be tested is incubated in culture with the cell. Alternatively, the cell can be a multidrug resistant cell in a transgenic animal, transgenic for a nucleic acid of the invention and the substance to be tested is administered to the transgenic animal. Furthermore, the cell can be a cell in culture isolated from a multidrug resistant transgenic animal of the invention. Additionally, a multidrug resistant cell line such as H69AR, or an equivalent cell line, can be used in the same methods for identifying a chemosensitizer of a therapeutic agent or for identifying a substance which is directly cytotoxic to a multidrug resistant cell. The isolated proteins of the invention are useful for making antibodies reactive against proteins having MRP activity, as described previously. Alternatively, the antibodies of the invention can be used to isolate a protein of the invention by standard immunoaffinity techiques. Furthermore, the antibodies of the invention, including bispecific antibodies and tetrameric antibody complexes, are useful for diagnostic puφoses and for therapeutic puφoses. In one embodiment of the invention, antibodies labelled with a detectable substance, such as a fluorescent marker, an enzyme or a radioactive marker, can be used to identify multidrug resistant tumor cells in a tumor sample or in vivo. Tumor tissue removed from a patient can be used as the tumor sample. In order to prevent rumor samples from being degraded, the samples can be stored at temperatures below -20 ° C. A tissue section, for example, a freeze-dried or fresh frozen section of tumor tissue removed from a patient, can also be used as the tumor sample. The samples can be fixed and the appropriate method of fixation is chosen depending upon the type of labelling used for the antibodies. Alternatively, a cell membrane fraction can be separated from the tumor tissue removed from a patient and can be used as the tumor sample. Conventional methods such as differential or density gradient centrifugation can be used to separate out a membrane fraction. A multidrug resistant tumor cell is identified by incubating an antibody of the invention, for example a monoclonal antibody, with a tumor cell to be tested for multidrug resistance. Binding of the antibody to the tumor cell is indicative of the presence on the tumor cell of a protein having MRP activity. The level of antibody binding to the tumor cell can be compared to the level of antibody binding to a normal control cell, and increased binding of the antibody to the tumor cell as compared to the normal cell can be used as an indicator of multidrug resistance. Binding of the antibody to a cell (e.g. a tumor cell to be tested or a normal control cell) can be determined by detecting a detectable substance with which the antibody is labelled. The detectable substance may be directly coupled to the antibody, or alternatively, the detectable substance may be coupled to another molecule which can bind the antibody. For example, an antibody of the invention which has a rabbit Fc region (e.g. which was prepared by immunization of a rabbit) can be detected using a second antibody directed against the rabbit Fc region, wherein the second antibody is coupled to a detectable substance. A multidrug resistant tumor cell can be detected as described above in vitro in a tumor sample prepared as described above. For example, a tumor section on a microscope slide can be reacted with antibodies using standard immunohistochemistry techniques. Additionally, if a single cell suspension of tumor cells can be achieved, tumor cells can be reacted with antibody and analyzed by flow cytometry. Alternatively, a multidrug resistant tumor cell can be detected in vivo in a subject bearing a tumor. Labelled antibodies can be introduced into the subject and antibodies bound to the tumor can be detected. For example, the antibody can be labelled with a radioactive marker whose presence and location in a subject can be detected by standard imaging techniques. The antibodies of the invention, and compositions thereof, can also be used to inhibit the multidrug resistance of a multidrug resistant cell. The invention provides a method for inhibiting the multidrug resistance of a multidrug resistant cell comprising inhibiting activity of a protein comprising an amino acid shown in SEQ ID NO: 2 expressed by the multidrug resistant cell. Preferably, the multidrug resistant cell is a tumor cell. In preferred embodiments, the molecule which binds to a protein comprising an amino acid sequence shown in SEQ ID NO: 2 is a monoclonal antibody, bispecific antibody or tetrameric immunological complex of the invention. Multidrug resistance can be inhibited by interfering with the MRP activity of the protein to which the molecule binds. For example, the ability of an MRP protein to transport drugs may be impaired. Accordingly, any molecule which binds to a protein having MRP activity and whose binding inhibits the MRP activity of the protein are encompassed by invention. Isolated proteins of the invention, comprising the amino acid sequence shown in SEQ ID NO: 2, can be used to identify molecules, including and in addition to the antibodies of the invention, which can bind to a protein having MRP activity in a standard binding assay. A multidrug resistant cell in which multidrug resistance is inhibited, by inhibiting the activity of an MRP protein, can further be treated with a therapeutic agent to which the cell is no longer resistant or less resistant due to inhibition of MRP activity in order to kill the cell. Molecules which bind to a protein comprising an amino acid sequence shown in SEQ ID NO: 2 can also be used in a method for killing a multidrug resistant cell which expresses the protein. Preferably, the multidrug resistant cell is a tumor cell. Destruction of a multidrug resistant cells can be accomplished by labelling the molecule with a substance having toxic or therapeutic acitivity. The term "substance having toxic or therapeutic acitivity" as used herein is intended to include molecules whose action can destroy a cell, such as a radioactive isotope, a toxin (e.g. diptheria toxin or ricin), or a chemotherapeutic drug, as well as cells whose action can destroy a cell, such as a cytotoxic cell. The molecule binding to multidrug resistant cells can be directly coupled to a substance having toxic or therapeutic acitivity (e.g. a ricin-linked monoclonal antibody), or may be indirectly linked to the substance. For example, a bispecific antibody which is capable of crosslinking a tumor cell and a cytotoxic cell can be used, thereby facilitating lysis of the tumor cell. A bispecific antibody can crosslink a tumor cell and the cytotoxic cell by binding to the Fc receptors of cytotoxic cells. The compositions and methods of the invention can be used to treat patients with tumors displaying multidrug resistance particularly those displaying resistance to anthracyclines, epipodophyllotoxins, vinca alkaloids, and hydrophobic drugs. The methods of the invention for inhibiting the multidrug resistance of a tumor cell and for killing a multidrug resistant tumor cell can be applied to patients having a multidrug resistant tumor. The compositions and methods can be particularly useful in treating breast cancer, leukemias, fibrosarcomas, cervical cancer, and lung cancer in particular, small cell lung cancers and non small cell lung cancers. The invention also provides a diagnostic kit for identifying multidrug resistant tumor cells comprising a molecule which binds to a protein comprising an amino acid sequence shown in SEQ ID NO: 2 for incubation with a sample of tumor cells; means for detecting the molecule bound to the protein, unreacted protein or unbound molecule; means for determining the amount of protein in the sample; and means for comparing the amount of protein in the sample with a standard. Preferably, the molecule is a monoclonal antibody. Other molecules which can bind a protein having MRP activity can be used, including the bispecific antibodies and tetrameric antibody complexes of the invention. The diagnostic kit can also contain an instruction manual for use of the kit. The invention further provides a diagnostic kit for identifying multidrug resistant tumor cells comprising a nucleotide probe complementary to the sequence, or an oligonucleotide fragment thereof, shown in SEQ ID NO: 1 for hybridization with mRNA from a sample of tumor cells; means for detecting the nucleotide probe bound to mRNA; means for determining the amount of mRNA in the sample; and means for comparing the amount of mRNA in the sample with a standard. The diagnostic kit can also contain an instruction manual for use of the kit. The invention is further illustrated by the following examples. However, the examples are merely intended to illustrate embodiments of the invention and are not to be construed to limit the scope of the invention. The contents of all references and published patents and patent applications cited throughout this application are hereby incoφorated by reference. Example 1: ISOLATION OF cDNA SEQUENCES DERIVED FROM mRNAS OVEREXPRESSED IN H69AR CELLS As part of a search of proteins responsible for the multidrug resistance displayed by H69AR cells, a randomly primed cDNA library constructed from H69AR mRNA was screened using differential hybridization with total cDNA prepared from H69 and H69AR mRNA. One of the clones isolated contained a 2.8 kb cDNA insert and gave a particularly strong differential signal when analyzed on northern blots (Figure IA). The analysis of lμg of poly(A + )RNA from each cell line was carried out using standard procedures. Poly(A + )RNA was obtained using a Fast Track™ mRNA isolation kit (Invitrogen) and 1 μg was electrophoresed on a denaturing formaldehyde agarose gel. The RNA was transferred to nitrocellulose membrane and prehybridized in 50% formamide, 5X SSPE(1X = 150 mM NaCl, 10 mM NaH 2 P0 4 , 1 mM EDTA, pH 7.4), 2.5X Denhardt's solution (50X = 1% bovine serum albumin, 1% polyvinylpyrrolidone, 1% ficoll) and sheared, denatured herring testes DNA (100 μg/ml) for 4-16 hours at 42°C. The blot was probed with a 1.8 kb EcoRI fragment of MRP, labelled to a specific activity of >5 x 10 8 cmp/μg DNA with α-[32P]-dCTP (3000 Ci/mmol; Dupont/NEN) by the random priming method [A.P. Feinberg, B. Vogelstein, Analyt. Biochem. 