Title of Invention

"METHODS OF MODIFYING EUKARYOTIC CELLS"

Abstract

An in vitro method of replacing, in whole or in part, in a non-human eukaryotic cell, an endogenous immunoglobulin variable region gene locus with an homologous or orthologous human gene locus comprising: a) obtaining a large cloned genomic fragment containing a first part of the homologous or orthologous human gene locus; b) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (a) to create a large targeting vector for use in the eukaryotic cells (LTVEC), which comprises a site-specific recombination site, a downstream homology arm containing the region immediately adjacent to, but not including, the J segments of the immunoglobulin variable gene locus region and an upstream homology arm within the variable gene locus; c) introducing the LTVEC of (b) into the eukaryotic cells to replace, in whole or in part, the endogenous immunoglobulin variable gene locus; d) using a quantitative assay to detect modification of allele (MOA) in the eukaryotic cells of (c) to identify those eukaryotic cells in which the endogenous immunoglobulin variable region gene locus has been replaced in whole or in part, with the first part of the homologous or orthologous human gene locus; e) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (a) to create a second LTVEC, which comprises a site-specific recombination site, an upstream homology arm containing the region adjacent to the most distal V gene segment, but not containing any V gene segments of the immunoglobulin variable gene locus region and a downstream homology arm; f) introducing the vector of (e) into the eukaryotic cells identified in step (d) such that, through recombination, the endogenous immunoglobulin variable region gene locus is replaced, in whole or in part, with the homologous or orthologous gene locus; and, (g) using a quantitative assay to detect MOA in the eukaryotic cells of (f) to identify those eukaryotic cells in which the endogenous immunogloblulin variable region gene locus has been replaced, in whole or in part, with the homologous or orthologous human gene locus.

Full Text

METHODS OF MODIFYING EUKARYOTIC CELLS This application claims priority to United States Patent Utility Application Serial No. 09/784,859, filed February 16,2001, which is incorporated by reference herein. Throughout this application various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application. Field of the Invention The field of this invention is a method for engineering and utilizing large DNA vectors to target, via homologous recombination, and modify, in any desirable fashion, endogenous genes and chromosomal loci in eukaryotic cells. These large DNA targeting vectors for eukaryotic cells, termed LTVECs, are derived from fragments of cloned genomic DNA larger than those typically used by other approaches intended to perform homologous targeting in eukaryotic cells. The field of the invention further provides for a rapid and convenient method of detecting eukaryotic cells in which the LTVEC has correctly targeted and modified the desired endogenous gene(s) or chromosomal locus(loci). The field also encompasses the use of these cells to generate organisms bearing the genetic modification, the organisms, themselves, and methods of use thereof. Introduction The use of LTVECs provides substantial advantages over current methods. For example, since these are derived from DNA fragments larger than those currently used to generate targeting vectors/ LTVECs can be more rapidly and conveniently generated from available libraries of large genomic DNA fragments (such as BAC and PAC libraries) than targeting vectors made using current technologies. In addition, larger modifications as well as modifications spanning larger genomic regions can be more conveniently generated than using current technologies. Furthermore, the present invention takes advantage of long regions of homology to increase the targeting frequency of "hard to target" loci, and also diminishes the benefit, if any, of using isogenic DNA in these targeting vectors. The present invention thus provides for a rapid, convenient, and streamlined method for systematically modifying virtually all the endogenous genes and chromosomal loci of a given organism. Background of the Invention Gene targeting by means of homologous recombination between homologous exogenous DNA and endogenous chromosomal sequences has proven to be an extremely valuable way to create deletions, insertions, design mutations, correct gene mutations, introduce transgenes, or make other genetic modifications in mice. Current methods involve using standard targeting vectors, with regions of homology to endogenous DNA typically totaling less than 10-20 kb, to introduce the desired genetic modification into mouse embryonic stem (ES) cells, followed by the injection of the altered ES cells into mouse embryos to transmit these engineered genetic modifications into the mouse germline (Smithies et al., Nature, 317:230-234,1985; Thomas et al., Cell, 51:503-512,1987; Koller et al., Proc Natl Acad Sci USA, 86:8927-8931, 1989; Kuhn et al., Science, 254:707-710,1991; Thomas et al.,. Nature, 346:847-850,1990; Schwartzberg et al., Science, 246:799-803,1989; Doetschman et al., Nature, 330:576-578,1987; Thomson et al., Cell, 5:313-321,1989; DeChiara et al., Nature, 345:78-80,1990; U.S. Patent No. 5,789,215, issued August 4,1998 in the name of GenPharm International) In these current methods, detecting the rare ES cells in which the standard targeting vectors have correctly targeted and modified the desired endogenous gene(s) or chromosomal locus(loci) requires sequence information outside of the homologous targeting sequences contained within the targeting vector. Assays for successful targeting involve standard Southern blotting or long PCR (Cheng, et al., Nature, 369:684-5,1994; Foord and Rose, PCR Methods Appl, 3:5149-61, 1994; Ponce and Micol, Nucleic Acids Res, 20:623,1992; U.S. Patent No. 5,436,149 issued to Takara Shuzo Co., Ltd.) from sequences outside the targeting vector and spanning an entire homology arm (see Definitions); thus, because of size considerations that limit these methods, the size of the homology arms are restricted to less than 10-20 kb in total (Joyner, The Practical Approach Series, 293,1999). The ability to utilize targeting vectors with homology arms larger than those used in current methods would be extremely valuable. For example, such targeting vectors could be more rapidly and conveniently generated from available libraries containing large genomic inserts (e.g. BAG or PAC libraries) than targeting vectors made using current technologies, in which such genomic inserts have to be extensively characterized and trimmed prior to use. In addition, larger modifications as well as modifications spanning larger genomic regions could be more conveniently generated and in fewer steps than using current technologies. Furthermore, the use of long regions of homology could increase the targeting frequency of "hard to target" loci in eukaryotic cells, since the targeting of homologous recombination in eukaryotic cells appears to be related to the total homology contained within the targeting vector (Deng and Capecchi, Mol Cell Biol, 12:3365-71,1992). In addition, the increased targeting frequency obtained using long homology arms could diminish any potential benefit that can be derived from using isogenic DNA in these targeting vectors. The problem of engineering precise modifications into very large genomic fragments, such as those cloned in BAG libraries, has largely been solved through the use of homologous recombination in bacteria (Zhang, et al., Nat Genet, 20:123-8,1998; Yang, et al., Nat Biotechnol, 15:859-65,1997; Angrand, et al., Nucleic Acids Res, 27:el6,1999; Muyrers, et al., Nucleic Acids Res, 27:1555-7,1999; Narayanan, et al., Gene Ther, 6:442-7,1999), allowing for the construction of vectors containing large regions of homology to eukaryotic endogenous genes or chromosomal loci. However, once made, these vectors have not been generally useful for modifying endogenous genes or chromosomal loci via homologous recombination because of the difficulty in detecting rare correct targeting events when homology arms are larger than 10-20 kb (Joyner, The Practical Approach Series, 293,1999). Consequently, vectors generated using bacterial homologous recombination from BAG genomic fragments must still be extensively trimmed prior to use as targeting vectors (Hill et al., Genomics, 64:111-3,2000). Therefore, there is still a need for a rapid and convenient methodology that makes possible the use of targeting vectors containing large regions of homology so as to modify endogenous genes or chromosomal loci in eukaryotic cells. In accordance with the present invention/ Applicants provide novel methods that enable the use of targeting vectors containing large regions of homology so as to modify endogenous genes or chromosomal loci in eukaryotic cells via homologous recombination. Such methods overcome the above-described limitations of current technologies. In addition, the skilled artisan will readily recognize that the methods of the invention are easily adapted for use with any genomic DNA of any eukaryotic organism including, but not limited to, animals such as mouse, rat, other rodent, or human, as well as plants such as soy, corn and wheat. Summary of the Invention In accordance with the present invention, Applicants have developed a novel, rapid, streamlined, and efficient method for creating and screening eukaryotic cells which contain modified endogenous genes or chromosomal loci. This novel methods combine, for the first time: 1. Bacterial homologous recombination to precisely engineer a desired genetic modification within a large cloned genomic fragment, thereby creating a large targeting vector for use in eukaryotic cells (LTVECs); 2. Direct introduction of these LTVECs into eukaryotic cells to modify the endogenous chromosomal locus of interest in these cells; and 3. An analysis to determine the rare eukaryotic cells in which the targeted allele has been modified as desired, involving an assay for modification of allele (MOA) of the parental allele that does not require sequence information outside of the targeting sequence, such as, for example, quantitative PCR. A preferred embodiment of the invention is a method for genetically modifying an endogenous gene or chromosomal locus in eukaryotic cells, comprising: a) obtaining a large cloned genomic fragment containing a DNA sequence of interest; b) using bacterial homologous recombination to genetically modify the large cloned genomic fragment of (a) to create a large targeting vector for use in the eukaryotic cells (LTVEC); c) introducing the LTVEC of (b) into the eukaryotic cells to modify the endogenous gene or chromosomal locus in the cells; and d) using a quantitative assay to detect modification of allele (MOA) in the eukaryotic cells of (c) to identify those eukaryotic cells in which the endogenous gene or chromosomal locus has been genetically modified. Another embodiment of the invention is a method wherein the genetic modification to the endogenous gene or chromosomal locus comprises deletion of a coding sequence, gene segment, or regulatory element; alteration of a coding sequence, gene segment, or regulatory element; insertion of a new coding sequence, gene segment, or regulatory element; creation of a conditional allele; or replacement of a coding sequence or gene segment from one species with an homologous or orthologous coding sequence from a different species. An alternative embodiment of the invention is a method wherein the alteration of a coding sequence, gene segment, or regulatory element comprises a substitution, addition, or fusion, wherein the fusion comprises an epitope tag or bifunctional protein. Yet another embodiment of the invention is a method wherein the quantitative assay comprises quantitative PCR, comparative genomic hybridization, isothermic DNA amplification, or quantitative hybridization to an immobilized probe, wherein the quantitative PCR comprises TaqMan® technology or quantitative PCR using molecular beacons. Another preferred embodiment of the invention is a method wherein the eukaryotic cell is a mammalian embryonic stem cell and in particular wherein the embryonic stem cell is a mouse, rat, or other rodent embryonic stem cell. Another preferred embodiment of the invention is a method wherein the endogenous gene or chromosomal locus is a mammalian gene or chromosomal locus, preferably a human gene or chromosomal locus or a mouse, rat, or other rodent gene or chromosomal locus. An additional preferred embodiment is one in which the LTVEC is capable of accommodating large DNA fragments greater than 20 kb, and in particular large DNA fragments greater than 100 kb. Another preferred embodiment is a genetically modified endogenous gene or chromosomal locus that is produced by the method of the invention. Yet another preferred embodiment is a genetically modified eukaryotic cell that is produced by the method of the invention. A preferred embodiment of the invention