132, 6 (1983)]. Hybridization was carried out for 16-20 hours at 42°C. Blots were washed three times in 0.1% SDS and 0.1X SSC(pH 7.0) for 30 minutes each at 52°C and then exposed to film. To estimate variation in RNA loading of the gel, the blot was reprobed with a 32 P-labelled β-actin cDNA (201pBv2.2)[H. Ueyama, H. Hamada, N. Battula, T. Kakunaga, Mol. Cell. Biol. 4, 1073 (1984)]. The autoradiograph shown in Figure IA is a 5 hour exposure with intensifying screens at -70°C. The size of the overexpressed mRNA in H69AR cells was estimated to be approximately 7 kb. Prolonged exposure of the film revealed low levels of this mRNA in H69 and H69PR cells. The concentration of this mRNA was increased 100 fold in H69AR cells relative to H69 cells. The level of this mRNA in H69PR, a drug sensitive revertant of H69AR, had decreased approximately 20-fold relative to that found in H69AR, further substantiating the correlation of overexpression of this particular mRNA with a multidrug resistance phenotype. Southern blot analyses of H69, H69AR and H69PR DNA indicated that the major mechanism underlying overexpression was gene amplification. Ten μg of each DNA was digested with EcoRI, electrophoresed through a 0.7% agarose gel and blotted onto a nitrocellulose membrane. The DNA was hybridized with a 1.8 kb EcoRI cDNA fragment of MRP, labelled by random priming with α-[ 32 P]-dCTP. The autoradiograph shown in Figure IB is a 6 hour exposure at -70°C. Based on the examination of several restriction digests and normalization of the amounts of DNA loaded, the MRP gene was amplified 40-50 fold in resistant H69AR cells and no differences in the copy number of the gene in H69 and H69PR cells were detected. The mRNA was also overexpressed 10-15 fold in a doxorubicin-selected multidrug resistant HeLa cell line that does not overexpress P-glycoprotein (Figure IC). S3 and J2c are drug sensitive and resistant HeLa cell lines obtained from the laboratory of Dr. R.M. Baker (Roswell Park Memorial Institute). Two μg of poly(A + )RNA from each cell line was electrophoresed, blotted and probed with MRP cDNA as described for Figure IA. The MRP and β-actin autoradiographs shown in Figure IC are 18 hour and 1 hour exposures, respectively, at -70°C. Southern blotting of DNA from S3 and J2c cells indicated that the MRP gene was amplified 10-15 fold in the resistant cell line. These findings provide further evidence of the association of elevated levels of this mRNA with multidrug resistance. The initial 2.8 kb cDNA clone was sequenced, allowing the isolation of overlapping clones by rescreening the H69AR cDNA library with synthetic oligonucleotides. A single, extended open reading frame of 1531 amino acids was defined encoding a protein designated as multidrug resistance associated protein (MRP). The translated GenBank and SwissProt databases were searched for similarities to MRP using the FASTA program. The search revealed that MRP is a novel member of the ATP-binding cassette (ABC) superfamily of transport systems. Members of this superfamily are involved in the energy dependent transport of a wide variety of molecules across cell membranes in both eukaryotes and prokaryotes. Included in this superfamily are the human multidrug transporter P-glycoprotein (MDR1) and the cystic fibrosis transmembrane conductance regulator (CFTR). Example 2: RELATIONSHIP OF MRP TO OTHER MEMBERS OF THE ABC TRANSPORTER SUPERFAMILY The relationship of MRP to the various members of the ABC transporter superfamily was examined using the PILEUP program from the Genetics Computer Group package (version 7) using a modified version of the progressive alignment method of Feng and Doolittle [J. Mol. Evol. 25, 351 (1987)]. A representative selection of a phylogenetically broad range of ABC proteins that are comprised of hydrophobic transmembrane regions followed by nucleotide binding regions, and whose sequences could be retrieved from GenBank and SwissProt databases, were included in this analysis. The analysis divides this family of proteins into two major subgroups (Figure 2). One of the major subgroups consists of the cluster containing MRP (Hum/MRP), the leishmania P-glycoprotein-related molecule (Lei/PgpA) and the CFTRs (Hum/CFTR, Bov/CFTR, Mus/CFTR and Squ/CFTR). The other subgroup consists of the P-glycoproteins, the MHC class Il-linked peptide transporters (Hum/Tap2, Mus/Tapl), the bacterial exporters (Eco/HlyB, Pas/LktB), the heterocyst differentiation protein (Ana/HetA), the malarial parasite transporter (Pfa/Mdrl) and the yeast mating factor exporter (Ysc/Ste6). The dendrogram in Figure 2 indicates that MRP is only distantly related to previously identified members of the ABC transporter superfamily. Although the analysis suggests that it is most closely related to Lei/PgpA, the similarity between MRP and Lei/PgpA resides predominantly in two regions, both containing signatures of nucleotide binding folds (NBFs) (Figure 3A). The alignment was generated using PILEUP as described in Figure 2. The MRP sequence shown was compiled from four overlapping lambda gtll cDNA clones. The alignment begins at a methionine residue in MRP that aligns with the initiator methionine of Lei/PgpA. The predicted initiator methionine of MRP itself is located 66 amino acids upstream. Identical and conserved amino acids are identified in Figure 3A by double and single dots, respectively. The Walker A and B motifs and the 'active transport' family signature that are characteristic of nucleotide binding folds (NBFs) of ABC transporters are indicated by single lines and denoted A, B, and C, respectively. The predicted transmembrane regions of each protein are indicated by double lines. The region in Lei/PgpA indicated by a dashed line has a mean hydrophobicity value approaching that of a transmembrane region. It has been proposed that the bipartite structure of P-glycoproteins reflects duplication of an ancestral gene that occurred prior to the evolutionary separation of animals and plants. However, comparison of the NH2- and COOH-terminal NBFs of MRP and Lei/PgpA revealed less similarity than typically found between the two corresponding regions of P-glycoproteins. To determine whether this was a common structural feature of MRP, Lei/PgpA and Hum/CFTR, their NH 2 - and COOH-terminal NBFs were aligned with each other and those of several P-glycoproteins. One such comparison using human P-glycoprotein (Hum/Mdrl) as an example is shown in Figure 3B. Shown in Figure 3B are the NH 2 -terminal (N) and COOH-terminal (C) halves of the deduced amino acid sequence of MRP corresponding to ltpgpA (Lei/PgpA) (amino acids 650-799 and 1303-1463), human CFTR (Hum/CFTR) (amino acids 441-590 and 1227-1385), and MDR1 (Hum/Mdrl)(amino acids 410-573 and 1053-1215). The sequences are presented as aligned by PILEUP. Reverse type indicates that 3 of 4 amino acids at that position are identical or conserved. The conserved motifs A, B, and C described in Figure 3A are underscored by a single line. The NH 2 -terminal NBFs of MRP, Hum/CFTR and Lei/PgpA share structural features that clearly distinguish them from the NH 2 -terminal NBF of Hum/Mdrl, particularly in the spacing of conserved motifs. This difference in spacing also contributes to the relatively low similarity between NH 2 and COOH-terminal NBFs in each of the three proteins. In addition, the COOH-terminal NBFs of MRP, Lei/PgpA and Hum/CFTR are more similar to each other than to either the COOH or NH-terminal NBFs of Hum/Mdrl. Similarity scores for the NH-terminal NBFs relative to MRP are: Lei/PgpA (0.93), Hum/CFTR (0.85) and Hum/Mdrl (0.60). Comparable COOH-terminal scores are Lei/PgpA (0.87), Hum/CFTR (0.84) and Hum/Mdrl (0.73). Similarity scores for NH- and COOH-terminal NBFs within the same protein are: MRP (0.61), Lei/PgpA (0.60), Hum/CFTR (0.62) and Hum/Mdrl (1.10). These observations, combined with the overall analysis shown in Figure 2, suggest that MRP, Lei/PgpA and CFTR evolved from a common ancestor containing both NH 2 - and COOH-terminal NBFs, which was distinct, or diverged from the ancestral gene of the P-glycoproteins prior to the animal/plant separation. Example 3: EXPRESSION OF MRP IN NORMAL TISSUES Despite knowledge of its structure and its ability to act as a drug efflux pump, the normal physiological role(s) of P-glycoprotein has not been elucidated. Some possible clues to its function have been provided by its distribution in normal tissues. P-glycoprotein is highly expressed in secretory organs and tissues, such as the adrenals, kidneys, lumenal epithelium of the colon and the murine gravid uterus. It has also been detected in the lung although this finding is variable. Based on the cell types in which it is expressed, it has been postulated that P-glycoprotein may be involved in steroid transport and/or protection against xenobiotics. Northern blot analyses of total RNA preparations from a range of human tissues shown that MRP is expressed at relatively high levels in lung, testis and peripheral blood mononuclear cells (PBMCs)(Figure 4). Lung and testis RNAs were obtained from Clontech Laboratories (Palo Alto, CA). PBMC RNA was prepared from cells isolated by centrifugation over Ficoll-Isopaque (specific gravity 1.078 g/ml; Pharmacia) of peripheral blood from healthy volunteers. Total RNAs from lung, testis and PBMCs (30 μg) and H69AR cells (10 μg) were analyzed as for Figure IA. The autoradiograph shown in Figure 4 is from a blot probed with a 0.9 kb EcoRI cDNA fragment of MRP and exposed for 38 hours for the normal tissue RNAs and for 24 hours for the H69AR RNA. The blot was stripped and reprobed with 32 P-labelled β-actin cDNA. The actin autoradiograph is a 24 hour exposure. MRP transcripts were below the level of detection in placenta, brain, kidney, salivary gland, uterus, liver and spleen. Example 4: MAPPING OF THE HUMAN MRP GENE The human CFTR and MDR1 genes have been mapped to chromosome 7 at bands q31 and q21, respectively. The possible evolutionary relationship of MRP to these proteins prompted examination of the possibility that the MRP gene may be linked to one of these previously identified loci. In situ hybridization of a 1.8 kb EcoRI fragment of MRP cDNA was performed using the method of Haφer and Saunders [Chromosoma 8, 431 (1981)]. Metaphase chromosomes on slides were denatured for 2 minutes at 70°C in 70% deionized formamide, 2X SSC and then dehydrated with ethanol. The hybridization mixture consisted of 50% deionized formamide, 10% dextran sulfate, 2X SSC (pH 6), 20 μg/ml sonicated salmon sperm DNA and 0.2 μg/ml 3 H-labelled MRP cDNA. The cDNA probe was labelled to a specific activity of 8.5 x 10 8 cpm/μg DNA with [ 3 H]-dTTP and [ 3 H]-dATP (New England Nuclear) using a Multiprime DNA Labelling System (Amersham) and denatured in the hybridization solution at 70°C for 5 minutes. Fifty μl of the probe solution was placed on each slide and incubated at 37°C overnight. After hybridization, the slides were washed in 50% deionized formamide, 2X SSC followed by 2X SSC (pH 7) and then dehydrated sequentially in ethanol. The slides were coated with Kodak NTB/2 emulsion and developed after exposure for 5 weeks at 4°C. Chromosomes were stained with a modified fluorescence, 0.25% Wright's stain procedure [C.C. Lin, P.N. Daper, M. Braekeleer, Cytogenet. Cell Genet. 39, 269 (1985)]. The positions of 200 silver grains directly over or touching well-banded metaphase chromosomes were recorded on the ISCN-derived idiogram of the human karyotype. A significant clustering of grains (40) was observed in the 16p region (p< 0.0001) and the peak of the distribution was at 16pl3.1, confirming that MRP was not linked to either CFTR or MDR genes. Approximately 160 metaphases were examined. These results are summarized in Figure 5. Example 5: EXPRESSION OF MRP IN A DRUG SENSITIVE CELL CONFERS DOXORUBICIN RESISTANCE ON THE CELL While increased concentrations of MRP and mRNA have been detected in multidrug resistant cell lines derived from a variety of tissues and several of these cell lines have also been shown to contain multiple copies of the MRP gene as a result of amplification and translocation of a region of chromosome 16 spanning the MRP gene at band pl3.1, it remained possible, in view of the multistep selection procedures used to derive the cell lines, that overexpression of the MRP gene is only one component of a set of alterations required to confer multidrug resistance. The ability of MRP alone to confer drug resistance on a drug sensitive cell line was determined by constructing an MRP expression vector, transfecting the expression vector into drug sensitive cells and assessing the relative drug resistance of the transfected cell populations. A DNA fragment corresponding to the complete coding region of MRP mRNA plus 86 nucleotides of 5' and 32 nucleotides of 3' untranslated sequence was assembled and transferred into the expression vector pRc/CMV under the control of the human cytomegalovirus promoter. A DNA fragment containing the complete coding region of MRP mRNA was assembled in the vector, pBluescript 11 KS + (Stratagene), using overlapping cDNA clones or PCR products generated from these clones. The fidelity of the MRP sequence was confirmed by DNA sequence analysis before moving the intact MRP fragment to the eukaryotic expression vector, pRc/CMV (Invitrogen). The integrity of the MRP fragment in the expression vector was assessed by detailed restriction mapping and DNA sequence analysis of the cloning sites. In the pRc/CMV vector, MRP expression is under the control of the enhancer/promoter sequence from the immediate early gene of human cytomegalovirus. The MRP transcript also contains part of the 3' untranslated region and the polyadenylation signal from bovine growth hormone mRNA which is provided by the vector. Thus, the pRc/CMV-MRP construct generates a transcript of 5.2 to 5.3 kb that includes the entire coding sequence (86 nucleotides of which are derived from MRP mRNA sequence), and approximately 250 nucleotides of 3' untranslated sequence (32 nucleotides of which are derived from MRP mRNA sequence). This vector also contains the bacterial aminoglycoside 3' phosphotransferase gene which confers resistance to geneticin (G418). HeLa cells were transfected with either the parental vector, or the vector containing the MRP coding region, using supercoiled DNA and a standard calcium phosphate transfection procedure. HeLa cells were transfected with the pRc/CMV vector or the vector containing the MRP coding sequence using a standard calcium phosphate transfection procedure [J. Sambrook, E.F. Firtsch, T. Maniatis, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY (1989)]. Approximately, 50,000 cells in each well of a 6-well tissue culture plate were exposed for 16 hours to 10 μg of supercoiled DNA in a calcium phosphate precipitate. After forty-eight hours, the growth medium was changed to include G418 at 200 μg/ml which selected for cells that expressed the neomycin resistance gene encoded by the pRc/CMV vector. Three weeks later, six independently transfected populations of cells were tested for resistance to doxorubicin using a tetrazolium salt microtiter plate assay (S.P.C. Cole, Cancer Chemother. Pharmacol.26, 250 (1990)). Those populations demonstrating increased relative resistance to the drug were expanded for testing for cross-resistance to other cytotoxic drugs, and analysis of MRP mRNA and protein levels. Poly(A) + RNA was isolated using the Micro-FastTrack RNA isolation kit (Invitrogen). The RNA was subjected to electrophoresis on a formaldehydeagarose gel and transferred to Zetaprobe membrane (Bio-Rad). The blots were hybridized with 32 P-labeled cDNA fragment probes complementary to the rnRNas for MRP, MDR1 [A.M. Van der Bliek, F. Baas, T. Ten Houte de Lange, P.M. Kooiman, T.Van der Velde Koerts, P. Borst, EMBO J. 6, 3325 (1987)], topoisomerase 11 α [T.D.Y. Chung, F.H. Drake, K.B. Tan, S.R. Per, S.T. Crooke, C.K. Mirabelli, Proc. Natl. Acad. Sci. U.S.A. 86 9431 (1989)], topoisomerase 11 β [ibid.], annexin 11 (S. P. C. Cole, M. J. Pinkoski, G. Bhardwaj, R. G. Deeley, Br. J. Cancer 65, 498 (1992)), and a region of the pRc/CMV vector encoding part of the 3' untranslated region and polyadenylation signal from the bovine growth hormone gene. Hybridization of the probes was quantified by densitometry of the autoradiographs on a Molecular Dynamics Computing Densitometer. Care was taken to compare autoradiographic exposures that were within the linear range of the film. In addition, variations in loading of RNA on the gels were estimated by probing blots with a 32 P-labeled glyceraldehyde-3-phosphate dehydrogenase (GAPDΗ) cDN A fragment (ATCC/NIH #57090), and by densitometric scanning of the ethidium bromide-stained ribosomal RNA bands on photographic negatives of the RNA gels. The relative amounts of MRP protein were assessed by immunoblot analysis of total cell extracts and membrane-enriched fractions. Cell pellets were resuspended at 5 X 10 7 cells/ml in buffer containing lOmM Tris-HCl, pH 7.4, lOmM KC1, 1.5 mM MgCl 2 , and protease inhibitors (2mM phenylmethylsulfonyfluoride, 50 μg/ml antipain, 2 μg/ml aprotinin, 200 μg/ml EDTA, 200 μg/ml benzamidine, 1 μg/ml pepstatin). After 10 min on ice, cells were homogenized with approximately 80 strokes of a Tenbroeck homogenizer. The homogenate was adjusted to 250 mM in sucrose before remaining intact cells and nuclei were removed by centrifugation at 800 X g at 4°C for 20 min. To prepare a membrane-enriched fraction, the supernatant was centrifuged at 100,000 X g at 4°C for 20 min in a Beckman TL-100 ultracentrifuge and the pellet resuspended in 10 mM Tris-HCl, pH 7.4, 125 mM sucrose, and the protease inhibitors listed above. For sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis and immunoblotting, appropriate amounts of protein were mixed 1:1 with solubilizing buffer (final concentration 4 M urea, 0.5% SDS, 50 mM dithiothreitol). Samples were loaded without heating onto a 7% resolving gel with a 4% stacking gel. Proteins were transferred to Immobilon-P PVDF membranes (Millipore) using 50 mM 3-(cyclohexylamino)- 1-propanesulfonate, pH 11.0. For detection of MRP, blots were incubated with an affinity-purified, rabbit polyclonal antibody raised against a synthetic peptide, the sequence of which was predicted from that of the cloned MRP cDNA and which is not found in any other known protein. Antibody binding was visualized with horseradish peroxidase-conjugated goat anti-rabbit IgG and enhanced chemiluminescence detection (Amersham). The affinity-purified anti-MRP antibody recognizes a glycosylated, integral membrane protein with an apparent molecular weight of 190 kilodaltons. In its deglycosylated form, the molecular weight of the protein decreases to 165- to 170 kilodaltons which is in agreement with the molecular weight of 171 kilodaltons predicted from the primary amino acid sequence of MRP. At this time, the level of G418 in the growth medium was increased to 400 or 800 μg/ml without any noticeable effect on the growth rate of cells transfected with either the parental vector or the vector containing the MRP coding sequence. Transfected populations have been grown continuously for up to four months in G418-containing medium without any change in the level of resistance to doxorubicin. Integration of these vectors into genomic DNA has the potential to alter the expression of endogenous genes that might adventitiously increase drug resistance. Consequently, chemotherapeutic drugs were not used as selecting agents. Populations of transfected cells were selected solely by their ability to grow in the presence of G418. Since cells overexpressing MRP do not display increased resistance to this antibiotic, variable levels of expression of MRP are to be expected in the transfected cell populations. The relative resistances to doxorubicin are shown for two examples of G418 resistant cell populations transfected with the MRP expression vector (T2 and T5), as well as untransfected HeLa cells and a population transfected with the parental vector (Cl) (Fig. 6). Key: HeLa cells (°); HeLa cells transfected with the expression vector pRc/CMV (Cl, °); HeLa cells transfected with the vector containing the MRP coding sequence (T2, □; T5, •*); and a clone isolated from the doxorubicin-resistant transfected T5 cells shown (ΔT5-5). Each point represents the mean of triplicate determinations in a single experiment and standard deviations were <5%. Similar results were obtained in three additional experiments. The IC 50 is indicated on the figure and is defined as the concentration of doxorubicin required to decrease by 50% the values obtained with untreated cells. In the examples shown, one of the populations transfected with the MRP expression vector (T2) displayed little change in doxorubicin resistance while resistance of the other (T5) was increased 15-fold. In addition, several clones from the resistant population were grown in the presence of G418 and their degree of doxorubicin resistance determined. Dose response curves for two of the transfectants (T2, T5) and for one of the clones (T5-5) were then compared to determine whether their resistance to doxorubicin correlated with the concentrations of MRP mRNA. The MRP mRNA produced from the expression vector has a predicted length of 5.2 to 5.3 kb including a poly(A) tail, thus allowing it to be distinguished from the longer, endogenous MRP mRNA by Northern analysis. A blot of poly(A) + RNA from the cell populations shown in Fig. 6 that was hybridized with a cloned cDNA probe corresponding to part of the MRP coding sequence, revealed a relatively abundant mRNA of approximately 5.3 Kb in the resistant transfectants and low levels of the endogenous MRP mRNA (Fig. 7A). The relative concentration of the 5.3 kb mRNA is 70- to 80-fold and 20- to 