is a non-human organism containing the genetically modified endogenous gene or chromosomal locus produced by the method of the invention. Also preferred in a non-human organism produced from the genetically modified eukaryotic cells or embryonic stem cells produced by the method of the invention. A preferred embodiment is a non-human organism containing a genetically modified endogenous gene or chromosomal locus, produced by a method comprising the steps of: a) obtaining a large cloned genomic fragment containing a DNA sequence of interest; b) using bacterial homologous recombination to genetically modify the large cloned genomic fragment of (a) to create a large targeting vector (LTVEC) for use in embryonic stem cells; c) introducing the LTVEC of (b) into the embryonic stem cells to modify the endogenous gene or chromosomal locus in the cells; d) using a quantitative assay to detect modification of allele (MOA) in the embryonic stem cells of (c) to identify those embryonic stem cells in which the endogenous gene or chromosomal locus has been genetically modified; e) introducing the embryonic stem cell of (d) into a blastocyst; and f) introducing the blastocyst of (e) into a surrogate mother for gestation. An additional preferred embodiment of the invention is a non-human organism containing a genetically modified endogenous gene or chromosomal locus, produced by a method comprising the steps of: a) obtaining a large cloned genomic fragment containing a DNA sequence of interest; b) using bacterial homologous recombination to genetically modify the large cloned genomic fragment of (a) to create a large targeting vector for use in eukaryotic cells (LTVEC); c) introducing the LTVEC of (b) into the eukaryotic cells to genetically modify the endogenous gene or chromosomal locus in the cells; d) using a quantitative assay to detect modification of allele (MOA) in the eukaryotic cells of (c) to identify those eukaryotic cells in which the endogenous gene or chromosomal locus has been genetically modified; e) removing the nucleus from the eukaryotic cell of (d); f) introducing the nucleus of (e) into an oocyte; and g) introducing the oocyte of (f) into a surrogate mother for gestation. Yet another preferred embodiment is a non-human organism containing a genetically modified endogenous gene or chromosomal locus, produced by a method comprising the steps of: a) obtaining a large cloned genomic fragment containing a DNA sequence of interest; b) using bacterial homologous recombination to genetically modify the large cloned genomic fragment of (a) to create a large targeting vector for use in eukaryotic cells (LTVEC); c) introducing the LTVEC of (b) into the eukaryotic cells to genetically modify the endogenous gene or chromosomal locus in the cells; d) using a quantitative assay to detect modification of allele (MOA) in the eukaryotic cells of (c) to identify those eukaryotic cells in which the endogenous gene or chromosomal locus has been genetically modified; e) fusing the eukaryotic cell of (d) with another eukaryotic cell; f) introducing the fused eukaryotic cell of (e) into a surrogate mother for gestation. In preferred embodiments, the non-human organism is a mouse, rat, or other rodent; the blastocyst is a mouse, rat, or other rodent blastocyst; the oocyte is a mouse, rat, or other rodent oocyte; and the surrogate mother is a mouse, rat, or other rodent. Another preferred embodiment is one in which the embryonic stem cell is a mammalian embryonic stem cell, preferably a mouse, rat, or other rodent embryonic stem cell. An additional preferred embodiment is the use of the genetically modified eukaryouc cells of the invention for the production of a non-human organism, and in particular, the use of the genetically modified embryonic stem cell of the invention for the production of a non-human organism. A preferred embodiment of the invention is a method for genetically modifying an endogenous gene or chromosomal locus of interest in mouse embryonic stem cells, comprising: a) obtaining a large cloned genomic fragment greater than 20 kb which contains a DNA sequence of interest, wherein the large cloned DNA fragment is homologous to the endogenous gene or chromosomal locus; b) using bacterial homologous recombination to genetically modify the large cloned genomic fragment of (a) to create a large targeting vector for use in the mouse embryonic stem cells, wherein the genetic modification is deletion of a coding sequence, gene segment, or regulatory element; c) introducing the large targeting vector of (b) into the mouse embryonic stem cells to modify the endogenous gene or chromosomal locus in the cells; and d) using a quantitative assay to detect modification of allele (MOA) in the mouse embryonic stem cells of (c) to identify those mouse embryonic stem cells in which the endogenous gene or chromosomal locus has been genetically modified, wherein the quantitative assay is quantitative PCR. Also preferred is a genetically modified mouse embryonic stem cell produced by this method; a mouse containing a genetically modified endogenous gene or chromosomal locus produced by this method; and a mouse produced from the genetically modified mouse embryonic stem cell. Another preferred" embodiment is a mouse containing a genetically modified endogenous gene or chromosomal locus of interest, produced by a method comprising the steps of: a) obtaining a large cloned genomic fragment greater than 20 kb which contains a DNA sequence of interest, wherein the large cloned DNA fragment is homologous to the endogenous gene or chromosomal locus; b) using bacterial homologous recombination to genetically modify the large cloned genomic fragment of (a) to create a large targeting vector for use in the mouse embryonic stem cells, wherein the genetic modification is deletion of a coding sequence, gene segment, or regulatory element; c) introducing the large targeting vector of (b) into the mouse embryonic stem cells to modify the endogenous gene or chromosomal locus in the cells; and d) using a quantitative assay to detect modification of allele (MOA) in the mouse embryonic stem cells of (c) to identify those mouse embryonic stem cells in which the endogenous gene or chromosomal locus has been genetically modified, wherein the quantitative assay is quantitative PCR; e) introducing the mouse embryonic stem cell of (d) into a blastocyst; and f) introducing the blastocyst of (e) into a surrogate mother for gestation. Also preferred is the use of the genetically modified mouse embryonic stem cell described above for the production of a mouse. One embodiment of the invention is a method of replacing, in whole or in part, in a non-human eukaryotic cell, an endogenous immunoglobulin variable region gene locus with an homologous or orthologous human gene locus comprising: a) obtaining a large cloned genomic fragment containing, in whole or in part, the homologous or orthologous human gene locus; b) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (a) to create a large targeting vector for use in the eukaryotic cells (LTVEC); c) introducing the LTVEC of (b) into the eukaryotic cells to replace, in whole or in part, the endogenous immunoglobulin variable gene locus; and d) using a quantitative assay to detect modification of allele (MOA) in the eukaryotic cells of (c) to identify those eukaryotic cells in which the endogenous immunoglobulin variable region gene locus has been replaced, in whole or in part, with the homologous or orthologous human gene locus. Another embodiment is a method of replacing, in whole or in part, in a non-human eukaryotic cell, an endogenous immunoglobulin variable region gene locus with an homologous or orthologous human gene locus further comprising the steps: e) obtaining a large cloned genomic fragment containing a part of the homologous or orthologous human gene locus that differs from the fragment of (a); f) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (e) to create a second LTVEC; • g) introducing the second LTVEC of (f) into the eukaryotic cells identified in step (d) to replace, in whole or in part, the endogenous irnmunoglobulin variable gene locus; and h) using a quantitative assay to detect modification of allele (MOA) in the eukaryotic cells of (g) to identify those eukaryotic cells in which the endogenous irnmunoglobulin variable region gene locus has been replaced, . in whole or in part, with the homologous or orthologous human gene locus. Another embodiment of the above method is a method wherein steps (e) through (h) are repeated until the endogenous irnmunoglobulin variable region gene locus is replaced in whole with an homologous or orthologous human gene locus. Another embodiment of the method is one in which the irnmunoglobulin variable gene locus is a locus selected from the group consisting of: a) a variable gene locus of the kappa light chain; b) a variable gene locus of the lambda light chain; and c) a variable gene locus of the heavy chain. A preferred embodiment is a method wherein the quantitative assay comprises quantitative PCR, FISH, comparative genomic hybridization, isothermic DNA amplification, or quantitative hybridization to an immobilized probe, and in particular wherein the quantitative PCR comprises TaqMan® technology or quantitative PCR using molecular beacons. Yet another preferred embodiment is a method of replacing, in whole or in part, in a mouse embryonic stem cell, an endogenous immunoglobulin variable region gene locus with its homologous or orthologous human gene locus comprising: a) obtaining a large cloned genomic fragment containing, in whole or in part, the homologous or orthologous human gene locus; b) using bacterial homologous recombination to genetically modify the large cloned genomic fragment of (a) to create a large targeting vector for use in the embryonic stem cells; c) introducing the large targeting vector of (b) into mouse embryonic stem cells to replace, in whole or in part, the endogenous immunoglobulin variable gene locus in the cells; and d) using a quantitative PCR assay to detect modification of allele (MOA) in the mouse embryonic stem cells of (d) to identify those mouse embryonic stem cells in which the endogenous variable gene locus has been replaced, in whole or in part, with the homologous or orthologous human gene locus. In another embodiment, the method further comprises: e) obtaining a large cloned genomic fragment containing a part of the homologous or orthologous human gene locus that differs from the fragment of (a); f) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (e) to create a large targeting vector for use in the embryonic stem cells; g) introducing the large targeting vector of (f) into the mouse embryonic stem cells identified in step (d) to replace, in whole or in'part, the endogenous immunoglobulin variable gene locus; and h) using a quantitative assay to detect modification of allele (MOA) in the mouse embryonic stem cells of (g) to identify those mouse embryonic stem cells in which the endogenous immunoglobulin variable region gene locus has been replaced, in whole or in part, with the homologous or orthologous human gene locus. Still another preferred embodiment is a method of wherein steps (e) through (h) above are repeated until the endogenous immunoglobulin variable region gene locus is replaced in whole with an homologous or orthologous human gene locus. Also preferred is a method wherein the immunoglobulin variable gene locus comprises a locus selected from the group consisting of a) a variable gene locus of the kappa light chain; b) a variable gene locus of the lambda light chain; and c) a variable gene locus of the heavy chain. Another preferred embodiment is a genetically modified immunoglobulin variable region gene locus produced by the methods described above; a genetically modified eukaryotic cell comprising a genetically modified immunoglobuiin variable region gene locus produced by the methods described above; a non-human organism comprising a genetically modified immunoglobulin variable region gene locus produced by the methods described above; and a mouse embryonic stem cell containing a genetically modified immunoglobulin variable region gene locus produced by the methods described above. Also preferred is an embryonic stem cell wherein the mouse heavy chain variable region locus is replaced/ in whole or in part, with a human heavy chain variable gene locus; an embryonic stem cell of claim wherein the mouse kappa light chain variable region locus is replaced, in whole or in part, with a human kappa light chain variable region locus; an embryonic stem cell wherein the mouse lambda light chain variable region locus is replaced, in whole or in part, with a human lambda light chain variable region locus; and an embryonic stem cell wherein the heavy and light chain variable region gene loci are replaced, in whole, with their human homologs or orthologs. Another preferred embodiment is a mouse produced from the embryonic stem cells described above. Yet another preferred embodiment is an antibody comprising a human variable region encoded by the genetically modified variable gene locus of described