30- fold higher in the resistant cell population (T5) and clone (T5-5), respectively, than that of endogenous MRP mRNA present in the control population (Cl). Relative levels of mRNAs were determined by densitometry and normalization to the levels of GAPDH mRNA. Expression of the 5.3 kb MRP mRNA in the transfected cell population which showed little change in resistance (T2) was only approximately half that of endogenous MRP mRNA. Similar RNA blots were also probed with a DNA fragment from the pRc/CMV plasmid that forms part of the 3' untranslated region of the vector encoded MRP mRNA. This probe hybridized only with the 5.3 kb MRP mRNA, confirming that it was transcribed from the vector and did not result from the increased expression of an endogenous MRP-related gene (Fig. 7B). Thus in cells transfected with the MRP expression vector the relative level of drug resistance increases with the concentration of MRP mRNA. The concentration of endogenous MRP mRNA in the multidrug resistant H69AR cells (labeled AR in the figures) is approximately 100-fold higher than in the H69 parental cells (labeled H69 in the figures) and the relative resistances of the two cell lines to doxorubicin also differ by 50- to 100-fold. Vector encoded MRP mRNA levels in the T5 HeLa cell population are 70- to 80-fold higher than endogenous MRP mRNA levels in the parental cells. However, drug resistance is increased only 15-fold. To investigate why the relative increase in drug resistance was lower in the transfectants than in H69AR cells, we compared the levels of MRP mRNA and protein in the two different cell types. Northern analysis revealed that the levels of endogenous MRP mRNA in H69 cells and HeLA cells transfected with the pRc/CMV parental vector were similar. The relative abundance of vector encoded MRP mRNA in the drug resistant transfectant cell population (T5) was also comparable to that of endogenous MRP mRNA in H69AR cells (Fig. 7C). However, a protein blot with affinity purified anti-MRP antibody indicated that the level of protein in the T5 HeLa cell transfectants was 5- to 8-fold lower than in H69AR cells (Fig. 7C). These findings are consistent with the 15-fold increase in resistance observed in the transfected T5 cells compared to the 50- to 100-fold increase in H69AR cells. The lower level of protein in the transfected cells is most likely attributable to a difference in translational efficiency between the vector encoded and endogenous MRP mRNAs, although a difference in rates of degradation of the protein between the two cell types cannot be excluded. Since H69AR cells were obtained by multistep selection, it is possible that additional alterations have occurred which may, either independently or in concert with MRP, influence their degree of resistance to some drugs. H69AR cells have been shown to have decreased levels of topoisomerase 11 a and β mRNA and protein which could enhance their resistance to anthracyclines and epipodophyllotoxins. They have also been shown to overexpress annexin 11 which may affect the trafficking of membrane proteins. CD. Evans, S. E. L. Mirski, M. K. Danks, W. T. Beck, S. P. C. Cole, Proc. Amer. Assoc. Cancer Res. 33, 2694 (1992). Annexin 11 has been shown to be involved in formation of fusogenic vesicles and in exocytosis. S. P. C. Cole, M. J. Pinkoski, G. Bhardwaj, R. G. Deeley, Br. J. Cancer 65, 498 (1992). It is unknown to what extent these additional changes influence the degree of resistance of H69AR cells or whether they are linked in any way to overexpression of MRP. However, overexpression of MRP in the transfected cells does not alter the levels of mRNAs specifying either topoisomerase 11 isoform (Fig. 8A) or annexin 11 (Fig. 8B), nor do the transfected HeLa cells display any alterations in the level of Mdrl mRNA. These observations strongly support the conclusion that increased resistance to doxorubicin in the transfected cells is directly attributable to overexpression of MRP. Example 6: EXPRESSION OF MRP IN A DRUG SENSITIVE CELL CONFERS MULTIDRUG RESISTANCE ON THE CELL To determine whether the increased doxorubicin resistance of transfected cells was accompanied by increased resistance to other classes of chemotherapeutic drugs, the cells were tested for cross-resistance to vincristine (a Vinca alkaloid), VP-16 (an epipodophyllotoxin) and cisplatin (Fig. 9). Cytotoxicity assays were performed on untransfected HeLa cells (•), HeLa cells transfected with the expression vector pRc/CMV (Cl, °), HeLa cells transfected with the expression vector pRc/CMV-MRP and maintained in G418 at 400 μg/ml for 4 months (T5, ■*), and T5 cells maintained at 800 μg/ml G418 for 1 month (T5-800/1, Δ) and 3 months (T5-800/3, D). Each point represents the mean of triplicate determinations in a single experiment and standard deviations were < 5%. Similar results with vincristine and VP-16 were obtained in two to three additional experiments. The IC 50 * S of the various cell lines are indicated on the figure. Dose response curves for several independently propagated cultures of MRP transfectants indicate that they are approximately 25-fold and 5- to 10-fold resistant to vincristine and VP-16, respectively, relative to untransfected HeLa cells or cells transfected with parental vector (Cl). The transfectants showed no increase in cisplatin resistance which is consistent with the pharmacological phenotype of H69AR cells and which is also characteristic of cells that overexpress P-glycoprotein. These results demonstrate for the first time that this phenotype can be conferred by a member of the ABC superfamily of transporters that is structurally very different from the P-glycoproteins. Example 7: PREPARATION OF ANTI-MRP ANTIBODIES AND USE THEREOF IN IMMUNOPRECIPITATION AND IMMUNOBLOTTING EXPERIMENTS MRP is encoded by a mRNA of approximately 6.5 kb with an extended open reading frame of 1531 amino acids. The protein is predicted to contain two nucleotide binding folds (NBFs) and 12 transmembrane regions, divided 8 and 4 between the NH2 and COOH-proximal halves of the molecule, respectively. To confirm that a protein of the predicted size and sequence is overexpressed in resistant H69AR cells, polyclonal antibodies were prepared against synthetic peptides based on the deduced amino acid sequence of MRP and used in immunoanalyses. One peptide of sequence AELQKAEAKKEE was selected from the highly divergent cytoplasmic linker domain of MRP (MRP-L, position 932-943) while the second peptide (GENLSVGQRQLVCLA) was chosen from the second nucleotide binding domain of MRP (MRP-2, position 1427-1441). Both peptides were synthesized on Ultrasyn D resin for direct immunization by the Biotechnology Service Centre at the Hospital for Sick Children (Toronto, Ont.). Approximately 400 μg of bound peptide was resuspended in distilled water and sonicated. The resulting suspension was emulsified in an equal volume of complete Freund's adjuvant (Difco) and injected s.c. at four sites in 3-month old female New Zealand White rabbits. At 2- to 3-week intervals, the same amount of immunogen emulsified in incomplete Freund's adjuvant was injected s.c. Rabbits were bled by arterial puncture beginning 2 weeks following the third immunization and their sera were tested for the presence of antibodies by an enzyme-linked immunosorbent assay (ELISA) and by immunoblotting. Rabbit antisera obtained after immunization with peptide MRP-L that were positive by ELISA or western blotting were concentrated by ammonium sulfate precipitation and purified by affinity chromatography. Affinity columns were constructed by coupling the MRP-L peptide to CNBr-activated Sepharose (5 μmole peptide/ml gel) according to the instructions of the supplier (Pharmacia LKB Biotechnology Inc.) followed by extensive washing with 10 mM Tris, pH 7.5. The ammonium sulfate precipitate was dissolved in phosphate-buffered saline, dialyzed extensively against the same buffer and then applied to the prewashed affinity column. The loaded column was washed first with 10 mM Tris pH 7.5 followed by 10 mM Tris, pH 7.5, 0.5 M NaCl before eluting the antibody with 0.1 M glycine, pH 2.5. Fractions were neutralized in collection tubes containing 1 M Tris, pH 8.0. The desired fractions were pooled, dialyzed extensively against phosphate-buffered saline and concentrated by Amicon concentrators/filtration. The final protein concentration of the purified antibody was adjusted to 0.7-1.5 mg/ml. Rabbit antisera obtained after immunization with peptide MRP-2 were used without further purification. ELISA positive antisera from these rabbits were used in immunoblot analyses. Polyacrylamide gel electrophoresis was carried out by the method of Laemmli with a 5 % or 7 % separating gel and a 4% stacking gel. Samples were diluted 1:1 in solubilizing buffer to a final concentration of 4 M urea, 0.5% SDS, 50 mM DTT and loaded on the gels without heating. For immunnblotting, proteins were transferred after gel electrophoresis to Immobilon-P PVDF membranes (Millipore, Mississauga. Ont.) using 50 mM CAPS, pH 11.0. Blots were incubated for 1 h in blocking solution (5 % normal goat serum/5% HyClone serum/1% BSA) in TBS-T (10 mM Tris, pH 7.5, 0.05 % Tween 20, 150 mM NaCl). Anti-MRP antibodies were added directly to the blocking solution and incubated for 2 h. The blot was washed 3 x 5 min in TBS-T and goat anti-rabbit IgG horseradish peroxidase-conjugate [affinity purified F(ab')2 fragment (Jackson ImmunoResearch) or whole molecule (ICN Biomedicals)] diluted in blocking buffer added. After a 1 h incubation, the blot was washed 5 x 5 min in TBS-T, and antibody binding detected by ECL (Amersham, UK) and exposure on Kodak XOMAT film. The antisera detected a 190 kD protein in resistant H69AR cells which was not detectable in sensitive H69 and revertant H69PR cells. The antisera were also used in immunoprecipitation experiments using cell membrane preparations of cells metabolically labelled with ^S-methionine. Cells were cultured in 50 μCi/ml 35 Smethionine (Tran ^S-label; cell labelling grade; specific activity, 710 Ci/mmol) (Dupont NEN) overnight in methionine-deficient RPMI 1640 medium (Sigma) or with 500 μCi/ml 32 P-orthophosphoric acid (Carrier free, 500 mCi/ml) (Dupont NEN) in phosphate-deficient RPMI 1640 medium (ICN) for 4 h. Crude radiolabelled 100,000 x g membrane-enriched fractions were prepared and immunoprecipitated as follows. Frozen or fresh cells (50 x 10 6 /ml) were suspended in 10 mM Tris-HCl, pH 7.4 containing 10 mM KCl, 1.5 mM MgCl 2 with protease inhibitors (2 mM phenylmethylsulfonylfluoride, 50 μg/ml antipain, 2 μg/ml aprotinin, 200 μg/ml EDTA, 200 μg/ml benzamidine, 0.5 μg/ml leupeptin, 1 μg/ml pepstatin) and 0.025 mg/ml RNase A and 0.05 mg/ml DNase 1. After 10 