above; an antibody further comprising a non-human constant region; and an antibody further comprising a human constant region. Also preferred is a transgenic mouse having a genome comprising entirely human heavy and light chain variable region loci operably linked to entirely endogenous mouse constant region loci such that the mouse produces a serum containing an antibody comprising a human variable region and a mouse constant region in response to antigenic stimulation; a transgenic mouse having a genome comprising human heavy and/or light chain variable region loci operably linked to endogenous mouse constant region loci such that the mouse produces a serum containing an antibody comprising a human variable region and a mouse constant region in response to antigenic stimulation; a transgenic mouse containing an endogenous variable region locus that has been replaced with an homologous or orthologous •human variable locus, such mouse being produced by a method comprising: a) obtaining one or more large cloned genomic fragments containing the entire homologous or orthologous human variable region locus; b) using bacterial homologous recombination to genetically modify the cloned genomic fragment(s) of (a) to create large targeting vector(s) for use in mouse embryonic stem cells; c) introducing the large targeting vector(s) of (b) into mouse embryonic stem cells to replace the entire endogenous variable region locus in the cells; and d) using a quantitative PCR assay to detect modification of allele (MOA) in the mouse embryonic stem cells of (c) to identify those mouse embryonic stem cells in which the entire endogenous variable region locus has been replaced with the homologous or orthologous human variable region locus; e) introducing the mouse embryonic stem cell of (d) into a blastocyst; and f) introducing the blastocyst of (e) into a surrogate mother for gestation. Another preferred embodiment is a transgenic mouse described above wherein the immunoglobulin variable region gene locus comprises one or more loci selected from the group consisting of: a) a variable gene locus of the kappa light chain; b) a variable gene locus of the lambda light chain; and c) a variable gene locus of the heavy chain. Also preferred are the methods described above wherein the mouse embryonic stem cell is derived from a transgenic mouse produced by the methods. Still yet another preferred embodiment of the invention is a method of making a human antibody comprising: a) exposing the mouse described above to antigenic stimulation, such that the mouse produces an antibody against the antigen; b) isolating the DNA encoding the variable regions of the heavy and light chains of the antibody; c) operably linking the DNA encoding the variable regions of (b) to DNA encoding the human heavy and light chain constant regions in a cell . capable of expressing active antibodies; d) growing the cell under such conditions as to express the human antibody; and e)'recovering the antibody. In another preferred embodiment, the cell described above is a CHO cell. Also preferred is a method of wherein the DNA of step (b) described above is isolated from a hybridoma created from the spleen of the mouse exposed to antigenic stimulation in step (a) described above. Also preferred is the method described above wherein the DNA is isolated by PCR. Another preferred embodiment is a method of replacing, in whole or in part, an endogenous immunoglobulin variable region gene locus with an homologous or orthologous gene locus comprising: a) creating a LTVEC comprising a site-specific recombination site, a downstream homology arm containing the region immediately adjacent to, but not including, the J segments of the immuno globulin variable gene locus region and an upstream homology arm within the variable gene locus; b) creating a LTVEC comprising a site-specific recombination site, an upstream homology arm containing the region adjacent to the most distal V gene segment, but not containing any V gene segments of the immunoglobulin variable gene locus region and a downstream homology arm within the variable gene locus; c) introducing the LTVEC s of (a) and (b) into the eukaryotic cell; d) using a quantitative assay to detect modification of allele (MOA) in the variable gene locus to identify those eukaryotic cells in (c) in which the site- specific recombination sites flank the endogenous variable region gene locus; e) creating a vector containing the site-specific recombination sequences flanking all or part of the orthologous or homologous gene locus; and f) introducing the vector of (e) into the eukaryotic cells identified in step (d) such that, through recombination, the endogenous immunoglobulin variable region gene locus is replaced, in whole or in part, with the homologous or orthologous gene locus. Also preferred is a transgenic mouse containing an endogenous immunoglobulin variable region locus that has been replaced with an homologous or orthologous human immunoglobulin variable region locus, such mouse being produced by a method comprising: a) creating a LTVEC comprising a site-specific recombination site and a downstream homology arm containing the region immediately adjacent to, but not including, the J segments of the mouse immunoglobulin variable gene locus region; b) creating a LTVEC comprising a site-specific recombination site and an upstream homology arm containing the region adjacent to the most distal mouse V gene segment, but not containing any V gene segments of the mouse immunoglobulin variable gene locus region; c) introducing the LTVEC s of (a) and (b) into the eukaryotic cell; using a quantitative assay to detect modification of allele (MOA) in the variable gene locus to identify those eukaryotic cells in (c) in which the site- specific recombination sites flank the endogenous immunoglobulin variable region gene locus; d) creating a vector containing the site-specific recombination sequences flanking all or part of the orthologous or homologous gene locus; e) introducing the vector of (e) into the eukaryotic cells identified in step (d) such that, through recombination, the endogenous immunoglobulin variable region gene locus is replaced, in whole or in part, with the homologous or orthologous gene locus; f) introducing the mouse embryonic stem cell of (d) into a blastocyst;; and introducing the blastocyst of (e) into a surrogate mother for gestation. Yet another preferred embodiment is a method of creating, in a eukaryotic cell, an endogenous gene locus flanked downstream by a site-specific recombination site comprising: a) creating a LTVEC comprising the site-specific recombination site, a downstream homology arm containing a region that flanks the 3' end of the endogenous gene locus region and an upstream homology arm within the locus; b) introducing the LTVEC of (a) into the eukaryotic cell; and c) using a quantitative assay to detect modification of allele (MOA) in the endogenous gene locus to identify those eukaryotic cells in (b) in which the endogenous gene locus is flanked downstream by the site-specific recombination site. Still another preferred embodiment is a method of creating, in a eukaryotic cell, an endogenous gene locus flanked upstream by a site-specific recombination site comprising: a) creating a LTVEC comprising the site-specific recombination site, an upstream homology arm containing a region that flanks the 5' end of the endogenous gene locus region and a downstream homology arm within the locus; b) introducing the LTVEC of (a) into the eukaryotic cell; and c) using a quantitative assay to detect modification of allele (MOA) in the endogenous gene locus to identify those eukaryotic cells in (b) in which the endogenous gene locus is flanked upstream by the site-specific recombination site. Also preferred is a method of creating, in a eukaryotic cell, an endogenous gene locus flanked by site-specific recombination sites comprising: a) creating a LTVEC comprising the site-specific recombination site, a downstream homology arm containing a region that flanks the 3' end of the endogenous gene locus region and an upstream homology arm within the locus; b) creating a LTVEC comprising the site-specific recombination site, an upstream homology arm containing a region that flanks the 5' end of the endogenous gene locus region and a downstream homology arm within the locus; c) introducing the LTVEC's of (a) and (b) into the eukaryotic cell; and • d) using a quantitative assay to detect modification of allele (MOA) in the endogenous gene locus to identify those eukaryotic cells in (c) in which the site-specific recombination sites are flanking the endogenous gene locus. Still another preferred embodiment is a method of creating, in a eukaryotic cell, an endogenous immunoglobulin variable gene locus flanked by a site-specific recombination site comprising: a) creating a LTVEC comprising a site-specific recombination site, a downstream homology arm containing the region immediately adjacent to, but not including, the J segments of the immunoglobulin variable gene locus region and an upstream homology arm within the variable gene locus; b) introducing the LTVEC of (a) into the eukaryotic cell; and c) using a quantitative assay to detect modification of allele (MOA) in the variable gene locus to identify those eukaryotic cells in (b) in which the site- specific recombination site flanks the downstream end of the endogenous immunovariable variable gene locus. Also preferred is a method of creating, in a eukaryotic cell/ an endogenous immunoglobulin variable gene locus flanked by site-specific recombination sites comprising: a) creating a LTVEC comprising a site-specific recombination site, an upstream homology arm containing the region adjacent to the most distal V gene segment, but not containing any V gene segments of the immunoglobulin variable gene locus region, and a downstream homology arm within the locus; b) introducing the LTVEC of (a) into the eukaryotic cell; and c) using a quantitative assay to detect modification of allele (MOA) in the variable gene locus to identify those eukaryotic cells in (c) in which the site- specific recombination sites flank the upstream end of the endogenous immunoglobulin variable region gene locus. Still another embodiment is a method of creating, in a eukaryotic cell, an endogenous immunoglobulin variable gene locus flanked by site-specific recombination sites comprising: a) creating a LTVEC comprising a site-specific recombination site, a downstream homology arm containing the region immediately adjacent to, but not including, the J segments of the immunoglobulin variable gene locus region, and an upstream homology arm within the locus; b) creating a LTVEC comprising a site-specific recombination site, an upstream homology arm containing the region adjacent to the most distal V gene segment, but not containing any V gene segments of the immunoglobulin variable gene locus region, and a downstream arm within the locus; c) introducing the LTVEC s of (a) and (b) into the eukaryotic cell; and d) using a quantitative assay to detect modification of allele (MOA) in the variable gene locus to identify those eukaryotic cells in (c) in which the site- specific recombination sites flank the endogenous immunoglobulin variable region gene locus. Brief Description of the Figures Figure 1: Schematic diagram of the generation of a typical LTVEC using bacterial homologous recombination. (hbl = homology box 1; hb2 = homology box 2; RE = restriction enzyme site). Figure 2: Schematic diagram of donor fragment and LTVEC for mouse OCR10. (hbl = homology box 1; lacZ = fi-galactosidase ORP; SV40 polyA = a DNA fragment derived from Simian Virus 40, containing a polyadenylation site and signal; PGKp = mouse phosphoglycerate kinase (PGK) promoter; EM7 = a bacterial promoter; neo = neomycin phosphotransferase; PGK polyA = 3' untranslated region derived from the PGK gene and containing a polyadenylation site and signal; hb2 = homology box 2) Figure 3A-3D: Sequence of the mouse OCR10 cDNA, homology box. 1 (hbl), homology box 2 (hb2), and TaqMan® probes and primers used in a quantitative PCR assay to detect modification of allele (MOA) in ES cells targeted using the mOCRlO LTVEC. hbl: base pairs 1 to 211 hb2: base pairs 1586 to 1801 TaqMan® probe and corresponding PCR primer set derived from mOCRlO exon 3: TaqMan® probe: nucleotides 4l3 to 439 - upper strand Primer ex3-5': nucleotides 390 to 410 - upper strand Primer ex3-3': nucleotides 445 to 461 - lower strand TaqMan® probe and corresponding PCR primer set derived from mOCRlO exon 4: TaqMan® probe: nucleotides 608 to 639 - upper strand Primer ex4-5': nucleotides 586 to 605 - upper strand Primer ex4-3': nucleotides 642 to 662 - lower strand Figure 4A-4D: Schematic diagram of the two LTVECs constructed to replace the mouse VDJ region with human VDJ region. Figure 4A: Large insert (BAG) clones spanning the entire VDJ region of the human heavy chain locus are isolated. Figure 4B: In this example, large insert (BAG) clones are isolated from the ends of the mouse VDJ region as a source of homology arms which are used to direct integration via homologous recombination of the human VDJ sequences in a two step process. Figure 4C-4D: In the first step, LTVEC1 (Figure 4D) is constructed by bacterial homologous recombination in E. coli. LTVEC1 contains, in order: a large mousse homology arm derived from the region upstream from the mouse DJ region, but whose absolute endpoints are not important; a cassette encoding a selectable marker functional in ES cells (PGK-neomycinR in this example); a loxP site; a large human insert spanning •from several V gene segments through the entire DJ region; and a mouse homology arm containing the region immediately adjacent to, but not including, the mouse J segments. In the second step, LTVEC2 (Figure 4C) is constructed by bacterial homologous recombination in E. coli. LTVEC2 contains, in order: a large mouse homology arm containing the region adjacent to the most distal mouse V gene segment, but not containing any mouse V gene segments; a large insert containing a large number of distal human V gene segments; a mutant loxP site called loxSll in the orientation opposite to that of the wild type loxP sites in LTVEC2 and LTVEC1 (this site will not recombine with wild type loxP sites but will readily recombine with other loxSll sites); a wild type loxP site; a second selectable marker (PGK-hygromycinR in this example); and a mouse homology arm derived from the V region, but whose absolute • endpoints are not important. Definitions A "targeting vector" is a DNA construct that contains sequences "homologous" to endogenous chromosomal nucleic acid sequences flanking a desired generic modification(s). The flanking homology sequences, referred to as "homology arms", direct the targeting vector to a specific chromosomal location within the genome by virtue of the homology that exists between the homology arms and the corresponding endogenous sequence and introduce the desired genetic modification by a process referred to as "homologous recombination". "Homologous" means two or more nucleic acid sequences that are either identical or similar enough that they are able to hybridize to each other or undergo intermolecular exchange. "Gene targeting" is the modification of an endogenous chromosomal locus by the insertion into, deletion of, or replacement of the endogenous sequence via homologous recombination using a targeting vector. A "gene knockout" is a genetic modification resulting from the disruption of the genetic information encoded in a chromosomal locus. A "gene knockin" is a genetic modification resulting from the replacement of the genetic information encoded in a chromosomal locus with a different DNA sequence. A "knockout organism" is an organism in which a significant proportion of the organism's cells harbor a gene knockout. A "knockin organism" is an organism in which a significant proportion of the organism's cells harbor a gene knockin. A "marker " or a "selectable marker" is a. selection marker that allows for the isolation of rare transfected cells expressing the marker from the majority of treated cells in the population. Such marker's gene's include, but are not limited to, neomycin phosphotransferase and hygromycin B phosphotransferase, or fluorescing proteins such as GFP. An "ES cell" is an embryonic stem cell. This cell is usually derived from the inner cell mass of a blastocyst-stage embryo. An "ES cell done" is a subpopulation of cells derived from a single cell of the ES cell population following introduction of DNA and subsequent selection. A "flanking DNA" is a segment of DNA that is collinear with and adjacent to a particular point of reference. "LTVECs" are large targeting vectors for eukaryotic cells that are derived from fragments of cloned genomic DNA larger than those typically used by other approaches intended to perform homologous targeting in eukaryotic cells. A "non-human organism" is an organism that is not normally accepted by the public as being human. "Modification of allele" (MOA) refers to the modification of the exact DNA sequence of one allele of a gene(s) or chromosomal locus (loci) in a genome. This modification of allele (MOA) includes, but is not limited to, deletions, substitutions, or insertions of as little as a single nucleotide or deletions of many kilobases spanning a gene(s) or chromosomal locus (loci) of interest, as well as any and all possible modifications between these two extremes. "Orthologous" sequence refers to a sequence from one species that is the functional equivalent of that sequence in another species. The description and examples presented infra are provided to illustrate the subject invention. One of skill in the art will recognize that these examples are provided by way of illustration only and are not included for the purpose of h'miting the invention. Detailed Description of the Invention Applicants have developed a novel, rapid, streamlined, and efficient method for creating and screening eukaryotic cells which contain modified endogenous genes or chromosomal loci. In these cells, the modification may be gene(s) knockouts, knockins, point mutations, or large genomic insertions or deletions or other modifications. By way of non-limiting example, these cells may be embryonic stem cells which are useful for creating knockout or knockin organisms and in particular/ knockout or knockin mice, for the purpose of determining the function of the gene(s) that have been altered, deleted and/or inserted. The novel methods described herein combine, for the first time: 1. Bacterial homologous recombination to precisely engineer a desired genetic modification within a large cloned genomic DNA fragment, thereby creating a large targeting vector for use in eukaryotic cells (LTVECs); 2. Direct introduction of these LTVECs into eukaryotic cells to modify the corresponding endogenous gene(s) or chromosomal locus(loci) of interest in these cells; and 3. An analysis to determine the rare eukaryotic cells in which the targeted allele has been modified as desired, involving a quantitative assay for modification of allele (MOA) of the parental allele. It should be emphasized that previous methods to detect successful homologous recombination in eukaryotic cells cannot be utilized in conjunction with the LTVECs of Applicants' invention because of the long homology arms present in the LTVECs. Utilizing a LTVEC to deliberately modify endogenous genes or chromosomal loci in eukaryotic cells'via homologous recombination is made possible by the novel application of an assay to determine the rare eukaryotic cells in which the targeted allele has been modified as desired, such assay involving a quantitative assay for modification of allele (MOA) of a parental allele, by employing, for example, quantitative PCR or other suitable quantitative assays for MOA. The ability to utilize targeting vectors with homology arms larger than those used in current methods is extremely valuable for the following reasons: 1. Targeting vectors are more rapidly and conveniently generated from available libraries containing large genomic inserts (e.g. BAG or PAC libraries) than targeting vectors made using previous technologies, in which the genomic inserts have to be extensively characterized and "trimmed" prior to use (explained in detail below). In addition, minimal sequence information needs to be known about the locus of interest, i.e. it is only necessary to know the approximately 80-100 nudeotides that are required to generate the homology boxes (described in detail below) and to generate probes that can be used in quantitative assays for MOA (described in detail below). 2. Larger modifications as well as modifications spanning larger genomic regions are more conveniently generated and in fewer steps than using previous technologies. For example, the method of the invention makes possible the precise modification of large loci that cannot be accommodated by traditional plasmid-based targeting vectors because of their size limitations. It also makes possible the modification of any given locus at multiple points (e.g. the introduction of specific mutations at different exons of a multi-exon gene) in one step, alleviating the need to engineer multiple targeting vectors and to perform multiple rounds of targeting and screening for homologous recombination in ES cells. 3. The use of long regions of homology (long homology arms) increase the targeting frequency of "hard to target" loci in eukaryotic cells, consistent with previous findings that targeting of homologous recombination in eukaryotic cells appears to be related to the total homology contained within the targeting vector. 4. The increased targeting frequency obtained using long homology arms apparently diminishes the benefit, if any, from using isogenic DNA in these targeting vectors. 5. The application of quantitative MOA assays for screening eukaryotic cells for homologous recombination not only empowers the use of LTVECs as targeting vectors (advantages outlined above) but also reduces the time for identifying correctly modified eukaryotic cells from the typical several days to a few hours. In addition, the application of quantitative MOA does not require the use of probes located outside the endogenous gene(s) or chromosomal locus(loci) that is being.modified, thus obviating the need to know the sequence flanking the modified gene(s) or locus(loci). This is a significant improvement in the way the screening has been performed in the past and makes it a much less labor-intensive and much more cost-effective approach to screening for homologous recombination events in eukaryotic cells. Methods Many of the techniques used to construct DNA vectors described herein are standard molecular biology techniques well known to the skilled artisan (see e.g., Sambrook, ]., E. F. Fritsch And T. Maruatis. Molecular Cloning: A Laboratory Manual/ Second Edition, Vols 1, 2, and 3,1989; Current Protocols in Molecular Biology, Eds. Ausubel et al., Greene Publ. Assoc., Wiley Interscience, NY). All DNA sequencing is done by standard techniques using an ABI373A DNA sequencer and Taq Dideoxy Terminator Cycle Sequencing Kit (Applied Biosystems, Inc., Foster City, CA). Step 1. Obtain a large genomic DNA clone containing the gene(s) or chromosomal locus (loci) of interest. A gene(s) or locus(loci) of interest can be selected based on specific criteria, such as detailed structural or functional data, or it can be selected in the absence of such detailed information as potential genes or gene fragments become predicted through the efforts of the various genome sequencing projects. Importantly, it should be noted that it is not necessary to know the complete sequence and gene structure of a gene(s) of interest to apply the method of the subject invention to produce LTVECs. In fact, the only sequence information that is required is approximately 80-100 nudeotides so as to obtain the genomic clone of interest as well as to generate the homology boxes used in making the LTVEC (described in detail below) and to make probes for use in quantitative MOA assays. Once a gene(s) or locus(loti) of interest has been selected, a large genomic clone(s) containing this gene(s) or locus(loci) is obtained. This clone(s) can be obtained in any one of several ways including, but not limited to, screening suitable DNA libraries (e.g. BAC, PAC, YAC, or cosmid) by standard hybridization or PCR techniques, or by any other methods familiar to the skilled artisan. Step 2. Append homology boxes 1 and 2 to a modification cassette and generation of LTVEC. Homology boxes mark the sites of bacterial homologous recombination that are used to generate LTVECs from large cloned genomic fragments (Figure 1). Homology boxes are short segments of DNA, generally double-stranded and at least 40 nudeotides in length, that are homologous to regions within the large cloned genomic fragment flanking the "region to be modified". The hornology boxes are appended to the modification cassette, so that following homologous recombination in bacteria, the modification cassette replaces the region to be modified (Figure 1). The technique of creating a targeting vector using bacterial homologous recombination can be performed in a variety of systems (Yang et al., Nat Biotechnol, 15:859-65,1997; Muyrers et al., Nucleic Acids Res, 27:1555-7,1999; Angrand et al., Nucleic Acids Res, 27:el6,1999; Narayanan et al., Gene Ther,. 6:442-7,1999; Yu, et al., Proc Natl Acad Sci U S A, 97:5978-83, 2000). One example of a favored technology currently in use is ET cloning (Zhang et al., Nat Genet, 20:123-8,1998; Narayanan et al., Gene Ther, 6:442-7,1999) and variations of this technology (Yu, et al., Proc Natl Acad Sti USA, 97:5978-83,2000). ET refers to the recE (Hall and Kolodner, Proc Natl Acad Sci USA, 91:3205-9,1994) and recT proteins (Kusano et al., Gene, 138:17-25,1994) that carry out the homologous recombination reaction. RecE is an exonuclease that trims one strand of linear double-stranded DNA (essentially the donor DNA fragment described infra.) 