min., the suspension was homogenized in a chilled Tenbroeck homogenizer with 80 strokes of the pestle. The homogenate was then centrifuged at 800 x g at 4°C for 15 min. to remove nuclei and remaining intact cells. A membrane-enriched fraction was prepared by ultracentrifugation of the supernatant at 100,000 x g at 4°C for 20 min. The pellets were resuspended in 10 mM Tris HC1, pH 7.6 with 125 mM sucrose and protease inhibitors as above. Protein concentrations were determined by the Peterson modification of the Lowry assay and aliquots were stored at -80°C. Proteins were solubilized in 1% CHAPS, 100 mM KCl, 50 mM Tris-HCl, pH 7.5, at a detergent to protein ratio of 20:1 for 1 h at 4°C with frequent vortexing followed by centrifugation at 100,000 x g for 20 min using a T100.3 rotor in a Beckman Ultracentrifuge. The supernatant (whatever percentage of protein is solubilized from an initial 40 μg of membrane protein) was incubated with affinity purified MRP-L antisera (25 mg solubilized in 1% CHAPS, 100 mM KCl, 50 mM Tris-HCl, pH 7.5) overnight at 4°C. The samples were made up 700 μl with 1 % CHAPS buffer then incubated with 50 μl (10% w/v) Protein A Sepharose C1-4B (Pharmacia) for 3 h at 4°C with gentle rocking. The samples were centrifuged for 10 sec at 14,000 x g and sequentially washed for 5 min with 1 ml each of Buffer 1 (10 mM Tris-HCl, pH 8.0, 0.5 mM NaCl, 0.5% Nonidet P-40, 0.05% SDS), Buffer 2 (10 mM Tris-HCl, pH 8.0, 0.15 M NaCl, 0.5% Nonidet P-40, 0.05% SDS, 0.5% deoxycholate) and Buffer 3 (10 mM Tris-HCl, pH 8.0, 0.05% SDS). The washed beads were incubated with 100 μl of 4 M urea, 0.5% SDS, 50 mM DTT for 1 h at room temperature with frequent vortexing. The samples were centrifuged and the supernatants analyzed on 7% polyacrylamide gels. The gels were fixed in isopropanokwateπacetic acid (25:65:10) for 30 min followed by the addition of the fluorographic reagent Amplify (Amersham). The gels were dried and then exposed to film overnight at -80°C. A 190 kD protein was detectable by immunoprecipitation of membrane-associated proteins from ^S-methionine labelled H69AR cells with the immunoreactive antisera. The apparent molecular weight of the immunodetectable 190 kD protein in the H69AR cell membranes is approximately 20 kD greater than the predicted 171 kD molecular weight of MRP based upon the deduced primary amino acid sequence. However, analysis of the MRP sequence indicates the presence of three potential N- glycosylation sites in regions predicted to be asymmetrically distributed about a membrane bilayer. To determine whether or not the 190 kD protein was N- glycosylated, two sets of experiments were carried out. First, resistant H69AR cells were grown in the presence of tunicamycin, a potent inhibitor of N-linked glycosylation. N-linked glycosylation was inhibited in H69AR cells by culturing in 15 μg/ml tunicamycin (Sigma) for 24 h. Treated cells were washed twice with phosphate-buffered saline and then whole cell lysates were prepared by homogenization in lysis buffer (20 mM Tris HC1, pH 7.5, 20 mM KCl, 3 mM MgC12. 0.5 mg/ml DNase 1, 0.25 mg/ml RNase A) with protease inhibitors as described above. Polyacrylamide gel eletrophoresis and immunoblotting of the whole cell lysates were carried out as before. In the second approach, H69AR 100,000 x g membranes were incubated with the deglycosylase PNGase F. Membrane-enriched fractions (200 μg protein) were diluted to a final concentration of 1 μg/ml in 50 mM Na phosphate buffer, pH 7.5, containing 25,000 NEB units PNGase F (New England Biolabs). After 8 h at 37°C, an additional 25,000 NEB units PNGase F was added followed by incubation overnight at 37°C. Sample buffer was added directly and SDS-PAGE and immunoblotting carried out as before. In both cases, a 170 kD protein was detected by immunoblot analyses which correlates well with the 171 kD predicted molecular weight of MRP. To confirm that MRP is an ATP-binding protein, as suggested by the presence of ATP-binding signature motifs, membranes from resistant H69AR and sensitive H69 cells were photolabelled with 32 P-8-azido ATP. Crude membrane- enriched fractions were resuspended at 1 μg/μl protein in 10 mM Tris-HCl, pH 7.6, buffer containing 1 mM MgCl 2 and protease inhibitors as described above. After the addition of 3-4 μCi 32P-8-azido-ATP (specific activity 2-10 Ci/mmol; ICN Biomedical, Mississauga, Ont.), incubation on ice was continued for 1-5 min. The azido-ATP was cross-linked to the protein on ice by irradiation at 366 nm about 10 cm from the light source for 8 min. using a Stratalinker set at 1100 μW. The labelled proteins were stored at -80°C until polyacrylamide gel electrophoresis or immunoprecipitations were carried out. Specificity of the labelling was confirmed by competition with cold excess ATP (Boehringer Mannheim, Laval, Que.) which was added to the membrane preparations prior to the addition of 32 P-8-azido-ATP. These studies revealed strong, specific labelling of a 190 kD protein in membranes from the H69AR cells that was not detected in drug sensitive H69 cells. Our results indicate that in H69AR cells, the MRP gene encodes an N-glycosylated ATP-binding protein of 190 kD. Forming part of the present disclosure is the appended Sequence Listing for the multidrug resistance protein of the present invention. EQUIVALENTS Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents of the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims. SEQUENCE LISTING (1) GENERAL INFORMATION: (i) APPLICANT: Cole, Susan P.C. Deeley, Roger G. (ii) TITLE OF INVENTION: MULTIDRUG RESISTANCE PROTEIN (iii) NUMBER OF SEQUENCES: 2 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: LAHIVE & COCKFIELD (B) STREET: 60 STATE STREET, SUITE 510 (C) CITY: BOSTON (D) STATE: MASSACHUSETTS (E) COUNTRY: USA (F) ZIP: 02109 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: PatentIn Release #1.0, Version #1.25 (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: US 000000 (B) FILING DATE: -0CT-1993 (C) CLASSIFICATION: (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: (B) REGISTRATION NUMBER: 000000 (C) REFERENCE/DOCKET NUMBER: PQI-002 (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: (617) 227-7400 (B) TELEFAX: (617) 227-5149 (2) INFORMATION FOR SEQ ID NO:1 : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 5011 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (ix) FEATURE: (A) NAME/KEY: CDS (B) LOCATION: 196..47! 61 SUBSTITUTE SHEET (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1 : CAGGCGGCGT TGCGGCCCCG GCCCCGGCTC CCTGCGCCGC CGCCGCCGCC GCCGCCGCCG 60 CCGCCGCCGC CGCCGCCAGC GCTAGCGCCA GCAGCCGGGC CCGATCACCC GCCGCCCGGT 120 GCCCGCCGCC GCCCGCGCCA GCAACCGGGC CCGATCACCC GCCGCCCGGT GCCCGCCGCC 180 GCCCGCGCCA CCGGC ATG GCG CTC CGG GGC TTC TGC AGC GCC GAT GGC TCC 231 Met Ala Leu Arg Gly Phe Cys Ser Ala Asp Gly Ser 1 5 10 GAC CCG CTC TGG GAC TGG AAT GTC ACG TGG AAT ACC AGC AAC CCC GAC 279 Asp Pro Leu Trp Asp Trp Asn Val Thr Trp Asn Thr Ser Asn Pro Asp 15 20 25 TTC ACC AAG TGC TTT CAG AAC ACG GTC CTC GTG TGG GTG CCT TGT TTT 327 Phe Thr Lys Cys Phe Gin Asn Thr Val Leu Val Trp Val Pro Cys Phe 30 35 40 TAC CTC TGG GCC TGT TTC CCC TTC TAC TTC CTC TAT CTC TCC CGA CAT 375 Tyr Leu Trp Ala Cys Phe Pro Phe Tyr Phe Leu Tyr Leu Ser Arg His 45 50 55 60 GAC CGA GGC TAC ATT CAG ATG ACA CCT CTC AAC AAA ACC AAA ACT GCC 423 Asp Arg Gly Tyr lie Gin Met Thr Pro Leu Asn Lys Thr Lys Thr Ala 65 70 75 TTG GGA TTT TTG CTG TGG ATC GTC TGC TGG GCA GAC CTC TTC TAC TCT 471 Leu Gly Phe Leu Leu Trp lie Val Cys Trp Ala Asp Leu Phe Tyr Ser 80 85 90 TTC TGG GAA AGA AGT CGG GGC ATA TTC CTG GCC CCA GTG TTT CTG GTC 519 Phe Trp Glu Arg Ser Arg Gly lie Phe Leu Ala Pro Val Phe Leu Val 95 100 105 AGC CCA ACT CTC TTG GGC ATC ACC ACG CTG CTT GCT ACC TTT TTA ATT 567 Ser Pro Thr Leu Leu Gly lie Thr Thr Leu Leu Ala Thr Phe Leu lie 110 115 120 CAG CTG GAG AGG AGG AAG GGA GTT CAG TCT TCA GGG ATC ATG CTC ACT 615 Gin Leu Glu Arg Arg Lys Gly Val Gin Ser Ser Gly lie Met Leu Thr 125 130 135 140 TTC TGG CTG GTA GCC CTA GTG TGT GCC CTA GCC ATC CTG AGA TCC AAA 663 Phe Trp Leu Val Ala Leu Val Cys Ala Leu Ala lie Leu Arg Ser Lys 145 150 155 ATT ATG ACA GCC TTA AAA GAG GAT GCC CAG GTG GAC CTG TTT CGT GAC 711 lie Met Thr Ala Leu Lys Glu Asp Ala Gin Val Asp Leu Phe Arg Asp 160 165 170 ATC ACT TTC TAC GTC TAC TTT TCC CTC TTA CTC ATT CAG CTC GTC TTG 759 lie Thr Phe Tyr Val Tyr Phe Ser Leu Leu Leu lie Gin Leu Val Leu 175 180 185 62 SUBSTITUTE SHEET TCC TGT TTC TCA GAT CGC TCA CCC CTG TTC TCG GAA ACC ATC CAC GAC 807 Ser Cys Phe Ser Asp Arg Ser Pro Leu Phe Ser Glu Thr lie His Asp 190 195 200 CCT AAT CCC TGC CCA GAG TCC AGC GCT TCC TTC CTG TCG AGG ATC ACC 855 Pro Asn Pro Cys Pro Glu Ser Ser Ala Ser Phe Leu Ser Arg lie Thr 205 210 215 220 TTC TGG TGG ATC ACA GGG TTG ATT GTC CGG GGC TAC CGC CAG CCC CTG 903 Phe Trp Trp lie Thr Gly Leu lie Val Arg Gly Tyr Arg Gin Pro Leu 225 230 235 GAG GGC AGT GAC CTC TGG TCC TTA AAC AAG GAG GAC ACG TCG GAA CAA 951 Glu Gly Ser Asp Leu Trp Ser Leu Asn Lys Glu Asp Thr Ser Glu Gin 240 245 250 GTC GTG CCT GTT TTG GTA AAG AAC TGG AAG AAG GAA TGC GCC AAG ACT 999 Val Val Pro Val Leu Val Lys Asn Trp Lys Lys Glu Cys Ala Lys Thr 255 260 265 AGG AAG CAG CCG GTG AAG GTT GTG TAC TCC TCC AAG GAT CCT GCC CAG 1047 Arg Lys Gin Pro Val Lys Val Val Tyr Ser Ser Lys Asp Pro Ala Gin 270 275 280 CCG AAA GAG AGT TCC AAG GTG GAT GCG AAT GAG GAG GTG GAG GCT TTG 1095 Pro Lys Glu Ser Ser Lys Val Asp Ala Asn Glu Glu Val Glu Ala Leu 285 290 295 300 ATC GTC AAG TCC CCA CAG AAG GAG TGG AAC CCC TCT CTG TTT AAG GTG 1143 lie Val Lys Ser Pro Gin Lys Glu Trp Asn Pro Ser Leu Phe Lys Val 305 310 315 TTA TAC AAG ACC TTT GGG CCC TAC TTC CTC ATG AGC TTC TTC TTC AAG 1191 Leu Tyr Lys Thr Phe Gly Pro Tyr Phe Leu Met Ser Phe Phe Phe Lys 320 325 330 GCC ATC CAC GAC CTG ATG ATG TTT TCC GGG CCG CAG ATC TTA AAG TTG 1239 Ala lie His Asp Leu Met Met Phe Ser Gly Pro Gin lie Leu Lys Leu 335 340 345 CTC ATC AAG TTC GTG AAT GAC ACG AAG GCC CCA GAC TGG CAG GGC TAC 1287 Leu lie Lys Phe Val Asn Asp Thr Lys Ala Pro Asp Trp Gin Gly Tyr 350 355 360 TTC TAC ACC GTG CTG CTG TTT GTC ACT GCC TGC CTG CAG ACC CTC GTG 1335 Phe Tyr Thr Val Leu Leu Phe Val Thr Ala Cys Leu Gin Thr Leu Val 365 370 375 380 CTG CAC CAG TAC TTC CAC ATC TGC TTC GTC AGT GGC ATG AGG ATC AAG 1383 Leu His Gin Tyr Phe His lie Cys Phe Val Ser Gly Met Arg lie Lys 385 390 395 ACC GCT GTC ATT GGG GCT GTC TAT CGG AAG GCC CTG GTG ATC ACC AAT 1431 Thr Ala Val lie Gly Ala Val Tyr Arg Lys Ala Leu Val lie Thr Asn 400 405 410 TCA GCC AGA AAA TCC TCC ACG GTC GGG GAG ATT GTC AAC CTC ATG TCT 1479 Ser Ala Arg Lys Ser Ser Thr Val Gly Glu lie Val Asn Leu Met Ser 415 420 425 63 GTG GAC GCT CAG AGG TTC ATG GAC TTG GCC ACG TAC ATT AAC ATG ATC 1527 Val Asp Ala Gin Arg Phe Met Asp Leu Ala Thr Tyr lie Asn Met lie 430 435 440 