5' to 3', thus leaving behind a linear double-stranded fragment with a 3' single-stranded overhang. This single-stranded overhang is coated by recT protein, which has single-stranded DNA (ssDNA) binding activity (Kovall and Matthews, Science, 277:1824-7,1997). ET cloning is performed using £. coli that transiently express the E. coli gene products of recE and recT (Hall and Kolodner, Proc Natl Acad Sci USA, 91:3205-9,1994; Clark et al., Cold Spring Harb Symp Quant Biol, 49:453-62, 1984; Noirot and Kolodner, J Biol Chem, 273:12274-80,1998; Thresher et al., J Mol Biol, 254:364-71,1995; Kolodner et al., Mol Microbiol, 11:23-30,1994; Hall et al., J Bacteriol, 175:277-87,1993) and the bacteriophage lambda (A) protein A,gam (Murphy, J Bacteriol, 173:5808-21,1991; Poteete et al., J Bacteriol, 170:2012-21,1988).' The Xgam protein is required for protecting the donor DNA fragment from degradation by the recBC exonuclease system (Myers and Stahl, Annu Rev Genet, 28:49-70,1994) and it is required for efficient ET-cloning in recBC+ hosts such as the frequently used E. coli strain DHlOb. The region to be modified and replaced using bacterial homologous recombination can range from zero nucleotides in length (creating an insertion into the original locus) to many tens of kilobases (creating a "deletion and/or a replacement of the original locus). Depending on the modification cassette/ the modification can result in the following: (a) deletion of coding sequences, gene segments, or regulatory elements; (b) alteration(s) of coding sequence, gene segments, or regulatory elements including substitutions, additions, and fusions (e.g. epitope tags or creation of bifunctional proteins such as those with GFP); (c) insertion of new coding regions, gene segments, or regulatory elements, such as those for selectable marker genes or reporter genes or putting new genes under endogenous transcriptional control; (d) creation of conditional alleles, e.g. by introduction of loxP sites flanking the region to be excised by Cre recombinase (Abremski and Hoess, J Biol Chem, 259:1509-14,1984), or FRT sites flanking the region to be excised by Flp recombinase (Andrews et al., Cell, 40:795-803, 1985; Meyer-Leon et al., Cold Spring Harb Symp Quant Biol, 49:797- 804,1984; Cox, Proc Natl Acad Sci USA, 80:4223-7,1983); or (e) replacement of coding sequences or gene segments from one species with orthologous coding sequences from a different species, e.g. replacing a murine genetic locus with the orthologous human genetic locus to engineer a mouse>where that particular locus has been 'humanized'. Any or all of these modifications can be incorporated into a LTVEC. A specific, non-limiting example in which an endogenous coding sequence is entirely deleted and simultaneously replaced with both a reporter gene as well as a selectable marker is provided below in Example 1, as are the advantages of the method of the invention as compared to previous technologies. Step 3 (optional). Verify that each LTVEC has been engineered correctly. Verify that each LTVEC has been engineered correctly by: a. Diagnostic PCR to verify the novel junctions created by the introduction of the donor fragment into the gene(s) or chromosomal locus(loci) of interest. The PCR fragments thus obtained can be sequenced to further verify the novel junctions created by the introduction of the donor fragment into the gene(s) or chromosomal locus(loci) of interest. b. Diagnostic restriction enzyme digestion to make sure that only the desired modifications have been introduced into the LTVEC during the bacterial homologous recombination process. c. Direct sequencing of the LTVEC, particularly the regions spanning the site of the modification to verify the novel junctions created by the introduction of the donor fragment into the gene(s) or chromosomal locus (loci) of interest. Step 4. Purification, preparation, and linearization of LTVEC DNA for introduction into eukaryotic cells. a. Preparation of LTVEC DNA: Prepare miniprep DNA (Sambrook, J., E. F. Fritsch And T. Maniatis. Molecular Cloning: A Laboratory Manual, Second Edition, Vols 1,2, and 3,1989; Tillett and Neilan, Biotechniques, 24:568-70,572,1998; http://www.qiagen.com/Hteratiire/handbooks/pIkmini/ plm_399.pdf) of the selected LTVEC and re-transform the miniprep LTVEC DNA into E. coli using electroporation (Sambrook, J., E. F. Fritsch and T. Maniatis, Molecular Cloning: A Laboratory Manual, Second Edition, Vols 1,2, and 3,1989). This step is necessary to get rid of the plasmid encoding the recombinogenic proteins that are utilized for the bacterial homologous recombination step (Zhang et al., Nat Genet, 20:123-8,1998; Narayanan et al., Gene Ther, 6:442-7,1999). It is useful to get rid of this plasmid (a) because it is a high copy number plasmid and may reduce the yields obtained in the large scale LTVEC preps; (b) to eliminate the possibility of inducing expression of the recombinogenic proteins; and (c) because it may obscure physical mapping of the LTVEC. Before introducing the LTVEC into eukaryotic cells, larger amounts of LTVEC DNA are prepared by standard methodology (http:/ /www. qiagen.com/literature /handbooks /plk/plklow.pdf; Sambrook, ]., E. F. Fritsch And T. Maniatis. Molecular Cloning: A Laboratory Manual, Second Edition, Vols 1,2, and 3,1989; Tillett and Neilan, Biotechniques, 24:568-70, 572,1998). However, this step can be bypassed if a bacterial homologous recombination method that utilizes a recombinogenic prophage is used, i.e. where the genes encoding the recombinogenic proteins are integrated into the bacterial chromosome (Yu, et al, Proc Natl Acad Sci U S A, 97:5978-83, 2000), is used. b. Linearizing the LTVEC DNA: To prepare the LTVEC for introduction into eukaryotic cells, the LTVEC is preferably linearized in a manner that leaves the modified endogenous gene(s) or chromosomal locus(loci) DNA flanked with long homology arms. This can be accomplished by linearizing the LTVEC, preferably in the vector backbone, with any suitable restriction enzyme that digests only rarely. Examples of suitable restriction enzymes include NotI, Pad, Sfil, Srfl, Swal, Fsel, etc. The choice of restriction enzyme may be determined experimentally (i.e. by testing several different candidate rare cutters) or, if the sequence of the LTVEC is known, by analyzing the sequence and choosing a suitable restriction enzyme based on the analysis. In situations where the LTVEC has a vector backbone containing rare sites such as CosN sites, then it can be deaved with enzymes recognizing such sites, for example A, terminase (Shizuya et al, Proc Natl Acad Sd USA, 89:8794-7,1992; Becker and Gold, Proc Natl Acad Sd USA, 75:4199-203,1978; Rackwitz et al., Gene, 40:259-66,1985). Step 5. Introduction of LTVEC into eukaryotic cells and selection of cells where successful introduction of the LTVEC has taken place. LTVEC DNA can be introduced into eukaryotic cells using standard methodology, such as transfection mediated by calcium phosphate, lipids, or electroporation (Sambrook, ]., E. F. Fritsch And T. Maniatis. Molecular Cloning: A Laboratory Manual, Second Edition, Vols 1, 2, and 3,1989). The cells where the LTVEC has been introduced successfully can be selected by exposure to selection agents, depending on the selectable marker gene that has been engineered into the LTVEC. As a non-limiting example, if the selectable marker is the neomytin phosphotransferase (neo) gene (Beck, et alv Gene, 19:327-36,1982), then cells that have taken up the LTVEC can be selected in G41S-containing media; cells that do not have the LTVEC will die whereas cells that have taken up the LTVEC will survive (Santerre, et al., Gene, 30:147-56,1984). Other suitable selectable markers include any drug that has activity in eukaryotic cells (Joyner, The Practical Approach Series,. 293,1999), such as hygromycin B (Santerre, et al., Gene, 30:147-56,1984; Bernard, et al., Exp Cell Res, 158:237-43,1985; Giordano and McAllister, Gene, 88:285-8,1990), Blasticidin S (Izumi, et al., Exp Cell Res, 197:229-33,1991), and other which are familiar to those skilled in the art. Step 6. Screen for homologous recombination events in eukaryotic cells using quantitative assay for modification of allele (MOA). Eukaryotic cells that have been successfully modified by targeting the LTVEC into the locus of interest can be identified using a variety of approaches that can detect modification of allele within the locus of interest and that do not depend on assays spanning the entire homology arm or arms. Such approaches can include but are not limited to: (a) quantitative PCR using TaqMan® (Lie and Petropoulos, Curr Opin Biotechnol, 9:43-8,1998); (b) quantitative MOA assay using molecular beacons (Tan, et al., Chemistry, 6:1107-11, 2000) (c) fluorescence in situ hybridization FISH (Laan, et al., Hum Genet, 96:275-80,1995) or comparative genomic hybridization (CGH) (Forozan, et al., Trends Genet, 13:405-9,1997; Thompson and Gray, J Cell Biochem Suppl, 139-43,1993; Houldsworth and Chaganti, Am J Pathol, 145:1253-60,1994); (d) isothermic DMA amplification (Lizardi, et al., Nat Genet, 19:225-32, 1998; Mirra and Church, Nucleic Acids Res, 27:e34,1999); and (e) quantitative hybridization to an immobilized probe(s) (Southern, J. Mol. Biol. 98:503,1975; Kafatos FC; Jones CW; Efstratiadis A, Nucleic Acids Res 7(6):1541-52,1979). Applicants provide herein an example in which TaqMan® quantitative PCR is used to screen for successfully targeted eukaryotic cells. In this non limiting example, TaqMan® is used to identify eukaryotic cells which have undergone homologous recombination wherein a portion of one of two endogenous alleles in a diploid genome has been replaced by another sequence. In contrast to traditional methods, in which a difference in restriction fragment length spanning the entire homology arm or arms indicates the modification of one of two alleles, the quantitative TaqMan® method will detect the modification of one allele by measuring the reduction in copy number (by half) of the unmodified allele. Specifically, the probe detects the unmodified allele and not the modified allele. Therefore,-the method is independent of the exact nature of the modification and not limited to the sequence replacement described in this example. TaqMan is used to quantify the number of copies of a DNA template in a genomic DNA sample, especially by comparison to a reference gene (Lie and Petropoulos, Curr Opin Biotechnol, 9:43-8,1998). The reference gene is quantitated in the same genomic DNA as the target gene(s) or locus(loci). Therefore, two TaqMan® amplifications (each with its respective probe) are performed. One TaqMan® probe determines the "Ct" (Threshold Cycle) of the reference gene, while the other probe determines the Ct of the region of the targeted gene(s) or locus(loci) which is replaced by successful targeting. The Ct is a quantity that reflects the amount of starting DNA for each of the TaqMan® probes, i.e. a less abundant sequence requires more cycles of PCR to reach the threshold cycle. Decreasing by half the number of copies of the template sequence for a TaqMan® reaction will result in an increase of about one Ct unit. TaqMan® reactions in cells where one allele of the target gene(s) or locus(loci) has been replaced by homologous recombination will result in an increase of one Ct for the target TaqMan® reaction without an increase in the Ct for the reference gene when compared to DNA from non-targeted cells. This allows for ready detection of the modification of one allele of the gene(s) of interest in eukaryotic cells using LTVECs. As stated above, modification of allele (MOA) screening is the use of any method that detects the modification of one allele to identify cells which have undergone homologous recombination. It is not a requirement that the targeted alleles be identical (homologous)'to each other, and in fact/ they may contain polymorphisms, as is the case in progeny resulting from crossing two different strains of mice. In addition, one special situation that is also covered by MOA screening is targeting of gaies which are normally present as a single copy in cells, such as some of the located on the sex chromosomes and in particular, on the Y chromosome. In this case, methods that will detect the modification of the single targeted allele, such as quantitative PCR, Southern blottings, etc., can be used to detect the targeting event. It is clear that the method of the invention can be used to generate modified eukaryotic cells even when alleles are polymorphic or when they are present in a single copy in the targeted cells. Step 8. Uses of genetically modified eukaryotic cells. (a) The genetically modified eukaryotic cells generated by the methods described in steps 1 through 7 can be employed in any in vitro or in vivo assay, where changing the phenotype of the cell is desirable. (b) The genetically modified eukaryotic cell generated by the methods described in steps 1 through 7 can also be used to generate an organism carrying the genetic modification. The genetically modified organisms can be generated by several different techniques including but not limited to: 1. Modified embryonic stem (ES) cells such as the frequently used rat and mouse ES cells. ES cells can be used to create genetically modified rats or mice by standard blastocyst injection technology or aggregation techniques (Robertson, Practical Approach Series, 254,1987; Wood; et al., Nature, 365:87-9,1993; Joyner, The Practical Approach Series, 293,1999), tetraploid blastocyst injection (Wang, et al., Mech Dev, 62:137-45,1997), or nuclear transfer and cloning (Wakayama, et al., Proc Natl Acad Sti U S A, 96:14984-9,1999). ES cells derived from other organisms such as rabbits (Wang, et al., Mech Dev, 62:137-45,1997; Schoonjans, et al., Mol Reprod Dev, 45:439-43,1996) or chickens (Pain, et al., Development, 122:2339-48,1996) or other species should also be amenable to genetic modification(s) using the methods of the invention. 