TGG TCA GCC CCC CTG CAA GTC ATC CTT GCT CTC TAC CTC CTG TGG CTG 1575 Trp Ser Ala Pro Leu Gin Val lie Leu Ala Leu Tyr Leu Leu Trp Leu 445 450 455 460 AAT CTG GGC CCT TCC GTC CTG GCT GGA GTG GCG GTG ATG GTC CTC ATG 1623 Asn Leu Gly Pro Ser Val Leu Ala Gly Val Ala Val Met Val Leu Met 465 470 475 GTG CCC GTC AAT GCT GTG ATG GCG ATG AAG ACC AAG ACG TAT CAG GTG 1671 Val Pro Val Asn Ala Val Met Ala Met Lys Thr Lys Thr Tyr Gin Val 480 485 490 GCC CAC ATG AAG AGC AAA GAC AAT CGG ATC AAG CTG ATG AAC GAA ATT 1719 Ala His Met Lys Ser Lys Asp Asn Arg lie Lys Leu Met Asn Glu lie 495 500 505 CTC AAT GGG ATC AAA GTG CTA AAG CTT TAT GCC TGG GAG CTG GCA TTC 1767 Leu Asn Gly lie Lys Val Leu Lys Leu Tyr Ala Trp Glu Leu Ala Phe 510 515 520 AAG GAC AAG GTG CTG GCC ATC AGG CAG GAG GAG CTG AAG GTG CTG AAG 1815 Lys Asp Lys Val Leu Ala lie Arg Gin Glu Glu Leu Lys Val Leu Lys 525 530 535 540 AAG TCT GCC TAC CTG TCA GCC GTG GGC ACC TTC ACC TGG GTC TGC ACG 1863 Lys Ser Ala Tyr Leu Ser Ala Val Gly Thr Phe Thr Trp Val Cys Thr 545 550 555 CCC TTT CTG GTG GCC TTG TGC ACA TTT GCC GTC TAC GTG ACC ATT GAC 1911 Pro Phe Leu Val Ala Leu Cys Thr Phe Ala Val Tyr Val Thr lie Asp 560 565 570 GAG AAC AAC ATC CTG GAT GCC CAG ACA GCC TTC GTG TCT TTG GCC TTG 1959 Glu Asn Asn lie Leu Asp Ala Gin Thr Ala Phe Val Ser Leu Ala Leu 575 580 585 TTC AAC ATC CTC CGG TTT CCC CTG AAC ATT CTC CCC ATG GTC ATC AGC 2007 Phe Asn lie Leu Arg Phe Pro Leu Asn lie Leu Pro Met Val lie Ser 590 595 600 AGC ATC GTG CAG GCG AGT GTC TCC CTC AAA CGC CTG AGG ATC TTT CTC 2055 Ser lie Val Gin Ala Ser Val Ser Leu Lys Arg Leu Arg lie Phe Leu 605 610 615 620 TCC CAT GAG GAG CTG GAA CCT GAC AGC ATC GAG CGA CGG CCT GTC AAA 2103 Ser His Glu Glu Leu Glu Pro Asp Ser lie Glu Arg Arg Pro Val Lys 625 630 635 GAC GGC GGG GGC ACG AAC AGC ATC ACC GTG AGG AAT GCC ACA TTC ACC 2151 Asp Gly Gly Gly Thr Asn Ser lie Thr Val Arg Asn Ala Thr Phe Thr 640 645 650 TGG GCC AGG AGC GAC CCT CCC ACA CTG AAT GGC ATC ACC TTC TCC ATC 2199 Trp Ala Arg Ser Asp Pro Pro Thr Leu Asn Gly lie Thr Phe Ser lie 64 655 660 665 CCC GAA GGT GCT TTG GTG GCC GTG GTG GGC CAG GTG GGC TGC GGA AAG 2247 Pro Glu Gly Ala Leu Val Ala Val Val Gly Gin Val Gly Cys Gly Lys 670 675 680 TTG TCC CTG CTC TCA GCC CTC TTG GCT GAG ATG GAC AAA GTG GAG GGG 2295 Leu Ser Leu Leu Ser Ala Leu Leu Ala Glu Met Asp Lys Val Glu Gly 685 690 695 700 CAC GTG GCT ATC AAG GGC TCC GTG GCC TAT GTG CCA CAG CAG GCC TGG 2343 His Val Ala lie Lys Gly Ser Val Ala Tyr Val Pro Gin Gin Ala Trp 705 710 715 ATT CAG AAT GAT TCT CTC CGA GAA AAC ATC CTT TTT GGA TGT CAG CTG 2391 lie Gin Asn Asp Ser Leu Arg Glu Asn lie Leu Phe Gly Cys Gin Leu 720 725 730 GAG GAA CCA TAT TAC AGG TCC GTG ATA CAG GCC TGT GCC CTC CTC CCA 2439 Glu Glu Pro Tyr Tyr Arg Ser Val lie Gin Ala Cys Ala Leu Leu Pro 735 740 745 GAC CTG GAA ATC CTG CCC AGT GGG GAT CGG ACA GAG ATT GGC GAG AAG 2487 Asp Leu Glu lie Leu Pro Ser Gly Asp Arg Thr Glu lie Gly Glu Lys 750 755 760 GGC GTG AAC CTG TCT GGG GGA CAG AAG CAG CGC GTG AGC CTG GCC CGG 2535 Gly Val Asn Leu Ser Gly Gly Gin Lys Gin Arg Val Ser Leu Ala Arg 765 770 775 780 GCC GTG TAC TCC AAC GCT GAC ATT TAC CTC TTC GAT GAT CCC CTC TCA 2583 Ala Val Tyr Ser Asn Ala Asp lie Tyr Leu Phe Asp Asp Pro Leu Ser 785 790 795 GCA GTG GAT GCC CAT GTG GGA AAA CAC ATC TTT GAA AAT GTG ATT GGC 2631 Ala Val Asp Ala His Val Gly Lys His lie Phe Glu Asn Val lie Gly 800 805 810 CCC AAG GGG ATG CTG AAG AAC AAG ACG CGG ATC TTG GTC ACG CAC AGC 2679 Pro Lys Gly Met Leu Lys Asn Lys Thr Arg lie Leu Val Thr His Ser 815 820 825 ATG AGC TAC TTG CCG CAG GTG GAC GTC ATC ATC GTC ATG AGT GGC GGC 2727 Met Ser Tyr Leu Pro Gin Val Asp Val He He Val Met Ser Gly Gly 830 835 840 AAG ATC TCT GAG ATG GGC TCC TAC CAG GAG CTG CTG GCT CGA GAC GGC 2775 Lys He Ser Glu Met Gly Ser Tyr Gin Glu Leu Leu Ala Arg Asp Gly 845 850 855 860 GCC TTC GCT GAG TTC CTG CGT ACC TAT GCC AGC ACA GAG CAG GAG CAG 2823 Ala Phe Ala Glu Phe Leu Arg Thr Tyr Ala Ser Thr Glu Gin Glu Gin 865 870 875 65 SUBSTITUTE SHEET GAT GCA GAG GAG AAC GGG GTC ACG GGC GTC AGC GGT CCA GGG AAG GAA 2871 Asp Ala Glu Glu Asn Gly Val Thr Gly Val Ser Gly Pro Gly Lys Glu 880 885 890 GCA AAG CAA ATG GAG AAT GGC ATG CTG GTG ACG GAC AGT GCA GGG AAG 2919 Ala Lys Gin Met Glu Asn Gly Met Leu Val Thr Asp Ser Ala Gly Lys 895 900 905 CAA CTG CAG AGA CAG CTC AGC AGC TCC TCC TCC TAT AGT GGG GAC ATC 2967 Gin Leu Gin Arg Gin Leu Ser Ser Ser Ser Ser Tyr Ser Gly Asp He 910 915 920 AGC AGG CAC CAC AAC AGC ACC GCA GAA CTG CAG AAA GCT GAG GCC AAG 3015 Ser Arg His His Asn Ser Thr Ala Glu Leu Gin Lys Ala Glu Ala Lys 925 930 935 940 AAG GAG GAG ACC TGG AAG CTG ATG GAG GCT GAC AAG GCG CAG ACA GGG 3063 Lys Glu Glu Thr Trp Lys Leu Met Glu Ala Asp Lys Ala Gin Thr Gly 945 950 955 CAG GTC AAG CTT TCC GTG TAC TGG GAC TAC ATG AAG GCC ATC GGA CTC 3111 Gin Val Lys Leu Ser Val Tyr Trp Asp Tyr Met Lys Ala He Gly Leu 960 965 970 TTC ATC TCC TTC CTC AGC ATC TTC CTT TTC ATG TGT AAC CAT GTG TCC 3159 Phe He Ser Phe Leu Ser He Phe Leu Phe Met Cys Asn His Val Ser 975 980 985 GCG CTG GCT TCC AAC TAT TGG CTC AGC CTC TGG ACT GAT GAC CCC ATC 3207 Ala Leu Ala Ser Asn Tyr Trp Leu Ser Leu Trp Thr Asp Asp Pro He 990 995 1000 GTC AAC GGG ACT CAG GAG CAC ACG AAA GTC CGG CTG AGC GTC TAT GGA 3255 Val Asn Gly Thr Gin Glu His Thr Lys Val Arg Leu Ser Val Tyr Gly 1005 1010 1015 1020 GCC CTG GGC ATT TCA CAA GGG ATC GCC GTG TTT GGC TAC TCC ATG GCC 3303 Ala Leu Gly He Ser Gin Gly He Ala Val Phe Gly Tyr Ser Met Ala 1025 1030 1035 GTG TCC ATC GGG GGG ATC TTG GCT TCC CGC TGT CTG CAC GTG GAC CTG 3351 Val Ser He Gly Gly He Leu Ala Ser Arg Cys Leu His Val Asp Leu 1040 1045 1050 CTG CAC AGC ATC CTG CGG TCA CCC ATG AGC TTC TTT GAG CGG ACC CCC 3399 Leu His Ser He Leu Arg Ser Pro Met Ser Phe Phe Glu Arg Thr Pro 1055 1060 1065 AGT GGG AAC CTG GTG AAC CGC TTC TCC AAG GAG CTG GAC ACA GTG GAC 3447 Ser Gly Asn Leu Val Asn Arg Phe Ser Lys Glu Leu Asp Thr Val Asp 1070 1075 1080 TCC ATG ATC CCG GAG GTC ATC AAG ATG TTC ATG GGC TCC CTG TTC AAC 3495 Ser Met He Pro Glu Val He Lys Met Phe Met Gly Ser Leu Phe Asn 1085 1090 1095 1100 66 SUBSTITUTE SHEET GTC ATT GGT GCC TGC ATC GTT ATC CTG CTG GCC ACG CCC ATC GCC GCC 3543 Val He Gly Ala Cys He Val He Leu Leu Ala Thr Pro He Ala Ala 1105 1110 1115 ATC ATC ATC CCG CCC CTT GGC CTC ATC TAC TTC TTC GTC CAG AGG TTC 3591 He He He Pro Pro Leu Gly Leu He Tyr Phe Phe Val Gin Arg Phe 1120 1125 1130 TAC GTG GCT TCC TCC CGG CAG CTG AAG CGC CTC GAG TCG GTC AGC CGC 3639 Tyr Val Ala Ser Ser Arg Gin Leu Lys Arg Leu Glu Ser Val Ser Arg 1135 1140 1145 TCC CCG GTC TAT TCC CAT TTC AAC GAG ACC TTG CTG GGG GTC AGC GTC 3687 Ser Pro Val Tyr Ser His Phe Asn Glu Thr Leu Leu Gly Val Ser Val 1150 1155 1160 ATT CGA GCC TTC GAG GAG CAG GAG CGC TTC ATC CAC CAG AGT GAC CTG 3735 He Arg Ala Phe Glu Glu Gin Glu Arg Phe He His Gin Ser Asp Leu 1165 1170 1175 1180 AAG GTG GAC GAG AAC CAG AAG GCC TAT TAC CCC AGC ATC GTG GCC AAC 3783 Lys Val Asp Glu Asn Gin Lys Ala Tyr Tyr Pro Ser He Val Ala Asn 1185 1190 1195 AGG TGG CTG GCC GTG CGG CTG GAG TGT GTG GGC AAC TGC ATC GTT CTG 3831 Arg Trp Leu Ala Val Arg Leu Glu Cys Val Gly Asn Cys He Val Leu 1200 1205 1210 TTT GCT GCC CTG TTT GCG GTG ATC TCC AGG CAC AGC CTC AGT GCT GGC 3879 Phe Ala Ala Leu Phe Ala Val He Ser Arg His Ser Leu Ser Ala Gly 1215 1220 1225 TTG GTG GGC CTC TCA GTG TCT TAC TCA TTG CAG GTC ACC ACG TAC TTG 3927 Leu Val Gly Leu Ser Val Ser Tyr Ser Leu Gin Val Thr Thr Tyr Leu 1230 1235 1240 AAC TGG CTG GTT CGG ATG TCA TCT GAA ATG GAA ACC AAC ATC GTG GCC 3975 Asn Trp Leu Val Arg Met Ser Ser Glu Met Glu Thr Asn He Val Ala 1245 1250 1255 1260 GTG GAG AGG CTC AAG GAG TAT TCA GAG ACT GAG AAG GAG GCG CCC TGG 4023 Val Glu Arg Leu Lys Glu Tyr Ser Glu Thr Glu Lys Glu Ala Pro Trp 1265 1270 1275 CAA ATC CAG GAG ACA CGT CCG CCC AGC AGC TGG CCC CAG GTG GGC CGA 4071 Gin He Gin Glu Thr Arg Pro Pro Ser Ser Trp Pro Gin Val Gly Arg 1280 1285 1290 GTG GAA TTC CGG AAC TAC TGC CTG CGC TAC CGA GAG GAC CTG GAC TTC 4119 Val Glu Phe Arg Asn Tyr Cys Leu Arg Tyr Arg Glu Asp Leu Asp Phe 1295 1300 1305 GTT CTC AGG CAC ATC AAT GTC ACG ATC AAT GGG GGA GAA AAG GTC GGC 4167 Val Leu Arg His He Asn Val Thr He Asn Gly Gly Glu Lys Val Gly 1310 1315 1320 SUBSTITUTE SHEET ATC GTG GGG CGG ACG GGA GCT GGG AAG TCG TCC CTG ACC CTG GGC TTA 4215 He Val Gly Arg Thr Gly Ala Gly Lys Ser Ser Leu Thr Leu Gly Leu 1325 1330 1335 1340 TTT CGG ATC AAC GAG TCT GCC GAA GGA GAG ATC ATC ATC GAT GGC ATC 4263 Phe Arg He Asn Glu Ser Ala Glu Gly Glu He He He Asp Gly He 1345 1350 1355 AAC ATC GCC AAG ATC GGC CTG CAC GAC CTC CGC TTC AAG ATC ACC ATC 4311 Asn He Ala Lys He Gly Leu His Asp Leu Arg Phe Lys He Thr He 1360 1365 1370 ATC CCC CAG GAC CCT GTT TTG TTT TCG GGT TCC CTC CGA ATG AAC CTG 4359 He Pro Gin Asp Pro Val Leu Phe Ser Gly Ser Leu Arg Met Asn Leu 1375 1380 1385 GAC CCA TTC AGC CAG TAC TCG GAT GAA GAA GTC TGG ACG TCC CTG GAG 4407 Asp Pro Phe Ser Gin Tyr Ser Asp Glu Glu Val Trp Thr Ser Leu Glu 1390 1395 1400 CTG GCC CAC CTG AAG GAC TTC GTG TCA GCC CTT CCT GAC AAG CTA GAC 4455 Leu Ala His Leu Lys Asp Phe Val Ser Ala Leu Pro Asp Lys Leu Asp 1405 1410 1415 1420 CAT GAA TGT GCA GAA GGC GGG GAG AAC CTC AGT GTC GGG CAG CGC CAG 4503 His Glu Cys Ala Glu Gly Gly Glu Asn Leu Ser Val Gly Gin Arg Gin 1425 1430 1435 CTT GTG TGC CTA GCC CGG GCC CTG CTG AGG AAG ACG AAG ATC CTT GTG 4551 Leu Val Cys Leu Ala Arg Ala Leu Leu Arg Lys Thr Lys He Leu Val 1440 1445 1450 TTG GAT GAG GCC ACG GCA GCC GTG GAC CTG GAA ACG GAC GAC CTC ATC 4599 Leu Asp Glu Ala Thr Ala Ala Val Asp Leu Glu Thr Asp Asp Leu He 1455 1460 1465 CAG TCC ACC ATC CGG ACA CAG TTC GAG GAC TGC ACC GTC CTC ACC ATC 4647 Gin Ser Thr He Arg Thr Gin Phe Glu Asp Cys Thr Val Leu Thr He 1470 1475 1480 GCC CAC CGG CTC AAC ACC ATC ATG GAC TAC ACA AGG GTG ATC GTC TTG 4695 Ala His Arg Leu Asn Thr He Met Asp Tyr Thr Arg Val He Val Leu 1485 1490 1495 1500 GAC AAA GGA GAA ATC CAG GAG TAC GGC GCC CCA TCG GAC CTC CTG CAG 4743 Asp Lys Gly Glu He Gin Glu Tyr Gly Ala Pro Ser Asp Leu Leu Gin 1505 1510 1515 CAG AGA GGT CTT TTC TAC AGC ATG GCC AAA GAC GCC GGC TTG GTG 4788 Gin Arg Gly Leu Phe Tyr Ser Met Ala Lys Asp Ala Gly Leu Val 1520 1525 1530 TGAGCCCCAG AGCTGGCATA TCTGGTCAGA ACTGCAGGGC CTATATGCCA GCGCCCCAGG 4848 GAGGAGTCAG TACCCCTGGT AAACCAAGCC TCCCACACTG AAACCAAAAC ATAAAAACCA 4908 AACCCAGACA ACCAAAACAT ATTCAAAGCA GCAGCCACCG CCATCCGGTC CCCTGCCTGG 4968 AACTGGCTGT GAAGACCCAG GAGAGACAGA GATGCGAACC ACC 5011 r Q (2) INFORMATION FOR SEQ ID NO:2 : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 1531 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2 : Met Ala Leu Arg Gly Phe Cys Ser Ala Asp Gly Ser Asp Pro Leu Trp 1 5 10 15 Asp Trp Asn Val Thr Trp Asn Thr Ser Asn Pro Asp Phe Thr Lys Cys 20 25 30 Phe Gin Asn Thr Val Leu Val Trp Val Pro Cys Phe Tyr Leu Trp Ala 35 40 45 Cys Phe Pro Phe Tyr Phe Leu Tyr Leu Ser Arg His Asp Arg Gly Tyr 50 55 60 He Gin Met Thr Pro Leu Asn Lys Thr Lys Thr Ala Leu