2. Modified protoplasts can be used to generate genetically modified plants (for example see US patent 5,350,689 "Zea mays plants and transgenic Zea mays plants regenerated from protoplasts or protoplast-derived cells", and US patent 5,508,189 "Regeneration of plants from cultured guard cell protoplasts" and references therein). 3. Nuclear transfer from modified eukaryotic cells to oocytes to generate cloned organisms with modified allele (Wakayama, et al., Proc Natl Acad Sri U S A, 96:14984-9,1999; Baguisi, et al., Nat Biotechnol, 17:456-61, 1999; Wilmut, et al., Reprod Fertil Dev, 10:639-43,1998; Wilmut, et al., Nature, 385:810-3,1997; Wakayama, et al., Nat Genet, 24:108-9,2000; Wakayama, et al., Nature, 394:369-74,1998; Rideout, et al., Nat Genet, 24:109-10,2000; Campbell, et al., Nature, 380:64-6,1996). 4. Cell-fusion to transfer the modified allele to another cell, including transfer of engineered chromosome(s), and uses of such cell(s) to generate organisms carrying the modified allele or engineered chromosome(s) (Kuroiwa, et al., Nat Biotechnol, 18:1086-1090,2000). 5. The method of the invention are also amenable to any other approaches that have been used or yet to be discovered. While many of the techniques used in practicing the individual steps of the methods of the invention are familiar to the skilled artisan, Applicants contend that the novelty of the method of the invention lies in the unique combination of those steps and techniques coupled with the never-before-described method of introducing a LTVEC directly into eukaryotic cells to modify a chromosomal locus, and the use of quantitative MOA assays to identify eukaryotic cells which have been appropriately modified. This novel combination represents a significant improvement over previous technologies for creating organisms possessing modifications of endogenous genes or chromosomal loci. Examples Example 1: Engineering mouse ES cells bearing a deletion of the OCR10 gene. a. Selection of a large genomic DNA clone containing mOCRlO. A Bacterial Artificial Chromosome (BAG) done carrying a large genomic DNA fragment that contained the coding sequence of the mouse OCR10 (mOCRlO) gene was obtained by screening an arrayed mouse genomic DNA BAG library (Incyte Genomics) using PCR. The primers employed to screen this library were derived from the mOCRlO gene cDNA sequence. Two primer pairs where used: (a) OCR10.RAA (5'-AGCTACCAGCTGCAGATGCGGGCAG -3') and OCR10.PVIrc (S'-CTCCCCAGCCTGGGTCTGAAAGATGACG-S1) which amplifies a 102 bp DNA; and (b) OCR10.TDY (S'-GACCTCACTTGCTACACTGACTAC-S1) and OCRlO.QETrc (5'-ACTTGTGTAGGCTGCAGAAGGTCTCTTG-3') which amplifies a 1500 bp DNA. This mOCRlO BAG contained approximately 180 kb of genomic DNA including the complete mOCRlO coding sequence. This BAG clone was used to generate an LTVEC which was subsequently used to delete a portion of the coding region of mOCRlO while simultaneously introducing a reporter gene whose initiation codon precisely replaced the initiation codon of OCR10, as well as insertion of a selectable marker gene useful for selection both in E. coli and mammalian cells following the reporter gene (Figure 2). The reporter gene (in this non-limiting example LacZ, the sequence of which is readily available to the skilled artisan), encodes the E. coli fi-galactosidase enzyme. Because of the position of insertion of LacZ (its initiating codon is at the same position as the initiation codon of mOCRlO) the expression of lacZ should mimic that of mOCRlO, as has been observed in other examples where similar replacements with LacZ were performed using previous technologies (see "Gene trap strategies in ES cells", by W Wurst and A. Gossler, in Joyner, The Practical Approach Series, 293,1999) The LacZ gene allows for a simple and standard enzymatic assay to be performed that can reveal its expression patterns in situ, thus providing a surrogate assay that reflects the normal expression patterns of the replaced gene(s) or chromosomal locus(loci). b. Construction of donor fragment and generation of LTVEC. The modification cassette used in the construction of the mOCRlO LTVEC is the lacZ-SV40 polyA-PGKp-EM7-neo-PGK polyA cassette wherein lacZ is a marker gene as described above, SV40 polyA is a fragment derived from Simian Virus 40 (Subramanian, et al., Prog Nucleic Acid Res Mol Biol, 19:157-64,1976/Thimmappaya, et al., J Biol Chem, 253:1613-8,1978; Dhar, et al., Proc Natl Acad Sci U S A, 71:371-5,1974; Reddy, et al., Science, 200:494-502,1978) and containing a polyadenylation site and signal (Subramanian, et al., Prog Nucleic Acid Res Mol Biol, 19:157-64,1976; Thimmappaya, et al., J Biol Chem, 253:1613-8,1978; Dhar, et al., Proc Natl Acad Sci U S A, 71:371-5,1974; Reddy, et al., Science, 200:494-502,1978), PGKp is the mouse phosphoglycerate kinase (PGK) promoter (Adra, et al., Gene, 60:65-74,1987) (which has been used extensively to drive expression of drug resistance genes in mammalian cells), EM7 is a strong bacterial promoter that has the advantage of allowing for positive selection in bacteria of the completed LTVEC construct by driving expression of the neomycin phosphotransferase (neo) gene, neo is a selectable marker that confers Kanamytin resistance in prokaryotic cells and G418 resistance in eukaryotic cells (Beck, et al., Gene, 19:327-36,1982), and PGK polyA is a 3' untranslated region derived from the PGK gene and containing a polyadenylation site and signal (Boer, et al., Biochem Genet, • 28:299-308,1990). To construct the mOCRlO LTVEC, first a donor fragment was generated consisting of a mOCRlO homology box 1 (hbl) attached upstream from the LacZ gene in the modification cassette and a mOCRlO homology box 2 (hb2) attached downstream of the neo-PGK polyA sequence in the modification cassette (Figure 2), using standard recombinant genetic engineering technology. Homology box 1 (hbl) consists of 211 bp of untranslated sequence immediately upstream of the initiating methionine of the mOCRlO open reading frame (mOCRlO ORF) (Figure 3A-3D). Homology box 2 (hb2) consists of last 216 bp of the mOCRlO ORF, ending at the stop codon (Figure 3A-3D). Subsequently, using bacterial homologous recombination (Zhang, et al., Nat Genet, 20:123-8,1998; Angrand, et al, Nucleic Acids Res, 27:616,1999; Muyrers, et al., Nucleic Acids Res, 27:1555-7,1999; Narayanan, et al., Gene Ther, 6:442-7,1999; Yu, et al., Proc Natl Acad Sci U S A, 97:5978-83,2000), this donor fragment was used to precisely replace the mOCRlO coding region (from initiation methionine to stop codon) with the insertion cassette, resulting in construction of the mOCRlO LTVEC (Figure 2). Thus, in this mOCRlO LTVEC, the mOCRlO coding sequence was replaced by the insertion cassette creating an approximately 20 kb deletion in the mOCRlO locus while leaving approximately 130 kb of upstream homology (upstream homology arm) and 32 kb of downstream homology (downstream homology arm). It is important to note that LTVECs can be more rapidly and conveniently generated from available BAG libraries than targeting vectors made using previous technologies because only a single bacterial homologous recombination step is required and the only sequence information required is that needed to generate the homology boxes. In contrast, previous approaches for generating targeting vectors using bacterial homologous recombination require that large targeting vectors be "trimmed" prior to their introduction in ES cells (Hill et al., Genomics, 64:111-3,2000). This trimming is necessary because of the need to generate homology arms short enough to accommodate the screening methods utilized by previous approaches. One major disadvantage of the method of Hill et al. is that two additional homologous recombination steps are required simply for trimming (one to trim the region upstream of the modified locus and one to trim the region downstream of the modified locus). To do this, substantially more sequence information is needed, including sequence information spanning the sites of trimming. In addition, another obvious advantage, illustrated by the above example, is that a very large deletion spanning the mOCRlO gene (approximately 20 kb) can be easily generated in a single step. In contrast, using previous technologies, to accomplish the same task may require several steps and may involve marking the regions upstream and downstream of the coding sequences with loxP sites in order to use the Cre recombinase to remove the sequence flanked by these sites after introduction of the modified locus in eukaryotic cells. This may be unattainable in one step, and thus may require the construction of two targeting vectors using different selection markers and two sequential targeting events in ES cells, one to introduce the loxP site at the region upstream of the coding sequence and another to introduce the loxP site at the region downstream' of the coding sequence. It should be further noted that the creation of large deletions often occurs with low efficiency using the previous targeting technologies in eukaryotic cells, because the frequency of achieving homologous recombination may be low when using targeting vectors containing large deletion flanked by relatively short homology arms. The high efficiency obtained using the method of the invention (see below) is due to the very long homology arms present in the LTVEC that increase the rate of homologous recombination in eukaryotic cells. c. Verification, preparation, and introduction of mOCB.10 LTVEC DNA into ES cells. The sequence surrounding the junction of the insertion cassette and the homology sequence was verified by DNA sequencing. The size of the mOCRlO LTVEC was verified by restriction analysis followed by pulsed field gel electrophoresis (PFGE) (Cantor, et al., Annu Rev Biophys Biophys Chem, 17:287-304,1988; Schwartz and Cantor, Cell, 37:67-75,1984). A standard large-scale plasmid preparation of the mOCRlO LTVEC was done, the plasmid DNA was digested with the restriction enzyme NotI, which cuts in the vector backbone of the mOCRlO LTVEC, to generate linear DNA. . Subsequently the linearized DNA was introduced into mouse ES cells by electroporation (Robertson, Practical Approach Series, 254,1987; Joyner, The Practical Approach Series, 293,1999; Sambrook, et al, Sambrook,}., E. P. Pritsch and T. Maniatis. Molecular Cloning: A Laboratory Manual, Second Edition, Vols 1,2, and 3,1989). ES cells successfully transfected with the mOCRlO LTVEC were selected for in G418-containing media using standard selection methods (Robertson, Practical Approach Series, 254,1987; Joyner, The Practical Approach Series, 293,1999). d. Identification of targeted ES cells clones using a quantitative modification of allele (MOA) assay. To identify ES cells in which one of the two endogenous mOCRlO genes had been replaced by the modification cassette sequence, DNA from individual ES cell clones was analyzed by quantitative PCR using standard TaqMan® methodology as described (Applied Biosystems, TaqMan® Universal PCR Master Mix, catalog number P/N 4304437; see also http://www.pebiodocs.com/pebiodocs/04304449.pdf). The primers and TaqMan® probes used are as described in Figure 3A-3D. A total of 69 independent ES cells clones where screened and 3 were identified as positive, i.e. as clones in which one of the endogenous mOCRlO coding sequence had been replaced by the modification cassette described above. Several advantages of the MOA approach are apparent: (i) It does not require the use of a probe outside the locus being modified, thus obviating the need to know the sequence flanking the modified locus. (ii) It requires very little time to perform compared to conventional Southern blot methodology which has been the previous method of choice (Robertson, Practical Approach Series, 254,1987, Joyner, The Practical Approach Series, 293,1999), thus reducing the time for identifying correctly modified cells from the typical several days to just a few hours. This is a significant improvement in the way screening has been performed in the past and makes it a much less labor-intensive and more cost-effective approach to screening for homologous recombination events in eukaryotic cells. Yet another advantage of the method of the invention is that it is also superior to previous technologies because of its ability to target difficult loci. Using previous technologies, it has been shown that for certain loci the frequency of successful targeting may by as low as 1 in 2000 integration events, perhaps even lower. Using the method of the invention, Applicants have demonstrated that such difficult loci can be targeted much more efficiently using LTVECs that contain long homology arms (i.e. greater than those allowed by previous technologies). As the non-limiting example described above demonstrates, the Applicants have targeted the OCR10 locus, a locus that has previously proven recalcitrant to targeting using conventional technology. Using the method of the invention, Applicants have shown that they have obtained successful targeting in 3 out of 69 ES cells clones in which the mOCRlO LTVEC (containing more than 160 kb of homology arms, and introducing a 20 kb deletion) had integrated, whereas using previous technology for ES cell targeting (Joyner, The Practical Approach Series, 293,1999) using a plasmid-based vector with homology arms shorter than 10-20 kb while also introducing a deletion of less than 15 kb, no targeted events were identified among more than 600 integrants of the vector. These data dearly demonstrate the superiority of the method of the invention over previous technologies. Example 2: Increased targeting frequency and abrogation of the need to use isogenic DNA when LTVECs are used as the targeting vectors. As noted above, the increased targeting frequency obtained using long homology arms should diminish the benefit, if any, derived from using genomic DNA in constructing LTVECs that is isogenic with (i.e. identical in sequence to) the DNA of the eukaryotic cell being targeted. To test this hypothesis, Applicants have constructed numerous LTVECs using genomic DNA derived from the same mouse substrain as the eukaryotic cell to be targeted (presumably isogenic), and numerous other LTVECs using genomic DNA derived from mouse substrains differing from that of the eukaryotic cell to be targeted (presumably non-isogenic). The two sets of LTVECs exhibited similar targeting frequencies, ranging from 1-13% (Table 1), indicating that the rate of successful targeting using LTVECs does not depend on isogenirity. TABLE 1 SUMMARY OF GENE TARGETING USING BAG CLONE VECTORS In summary, the approach of creating LTVECs and directly using them as targeting vectors combined with MOA screening for homologous recombination events in ES cells creates a novel method for engineering genetically modified loci that is rapid, inexpensive and represents a significant improvement over the tedious, time-consuming methods previously in use. It thus opens the possibility of a rapid large scale in vivo functional genomics analysis of essentially any and all genes in an organism's genome in a fraction of the time and cost necessitated by previous-methodologies. Example 3: Use of LTVEC's to produce chimeric and human antibodies a. Introduction Antibodies are composed of two chains, the light and heavy chains, each of which are composed of two domains, the variable and constant domains. The variable region of the antibody protein is the N-terminal portion of the antibody, which binds the antigen. The heavy chain variable domain is encoded by the DNA of the heavy chain variable gene locus/ which is composed of the variable (V), the diversity p), and the joining (J) gene segments. The light chain variable domains are encoded by the DNA of the light chain variable gene loci, kappa and lambda, which are composed of the variable (V) and joining (J) gene segments. The rearrangement of variable region (VDJ/VJ) genes during the initial development of B cells is the primary mechanism whereby the immune system produces antibodies capable of recognizing the huge number of antigens that it may encounter. Essentially, through DNA rearrangements during B cell development, a huge repertoire of variable (VDJ/VJ) region sequences are assembled which are subsequently joined to a constant (C) region to produce complete heavy and light chains which assemble to form an antibody. After functional antibodies have been assembled, somatic hypermutation which occurs in the secondary lymphoid organs, introduces further diversity which enables the organism to select and optimize the affinity of the antibody. The production of antibodies to various antigens in non-human species initially provided great promise for the large scale production of antibodies that could be used as human therapeutics. Species differences, however, leads to the production of antibodies by humans which inactivate the foreign antibodies and cause allergic reactions. Attempts were subsequently made to "humanize" the antibodies, thus making them less likely to be recognized as foreign in humans. Initially, this process involved combining the antigen binding portions of antibodies derived from mice with the constant region of human antibodies, thereby creating recombinant antibodies that were less immunogenic in humans. A second approach which was developed was phage display, whereby human V regions are cloned into a phage display library and regions with the appropriate binding characteristics are joined to human constant regions to create human antibodies. This technology is limited, however, by the lack of antibody development and affinity maturation which naturally occurs in B cells. More recently, endogenous genes have been knocked out of mice, and the genes replaced with their human counterparts to produce entirely human antibodies. Unfortunately, the use of these constructs has highlighted the importance of an endogenous constant region in the development and optimization of antibodies in B cells. Mice producing fully human antibodies have reduced immune responses. This may because human antibodies produced by transgenic mice with entirely human constructs have reduced affinity as compared to their mouse counterparts. Reduced affinity could effect B-cell maturation and survival. Accordingly, the much acclaimed methods of producing humanized antibodies in mice and other organisms, wherein endogenous variable and constant regions of the mice are knocked out and replaced with their human counterparts, has not resulted in optimal antibodies. The use of chimeric antibodies, which utilize human variable regions (VDJ/VJ) with mouse constant regions through B cell maturation, followed by subsequent engineering of the antibodies to replace the mouse constant regions with their human counterparts, has been suggested (U.S. Patent No. 5,770,429 issued June 23,1998). However, the only methodology that has existed to date for making such chimeras has been trans-switching, wherein the formation of the chimeras is only a rare event which occurs only in heavy chains. Heretofore, there has been no mechanism to produce, in transgenic animals, large scale replacement of the entire variable gene encoding segments with human genes, thereby producing chimeras in both the heavy and light chains. Utilizing applicants technology, as disclosed herein, chimeric antibodies are generated which can then be altered, through standard technology, to create high affinity human antibodies. b. Brief Description A transgenic mouse is created that produces hybrid antibodies containing human variable regions (VDJ/VJ) and mouse constant regions. This is accomplished by a direct, in situ replacement of the mouse variable region (VDJ/VJ) genes with their human counterparts. The resultant hybrid immunoglobulin loci will undergo the natural process of rearrangements during B-cell development to produce the hybrid antibodies. Subsequently, fully-human antibodies are made by replacing the mouse constant regions with the desired human counterparts. This approach will give rise to therapeutic antibodies much more efficiently than previous methods, e.g. the "humanization" of mouse monoclonal antibodies or the generation of fully human antibodies in HuMAb mice. Further, this method will succeed in producing therapeutic antibodies for many antigens for which previous methods have failed. This mouse will create antibodies that are human (VDJ/VJ)-mouse constant region, which will have the following benefits over the previously available HuMAb mice that produce totally human antibodies, Antibodies generated by the new mouse will retain murine Fc regions which will interact more efficiently with the other components of the mouse B cell receptor complex, including the signaling components required for appropriate B cell differentiation (such as Iga and Igb). Additionally, the murine Fc regions will be more specific than human Fc regions in their interactions with Fc receptors on mouse cells, complement molecules/ etc. These interactions are important for a strong and specific immune response, for the proliferation and maturation of B cells, and for the affinity maturation of antibodies. Because there is a direct substitution of the human V-D-J/V-J regions for the equivalent regions of the mouse loci all of the sequences necessary for proper transcription, recombination, and/or class switching will remain intact For example, the murine immunoglobulin heavy chain intronic enhancer, Em, has been shown to be critical for V-D-J recombination as well as heavy chain gene expression during the early stages of B cell development [Ronai, D. Berru, M., and Shulman, M. J. Mol Cell Biol 19:7031-7040 (1999)], whereas the immunoglobulin heavy chain 3' enhancer region appears to be critical for class switching [Pan, Q., Petit-Frere, C, Stavnezer, Jv and Hammarstrom/ L. Eur J Immunol 30:1019-1029 (2000)] as well as heavy chain gene expression at later stages of B cell differentiation [Ong,}., Stevens, S., Roeder, R. G., and Eckhardt, L. A. J Immunol 160:4896-4903 (1998)]. Given these various, yet crucial, functions of the transcriptional control elements, it is desirable to maintain these sequences intact. The required recombination events which occur at the immunoglobulin loci during the normal course of B cell differentiation may increase the frequency of aberrant, non-productive irnmunoglobulin rearrangements when these loci are inserted at improper chromosomal locations, or in multiple copies, as in currently available mice. With reductions in productive immunoglobulin rearrangement and, therefore, appropriate signaling at specific steps of B cell development the aberrant cells are eliminated. Reductions of B cell numbers at early stages of development significantly decreases the final overall B cell population and greatly limits the immune responses of the mice. Since there will be only one, chimeric, heavy or light chain locus (as opposed to mutated immunoglobulin loci and with human transgenic loci integrated at distinct chromosomal locations for heavy and light chains in the currently available mice) there should be no trans-splicing or trans-rearrangements of the loci which could result in non-productive rearrangements or therapeutically irrelevant chimeric antibodies (Willers, J., Kolb, C. and Weiler, E. Immunobiology 200:150-164 (2000); Fujieda, Sv Lin, Y. Qv Saxon, A., and Zhang, K. J Immunol 157:3450-3459 (1996)). The substitutions of the human V-D-J or V-J regions into the genuine murine chromosomal immunoglobulin, loci should be substantially more stable, with increased transmission rates to progeny and decreased mosaicism of B cell genotypes compared with the currently available mice (Tomizuka, K., Shinohara, T., Yoshida, H., Uejima, H., Ohguma, A., Tanaka, Sv Sato, K., Oshimura, M., and Ishida, I. Proc Nati Acad Sci (USA) 97:722-727 (2000)). Furthermore, introduction of the human variable regions (VDJ/VJ) at the genuine murine loci in vivo vvill maintain .the appropriate global regulation of chromatin accessibility previously shown to be important for appropriately timed recombination events (Haines, B. B., and Brodeur, P. H. Eur J Immunol 28:4228-4235 (1998)). Approximately 1/3 of human antibodies contain lambda light chains, as compared to mice in which only 1/20 of murine antibodies contain lambda light chains. Therefore, replacing murine lambda light chain V-J sequences with lambda light chain V-J sequences derived from the human locus will serve to increase the repertoire of antibodies as well as more closely match the genuine human immune response, thus increasing the likelihood of obtaining therapeutically useful antibodies. An additional benefit of integrating the human sequences into the genuine murine immunoglobulin loci is that no novel integration sites are introduced which might give rise to mutagenic disruptions at the insertion site and preclude the isolation of viable homozygous mice. This will greatly simplify the production and maintenance of a breeding mouse colony. The following provides a novel method for producing antibodies with all of the above advantages. One skilled in the art will recognize that the generaf method described herein can be modified to produce equivalent results. c. Materials and Methods: Precise replacement of the mouse heavy chain locus variable region (VDJ) with its human counterpart is exemplified using a combination of homologous and site-specific recombination in the following example, which utilizes a. two step process. One skilled in the art will recognize that replacement of the mouse locus with the homologous or orthologous human locus may be accomplished in one or more steps. Accordingly, the invention contemplates