Gly Phe Leu 65 70 75 80 Leu Trp He Val Cys Trp Ala Asp Leu Phe Tyr Ser Phe Trp Glu Arg 85 90 95 Ser Arg Gly He Phe Leu Ala Pro Val Phe Leu Val Ser Pro Thr Leu 100 105 110 Leu Gly He Thr Thr Leu Leu Ala Thr Phe Leu He Gin Leu Glu Arg 115 120 125 Arg Lys Gly Val Gin Ser Ser Gly He Met Leu Thr Phe Trp Leu Val 130 135 140 Ala Leu Val Cys Ala Leu Ala He Leu Arg Ser Lys He Met Thr Ala 145 150 155 160 Leu Lys Glu Asp Ala Gin Val Asp Leu Phe Arg Asp He Thr Phe Tyr 165 170 175 Val Tyr Phe Ser Leu Leu Leu He Gin Leu Val Leu Ser Cys Phe Ser 180 185 190 Asp Arg Ser Pro Leu Phe Ser Glu Thr He His Asp Pro Asn Pro Cys 195 200 205 Pro Glu Ser Ser Ala Ser Phe Leu Ser Arg He Thr Phe Trp Trp He 210 215 22θ' Thr Gly Leu He Val Arg Gly Tyr Arg Gin Pro Leu Glu Gly Ser Asp 225 230 235 240 Leu Trp Ser Leu Asn Lys Glu Asp Thr Ser Glu Gin Val Val Pro Val 245 250 255 69 SUB Leu Val Lys Asn Trp Lys Lys Glu Cys Ala Lys Thr Arg Lys Gin Pro 260 265 270 Val Lys Val Val Tyr Ser Ser Lys Asp Pro Ala Gin Pro Lys Glu Ser 275 280 285 Ser Lys Val Asp Ala Asn Glu Glu Val Glu Ala Leu He Val Lys Ser 290 295 300 Pro Gin Lys Glu Trp Asn Pro Ser Leu Phe Lys Val Leu Tyr Lys Thr 305 310 315 320 Phe Gly Pro Tyr Phe Leu Met Ser Phe Phe Phe Lys Ala He His Asp 325 330 335 Leu Met Met Phe Ser Gly Pro Gin He Leu Lys Leu Leu He Lys Phe 340 345 350 Val Asn Asp Thr Lys Ala Pro Asp Trp Gin Gly Tyr Phe Tyr Thr Val 355 360 365 Leu Leu Phe Val Thr Ala Cys Leu Gin Thr Leu Val Leu His Gin Tyr 370 375 380 Phe His He Cys Phe Val Ser Gly Met Arg He Lys Thr Ala Val He 385 390 395 400 Gly Ala Val Tyr Arg Lys Ala Leu Val He Thr Asn Ser Ala Arg Lys 405 410 415 Ser Ser Thr Val Gly Glu He Val Asn Leu Met Ser Val Asp Ala Gin 420 425 430 Arg Phe Met Asp Leu Ala Thr Tyr He Asn Met He Trp Ser Ala Pro 435 440 445 Leu Gin Val He Leu Ala Leu Tyr Leu Leu Trp Leu Asn Leu Gly Pro 450 455 460 Ser Val Leu Ala Gly Val Ala Val Met Val Leu Met Val Pro Val Asn 465 470 475 480 Ala Val Met Ala Met Lys Thr Lys Thr Tyr Gin Val Ala His Met Lys 485 490 495 Ser Lys Asp Asn Arg He Lys Leu Met Asn Glu He Leu Asn Gly He 500 505 510 Lys Val Leu Lys Leu Tyr Ala Trp Glu Leu Ala Phe Lys Asp Lys Val 515 520 525 Leu Ala He Arg Gin Glu Glu Leu Lys Val Leu Lys Lys Ser Ala Tyr 530 535 540 70 SUBSTITUTE SHEET Leu Ser Ala Val Gly Thr Phe Thr Trp Val Cys Thr Pro Phe Leu Val 545 550 555 560 Ala Leu Cys Thr Phe Ala Val Tyr Val Thr He Asp Glu Asn Asn He 565 570 575 Leu Asp Ala Gin Thr Ala Phe Val Ser Leu Ala Leu Phe Asn He Leu 580 585 590 Arg Phe Pro Leu Asn He Leu Pro Met Val He Ser Ser He Val Gin 595 600 605 Ala Ser Val Ser Leu Lys Arg Leu Arg He Phe Leu Ser His Glu Glu 610 615 620 Leu Glu Pro Asp Ser He Glu Arg Arg Pro Val Lys Asp Gly Gly Gly 625 630 635 640 Thr Asn Ser He Thr Val Arg Asn Ala Thr Phe Thr Trp Ala Arg Ser 645 650 655 Asp Pro Pro Thr Leu Asn Gly He Thr Phe Ser He Pro Glu Gly Ala 660 665 670 Leu Val Ala Val Val Gly Gin Val Gly Cys Gly Lys Leu Ser Leu Leu 675 680 685 Ser Ala Leu Leu Ala Glu Met Asp Lys Val Glu Gly His Val Ala He 690 695 700 Lys Gly Ser Val Ala Tyr Val Pro Gin Gin Ala Trp He Gin Asn Asp 705 710 715 720 Ser Leu Arg Glu Asn He Leu Phe Gly Cys Gin Leu Glu Glu Pro Tyr 725 730 735 Tyr Arg Ser Val He Gin Ala Cys Ala Leu Leu Pro Asp Leu Glu He 740 745 750 Leu Pro Ser Gly Asp Arg Thr Glu He Gly Glu Lys Gly Val Asn Leu 755 760 765 Ser Gly Gly Gin Lys Gin Arg Val Ser Leu Ala Arg Ala Val Tyr Ser 770 775 780 Asn Ala Asp He Tyr Leu Phe Asp Asp Pro Leu Ser Ala Val Asp Ala 785 790 795 800 His Val Gly Lys His He Phe Glu Asn Val He Gly Pro Lys Gly Met 805 810 815 Leu Lys Asn Lys Thr Arg He Leu Val Thr His Ser Met Ser Tyr Leu 820 825 830 Pro Gin Val Asp Val He He Val Met Ser Gly Gly Lys He Ser Glu 835 840 845 71 Met Gly Ser Tyr Gin Glu Leu Leu Ala Arg Asp Gly Ala Phe Ala Glu 850 855 860 Phe Leu Arg Thr Tyr Ala Ser Thr Glu Gin Glu Gin Asp Ala Glu Glu 865 870 875 880 Asn Gly Val Thr Gly Val Ser Gly Pro Gly Lys Glu Ala Lys Gin Met 885 890 895 Glu Asn Gly Met Leu Val Thr Asp Ser Ala Gly Lys Gin Leu Gin Arg 900 905 910 Gin Leu Ser Ser Ser Ser Ser Tyr Ser Gly Asp He Ser Arg His His 915 920 925 Asn Ser Thr Ala Glu Leu Gin Lys Ala Glu Ala Lys Lys Glu Glu Thr 930 935 940 Trp Lys Leu Met Glu Ala Asp Lys Ala Gin Thr Gly Gin Val Lys Leu 945 950 955 960 Ser Val Tyr Trp Asp Tyr Met Lys Ala He Gly Leu Phe He Ser Phe 965 970 975 Leu Ser He Phe Leu Phe Met Cys Asn His Val Ser Ala Leu Ala Ser 980 985 990 Asn Tyr Trp Leu Ser Leu Trp Thr Asp Asp Pro He Val Asn Gly Thr 995 1000 1005 Gin Glu His Thr Lys Val Arg Leu Ser Val Tyr Gly Ala Leu Gly He 1010 1015 1020 Ser Gin Gly He Ala Val Phe Gly Tyr Ser Met Ala Val Ser He Gly 1025 1030 1035 1040 Gly He Leu Ala Ser Arg Cys Leu His Val Asp Leu Leu His Ser He 1045 1050 1055 Leu Arg Ser Pro Met Ser Phe Phe Glu Arg Thr Pro Ser Gly Asn Leu 1060 1065 1070 Val Asn Arg Phe Ser Lys Glu Leu Asp Thr Val Asp Ser Met He Pro 1075 1080 1085 Glu Val He Lys Met Phe Met Gly Ser Leu Phe Asn Val He Gly Ala 1090 1095 1100 Cys He Val He Leu Leu Ala Thr Pro He Ala Ala He He He Pro 1105 1110 1115 1120 Pro Leu Gly Leu He Tyr Phe Phe Val Gin Arg Phe Tyr Val Ala Ser 1125 1130 1135 Ser Arg Gin Leu Lys Arg Leu Glu Ser Val Ser Arg Ser Pro Val Tyr 1140 1145 1150 72 SUBSTITUTE SHEET Ser His Phe Asn Glu Thr Leu Leu Gly Val Ser Val He Arg Ala Phe 1155 1160 1165 Glu Glu Gin Glu Arg Phe He His Gin Ser Asp Leu Lys Val Asp Glu 1170 1175 1180 Asn Gin Lys Ala Tyr Tyr Pro Ser He Val Ala Asn Arg Trp Leu Ala 1185 1190 1195 1200 Val Arg Leu Glu Cys Val Gly Asn Cys He Val Leu Phe Ala Ala Leu 1205 1210 1215 Phe Ala Val He Ser Arg His Ser Leu Ser Ala Gly Leu Val Gly Leu 1220 1225 1230 Ser Val Ser Tyr Ser Leu Gin Val Thr Thr Tyr Leu Asn Trp Leu Val 1235 1240 1245 Arg Met Ser Ser Glu Met Glu Thr Asn He Val Ala Val Glu Arg Leu 1250 1255 1260 Lys Glu Tyr Ser Glu Thr Glu Lys Glu Ala Pro Trp Gin He Gin Glu 1265 1270 1275 1280 Thr Arg Pro Pro Ser Ser Trp Pro Gin Val Gly Arg Val Glu Phe Arg 1285 1290 1295 Asn Tyr Cys Leu Arg Tyr Arg Glu Asp Leu Asp Phe Val Leu Arg His 1300 1305 1310 He Asn Val Thr He Asn Gly Gly Glu Lys Val Gly He Val Gly Arg 1315 1320 1325 Thr Gly Ala Gly Lys Ser Ser Leu Thr Leu Gly Leu Phe Arg He Asn 1330 1335 1340 Glu Ser Ala Glu Gly Glu He He He Asp Gly He Asn He ' Ala Lys 1345 1350 1355 1360 He Gly Leu His Asp Leu Arg Phe Lys He Thr He He Pro Gin Asp 1365 1370 1375 Pro Val Leu Phe Ser Gly Ser Leu Arg Met Asn Leu Asp Pro Phe Ser 1380 1385 1390 Gin Tyr Ser Asp Glu Glu Val Trp Thr Ser Leu Glu Leu Ala His Leu 1395 1400 1405 Lys Asp Phe Val Ser Ala Leu Pro Asp Lys Leu Asp His Glu Cys Ala 1410 1415 1420 Glu Gly Gly Glu Asή Leu Ser Val Gly Gin Arg Gin Leu Val Cys Leu 1425 1430 1435 1440 Ala Arg Ala, Leu Leu Arg Lys Thr Lys He Leu Val Leu Asp Glu Ala 1445 1450 1455 73 SU Thr Ala Ala Val Asp Leu Glu Thr Asp Asp Leu He Gin Ser Thr He 1460 1465 1470 Arg Thr Gin Phe Glu Asp Cys Thr Val Leu Thr He Ala His Arg Leu 1475 1480 1485 Asn Thr He Met Asp Tyr Thr Arg Val He Val Leu Asp Lys Gly Glu 1490 1495 1500 He Gin Glu Tyr Gly Ala Pro Ser Asp Leu Leu Gin Gin Arg Gly Leu 1505 1510 1515 1520 Phe Tyr Ser Met Ala Lys Asp Ala Gly Leu Val 1525 1530 74 SUBSTITUTE SHEET

Claims: WE CLAIM: 1. An isolated nucleic acid comprising a nucleotide sequence encoding a protein having biological activity of multidrug resistance-associated protein, MRP, which confers multidrug resistance on a drug sensitive mammalian cell when the protein is expressed in the cell, said multidrug resistance not being substantially reversed by chemosensitizers which reverse P-glycoprotein-mediated multidrug resistance. 2. The isolated nucleic acid of claim 1, wherein the protein comprises an amino acid sequence shown in SEQ ID NO: 2. 3. The isolated nucleic acid of claim 1, wherein the protein is at least 50% homologous with a sequence comprising an amino acid sequence shown in SEQ ID NO: 2. 4. The isolated nucleic acid of claim 1, wherein the protein is encoded by a nucleic acid which hybridizes under high or low stringency conditions to a nucleic acid which encodes a protein comprising an amino acid sequence shown in SEQ ID NO: 2. 5. An isolated nucleic acid which confers multidrug resistance on a cell comprising a nucleotide sequence shown in SEQ ID NO: 1. 6. The isolated nucleic acid of claim 5 comprising a coding region of the nucleotide sequence shown in SEQ ID NO: 1. 7. An isolated nucleic acid which confers multidrug resistance on a cell comprising a nucleotide sequence encoding a protein comprising an amino acid sequence represented by the formula V-W-X-Y-Z, wherein V are amino acid residues corresponding to amino acid residues from about 1 to 660 of SEQ ID NO: 2, W are amino acid residues of a nucleotide binding fold substantially homologous with amino acid residues from about 661 to 810 of SEQ ID NO: 2, X are amino acid residues corresponding to amino acid residues from about 811 to 1309 of SEQ ID NO: 2, Y are amino acid residues of a nucleotide binding fold substantially homologous with amino acid residues from about 1310 to 1469 of SEQ ID NO: 2 and Z are amino acid residues corresponding to amino acid residues from about 1470 to 1531 of SEQ ID NO: 2. 8. An isolated nucleic acid which is antisense to a nucleic acid as claimed in claim 1. 75 SUBSTITUTE SHEET 9. The isolated nucleic acid of claim 8 which is antisense to a coding region of a nucleic acid as claimed in claim 1. 10. The isolated nucleic acid of claim 8 which is antisense to a noncoding region of a nucleic acid as claimed in claim 1. 11. An isolated nucleic acid which is antisense to a nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1. 12. The isolated nucleic acid of claim 11 which is antisense to a coding region of a nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1. 13. The isolated nucleic acid of claim 11 which is antisense to a noncoding region of a nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1. 14. A recombinant expression vector suitable for transformation of a host cell comprising a nucleic acid as claimed in claim 1 and a regulatory sequence operatively linked to the nucleic acid. 15. A recombinant expression vector suitable for transformation of a host cell comprising a DNA molecule having a nucleotide sequence shown in SEQ ID NO: 1 and a regulatory sequence operatively linked to the DNA molecule. 16. The recombinant expression vector of claim 15 wherein the DNA molecule is operatively linked to the regulatory sequence to allow expression of an RNA molecule which is antisense to a nucleotide sequence shown in SEQ ID NO: 1. 17. A transformant host cell including a recombinant expression vector as claimed in claim 14. 18. A transformant host cell including a recombinant expression vector as claimed in claim 15. 19. The transformant host cell of claim 18 which is drug sensitive prior to transformation. 20. The transformant host cell of claim 19 which is a hematopoietic stem cell. 21. The transformant host cell of claim 19 which is a cardiac muscle cell. 76 ET 22. A transformant host cell including a recombinant expression vector as claimed in claim 16. 23. A method for preparing an isolated multidrug resistance-associated protein comprising an amino acid sequence shown in SEQ ID NO: 2 comprising culturing a transformant host cell including a recombinant expression vector as claimed in claim 15 in a suitable medium until a multidrug resistance-associated protein is formed and isolating the protein. 24. An isolated protein having biological activity of a multidrug resistance-associated protein, MRP, which confers multidrug resistance on a drug sensitive mammalian cell when the protein is expressed in the cell, said multidrug resistance not being substantially reversed by chemosensitizers which reverse P-glycoprotein-mediated multidrug resistance. 25. The isolated protein of claim 24 comprising an amino acid sequence shown in SEQ ID NO: 2. 26. The isolated protein of claim 24, wherein the isolated protein is encoded by a nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1. 27. The isolated protein of claim 24 which is at least 50% homologous with a protein comprising an amino acid sequence shown in SEQ ID NO: 2. 28. The isolated protein of claim 24, wherein the isolated protein is encoded by a nucleic acid which hybridizes under high or low stringency conditions to a nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1. 29. The isolated protein of claim 25 or portion thereof which is immunogenic. 30. An antibody specific for an epitope of a protein as claimed in claim 24. 31. An antibody specific for an epitope of a protein comprising an amino acid sequence shown in SEQ ID NO: 2. 32. The antibody of claim 31 which is a monoclonal antibody. 33. The antibody of claim 31 which binds an unconserved region of the protein. 77 34. The monoclonal antibody of claim 32 which is coupled to a detectable substance. 35. The monoclonal antibody ' of claim 32, wherein the detectable substance is selected from a group consisting of a fluorescent substance, an enzyme and a radioactive substance. 36. The monoclonal antibody of claim 32 which is coupled to a substance having toxic or therapeutic activity. 37. A bispecific antibody capable of binding to a tumor cell which expresses a protein comprising an amino acid sequence shown in SEQ ID NO: 2 and to a detectable substance or a substance having toxic or therapeutic activity. 38. The bispecific antibody of claim 37 wherein the substance having toxic activity is a cytotoxic cell and the bispecific antibody is capable of crosslinking the tumor cell and the cytotoxic cell thereby facilitating lysis of the tumor cell. 39. The bispecific antibody of claim 38 which is capable of binding to Fc receptors of cytotoxic cells. 40. A tetrameric antibody complex of a first monoclonal antibody which is capable of binding to a tumor cell expressing a protein comprising an amino acid sequence as shown in SEQ ID NO: 2 and a second monoclonal antibody which is capable of binding to a detectable substance or a substance having toxic or therapeutic activity wherein said first and second monoclonal antibodies are from a first animal species, conjugated to form a cyclic tetramer with two monoclonal antibodies of a second animal species directed against Fc fragments of the first and second monoclonal antibodies of the first animal species. 41. A composition suitable for administration comprising a monoclonal antibody as claimed in claim 32 in a pharmaceutically acceptable carrier. 42. The composition of claim 41 wherein the monoclonal antibody is coupled to a detectable substance or a substance having toxic or therapeutic activity. 43. A composition suitable for administration comprising a bispecific antibody as claimed in claim 37 in a pharmaceutically acceptable carrier. 78 SUBSTITUTE SHEET 44. A composition suitable for administration comprising a tetrameric antibody complex as claimed in claim 40 in a pharmaceutically acceptable carrier. 45. A nonhuman transgenic animal which contains cells transfected with nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1. 46. The nonhuman transgenic animal of claim 45 which is a mouse. 47. A nonhuman knockout animal which contains cells having an altered gene containing a nucleotide sequence shown in SEQ ID NO: 1. 48. The nonhuman knockout animal of claim 47 which is a mouse. 49. A method for identifying a multidrug resistant tumor cell comprising contacting the tumor cell with a molecule which binds to a protein comprising an amino acid sequence shown in SEQ ID NO: 2, wherein the molecule is labelled with a detectable substance, and detecting the detectable substance bound to the tumor cell. 50. The method of claim 49 wherein the molecule is a monoclonal antibody. 51. The method of claim 49 wherein the detectable substance is selected from a group consisting of a fluorescent substance, an enzyme and a radioactive substance. 52. The method of claim 49 wherein the molecule is a bispecific antibody. 53. The method of claim 49 wherein the molecule is a tetrameric antibody complex. 54. A method for inhibiting drug resistance of a multidrug resistant cell comprising inhibiting activity of a protein comprising an amino acid shown in SEQ ID NO: 2 expressed by the multidrug resistant cell. 55. The method of claim 54 wherein activity of the protein is inhibited by a molecule which binds to the protein. 56. The method of claim 55 wherein the molecule which binds to the protein is a monoclonal antibody. 79 SUBSTITUTE SHEET 57. The method of claim 54 wherein activity of the protein is inhibited by introducing into the multidrug resistant cell a nucleic acid which is antisense to a nucleic acid which encodes the protein. 58. The method of claim 54 wherein the multidrug resistant cell is a tumor cell. 59. The method of claim 54 further comprising exposing the multidrug resistant cell to a therapeutic agent for which resistance of the multidrug resistant cell is inhibited to kill the multidrug resistant cell. 60. A method for killing a multidrug resistant cell which expresses a protein comprising an amino acid sequence shown in SEQ ID NO: 2 comprising contacting the cell with a molecule labelled with a toxic or therapeutic substance which binds to the protein to kill the cell. 61. The method of claim 60, wherein the molecule is a monoclonal antibody. 62. The method of claim 60, wherein the molecule is a bispecific antibody as claimed in claim 37. 63. The method of claim 60, wherein the molecule is a tetrameric antibody complex as claimed in claim 40. 64. The method of claim 60, wherein the multidrug resistant cell is a tumor cell. 65. A method for protecting a drug sensitive cell from cytotoxity due to exposure to a drug comprising transfecting the cell with a nucleic acid comprising a nucleotide sequence shown in SEQ ID NO: 1 to confer drug resistance on the cell. 66. The method of claim 65 wherein the nucleic acid is a recombinant expression vector. 67. The method of claim 65 wherein the cell is a hematopoietic stem cell. 68. The method of claim 65 wherein the cell is a cardiac muscle cell. 69. A method for identifying a substance which is a chemosensitizer of a therapeutic agent comprising: 80 SUBSTITUTE SHEET a) incubating the therapeutic agent with a cell transfected with a nucleic acid which confers resistance to the therapeutic agent on the cell, with and without a substance to be tested; b) determining resistance of the cell to the therapeutic agent when incubated with and without the substance to be tested; and c) identifying a substance which is a chemosensitizer of a therapeutic agent by the ability of the substance to decrease the resistance of the cell to the therapeutic agent when incubated with the substance as compared to the resistance of the cell to the therapeutic agent when incubated without the substance. 70. The method of claim 69 wherein the cell is sensitive to the therapeutic agent before the cell is transfected with the nucleic acid. 71. The method of claim 69 wherein the nucleic acid comprises a recombinant expression vector comprising a nucleotide sequence shown in SEQ ID NO: 1. 72. The method of claim 69 wherein the therapeutic agent is selected from a group consisting of anthracyclines, epipodophyllotoxins and Vinca alkaloids. 73. The method of claim 69 wherein the cell is in a nonhuman transgenic animal and the therapeutic agent and substance to be tested are incubated with the cell by administering the therapeutic agent and substance to be tested to the nonhuman transgenic animal. 74. A method for identifying a substance which is cytotoxic to a multidrug resistant cell comprising incubating a substance to be tested with a cell transfected with nucleic acid which confers multidrug resistance on the cell and determining cytotoxicity of the substance for the multidrug resistant cell. 75. The method of claim 74 wherein the multidrug resistant cell is multidrug sensitive before the cell is transfected with the nucleic acid. 76. The method of claim 74 wherein the nucleic acid comprises a recombinant expression vector comprising a nucleotide sequence shown in SEQ ID NO: 1. SUBSTITUTE SHEET 77. The method of claim 74 wherein the cell is in a nonhuman transgenic animal and the substance to be tested is incubated with the cell by administering the substance to be tested to the nonhuman transgenic animal. 78. A diagnostic kit for identifying multidrug resistant tumor cells comprising a molecule which binds to a protein comprising an amino acid sequence shown in SEQ ID NO: 2 for incubation with a sample of tumor cells; means for detecting the molecule bound to the protein, unreacted protein or unbound molecule; means for determining the amount of protein in the sample; and means for comparing the amount of protein in the sample with a standard. 79. The diagnostic kit of claim 78 wherein the molecule is a monoclonal antibody. 80. A diagnostic kit for identifying multidrug resistant tumor cells comprising a nucleotide probe complementary to the sequence, or an oligonucleotide fragment thereof, shown in SEQ ID NO: 1 for hybridization with mRNA from a sample of tumor cells; means for detecting the nucleotide probe bound to mRNA; means for determining the amount of mRNA in the sample; and means for comparing the amount of mRNA in the sample with a standard. 82 SUBSTITUTE SHEET

Inventors: COLE SUSAN P C, DEELEY ROGER G, COLE, SUSAN, P., C., DEELEY, ROGER, G.
Applicants: UNIV KINGSTON, QUEEN'S UNIVERSITY AT KINGSTON
Application year: 1993
Main classification: C12N15
IPC codes: A01K67, A61K38, A61K39, A61K47, A61K48, C07K14, C07K16, C12N5, C12N15, C12P21, C12R1, G01N33