replacement of the murine locus, in whole or in part, with each integration via homologous recombination. Large insert (BAG) clones spanning the entire VDJ region of the human heavy chain locus are isolated (Figure 4A). The sequence of this entire region is available in the following GenBank files (AB019437, AB019438, AB019439, AB019440, AB019441, X97051 and X54713). In this example, large insert (BAG) clones are isolated from the ends of the mouse VDJ region as a source of homology arms (Figure 4B) which are used to direct integration via homologous recombination of the human VDJ sequences in a two step process. In the first step, LTVEC1 (Figure 4D) is constructed by bacterial homologous recombination in E. coli. LTVEC1 contains, in order: a large mouse homology arm derived from the region upstream from the mouse DJ region but whose absolute endpoints are not important; a cassette encoding a selectable marker functional in ES cells (PGK-neomycinR in this example); a loxP site; a large human insert spanning from several V gene segments through the entire DJ region; and a mouse homology arm containing the region immediately adjacent to, but not including, the mouse J segments. The 5' end of the downstream arm and the placement of the loxP sites define the 3' end of the region to be replaced in the locus. Mouse ES cells will be transformed by standard techniques, for example, electroporation, with linearized LTVEC1. Because direct introduction of LTVEC1 results in a modification of the endogenous variable gene locus, neomycin resistant colonies can be screened for correct targeting using a MOA assay. These targeted ES cells can give rise to mice that produce antibodies with hybrid heavy chains. However, it will be preferable to proceed with subsequent steps that will eliminate the remainder of the mouse variable segments. In the second step, LTVEC2 (Figure 4C) is constructed by bacterial homologous recombination in E. coli. LTVEC2 contains, in order: a large mouse homology arm containing the region adjacent to the most distal mouse V gene segment, but not containing any mouse V gene segments; a large insert containing a large number of distal human V gene segments; a mutant loxP site called loxSll [Hoess, R.H., Wierzbicki,A. and Abremski,K. Nucleic Acids Res. 14:2287-2300 (1986)] in the orientation opposite to that of the wild type loxP sites in LTVEC2 and LTVEC1 (this site will not recombine with wild type loxP sites but will readily recombine with other loxSll sites); a wild type loxP site; a second selectable marker (PGK-hygromycinR in this example); and a mouse homology arm derived from the V region, but whose absolute endpoints are not important. The 3' end of the upstream homology arm and the placement of the loxP sites define the 5' end of the region to be replaced in the locus. Mouse ES cells that were correctly targeted with LTVEC1 will then be transformed by standard techniques with linearized LTVEC2, and hygromycin resistant colonies will be screened for correct targeting using a MOA assay for modifications in the endogenous variable gene locus. Correctly targeted ES cells resulting from this transformation will hereafter be referred to as "double targeted ES cells". Subsequent transient expression of CRE recombinase in the double targeted ES cells will result in deletion of the remainder of the mouse V region. Alternatively, the double targeted ES cells can be injected into host blastocysts for the production of chimeric mice. Breeding of the resultant chimeric mice with mice expressing CRE recombinase early in development will result in deletion of the remainder of the mouse V region in the progeny Fl. This later alternative increases the likelihood that the hybrid heavy chain locus will be passed through the germline because it involves culruring the ES cells for fewer generations. The inclusion of loxSll in LTVEC2 will allow for the insertion of additional human V gene segments into the hybrid locus. One approach would be to use bacterial homologous recombination to flank a large genomic DNA done containing many additional human V gene segments with loxSll and loxP sites. Co-transformation of such a modified large genomic DNA done into double targeted ES cells with a plasmid that transiently expresses CRE recombinase will result in the introduction of the additional V gene segments by cassette exchange (Bethke,B. and Sauer,B. Nudeic Adds Res. 25:2828-2834 (1997)). A second approach to the incorporation of additional V gene segments is to independently target a large genomic DNA done containing many additional human V gene segments into the mouse locus using, for instance, the same mouse homology arms induded in LTVEC2. In this case, the additional human V gene segments would be flanked by loxSll and loxP sites, and the targeted ES cells would be used to create a mouse. The mice derived from double targeted ES cells and the mice derived from the ES cells containing the additional V gene segments would be bred with a third mouse that directs expression of CRE recombinase during meiosis. The dose proximity of the two recombinant lod during meiotic pairing would result in a high frequency of CRE induced inter-chromosomal recombination as has been seen in other systems (Herault,Y., Rassoulzadegan, M., Cuzin, F. and Duboule, D. Nature Genetics 20: 381-384 (1998)). Another approach is similar to that outlined above but, rather than introducing the loxP and loxSll sites with human LTVECs 1 and 2, they are introduced on mouse LTVECs and then CRE is used to specifically target in the human lod by cassette exchange via flanking loxP and loxSll sites. The methodology outlined below demonstrates how the LTVEC technology may be used to place flanking site specific recombination sites at the ends of any endogenous gene of interest in any non-human animal. Mouse LTVEC 1 contains a cassette inserted by bacterial recombination downstream of, and adjacent to, the } region. This cassette contains a loxP site and a bacterial/mammalian selectable marker, such as hygromydn resistance. LTVEC1 contains, in order: a large homology arm derived from the region upstream from, the mouse DJ region (but within the variable gene locus), but whose absolute endpoints are not important; a cassette encoding a selectable marker functional in ES cells (PGK-hygromycinRin this example); a loxP site; and a homology arm containing the region immediately adjacent to, but not including, the mouse J segments. The 5' end of the downstream homology arm and the placement of the loxP sites define the 3' end of the region to be replaced in the locus. Modification of the 3' end of the endogenous variable gene at the site of cassette insertion allows for the detection of correctly inserted LTVEC1 in the ES cells by an MOA assay. Drug resistance markers are flanked by FRT sites. The introduction of FRT sites allows the removal of any remaining drug resistance markers by FLPe either in ES cells or by crossing the resulting mice to a mice that expresses FLPe in cells that have germ-line potential. LTVEC2 is constructed by bacterial recombination to insert a cassette upstream of the most distal V region of the loci. This cassette contains a loxSll site and a bacteria/mammalian selectable marker, such as neomycin resistance. LTVEC2 contains, in order: a large homology arm containing the region adjacent to the most distal mouse V gene segment, but not containing any mouse V gene segments; a loxSll site in the orientation opposite to that of the wild type loxP sites in LTVEC2 and LTVEC1; a wild type loxP site; a second selectable marker (PGK-neomycinR in this example); and a mouse homology arm derived from the V region (and therefore within the variable gene locus), but whose absolute endpoints are not important. The 3' end of the upstream homology arm and the placement of the loxP sites define the 5' end of the region to be replaced in the locus. Modification of the 5' end of the endogenous variable gene at the site of cassette insertion allows for the detection of correctly inserted LTVEC2 in the ES cells by an MOA assay. These LTVECs are introduced together or sequentially into ES cells using standard techniques and screened for correct targeting using an MOA assay. A human BAG containing the VDJ/VJ region, in part or in whole, is modified by bacterial recombination to insert cassettes that flank the human sequences with loxSll and loxP sites. The upstream cassette is inserted just upstream of the region that will replace the mouse variable region, and contains, in order, a loxSll site followed by a bacteria/mammalian selectable marker, such as puromycin resistance. The downstream cassette is inserted downstream of, and adjacent to, the J region and contains, in order, a loxP site followed by a selectable marker for bacteria, such as spectinomycin resistance. Several methods may be used to insert a larger piece of the human variable region than occurs on a single BAG isolated from a library. A few of these are described below. The loxP and loxSll sites can be inserted separately, by bacterial recombination, onto overlapping BACs, which recombine with each other when transformed into ES cells. In this case, the upstream BAG has one cassette, recombined just upstream of the region that will replace the mouse variable region, that has a Iox511 site followed by a bacterial/mammalian * selectable marker, such as neomycin resistance. The downstream BAG has one cassette, recombined just downstream of, and adjacent to, the } region, that contains a bacterial/mammalian selectable marker, such as puromycin resistance followed by a loxP site. If these two BACs are not overlapping, additional BACs that link the upstream and downstream BACs by overlapping homology are incorporated into the scheme. These are modified by bacterial recombination to contain bacterial/mammalian selectable markers, such as puromycin resistance, and the upstream and downstream BACs are modified to contain loxP and loxSll cassettes that carrying neomycin and hygromycin resistance markers. The human BAC(s) are co-transformed with' CRE recombinase into the ES cell line containing the variable-region-flanking loxSll and loxP recombination sites. If overlapping BACs are used, homologous recombination occurs between them to create a larger DNA fragment, and the flanking loxP and loxSll sites target this large fragment into the mouse locus. Cells are selected for puromycin resistance and screened for the replacement of the mouse variable region. Alternatively, the mouse sequences can first be deleted via the two loxP sites and then the human sequences can be introduced via the remaining loxSll and loxP sites. A fourth BAG can be inserted if LTVEC1 also contains a third site specific recombination site, e.g. Iox2272 (Anal Biochem 2001 Marl5;290(2):260-71) just downstream of the bacterial/mammalian resistance gene, such as puromycin resistance, creating a LTVEC with, in order, the puromycin resistance gene, a loxP site, and a Iox2272 site, followed by the human sequences. After this BAG is integrated into the mouse immunoglobulin locus, the Iox511/lox2272 sites can serve as a recipient in a second round of cassette exchange, wherein the puromycin resistance gene is replaced by an additional upstream portion of the human immunoglobulin locus variable region and a different bacterial/mammalian resistance gene flanked by loxSll and Iox2272 sites. Another method for inserting a larger stretch of the human variable region is to combine sequences from multiple BACs in viiro using rare restriction endonuclease cleavage sites. This is accomplished by using bacterial homologous recombination to insert a loxP site and spectinomycin resistance gene just downstream of the last J of the most downstream BAG and inserting a second bacterial selectable marker and a rare I-Ceul site at the upstream end of the human sequences of the downstream BAG. A loxSll site and a bacterial/mammalian selectable marker, e.g. puromycin resistance, is inserted at the upstream end of a second BAG containing a region of the human variable region upstream from the sequences in the first BAG. An I-Ceul site is inserted at the downstream end of the second BAG. After digestion of both BACs with I-Ceul and Notl, which is unique in the vector portion of both modified BACs, the two BACs are ligated and a recombinant is selected in bacteria for puromycin and spectinomycin resistance. The resultant larger BAG contains, in order, a loxSll site, upstream human sequences, a I-Ceul site, downstream human sequences, a loxP site and a spectinomycin resistance gene. The region between the loxSll site and the loxP site are inserted into the mouse immunoglobulin locus by cassette exchange and puromycin selection as described above. A third method for inserting a larger stretch of the human variable region is to combine sequences from multiple BACs as described above, but using bacterial homologous recombination instead of restriction digestion/ligation. The same selection for recombinant BACs is applied in bacteria, except only one of the two BACs would be digested, its ends after digestion would be designed to be homologous to the other "recipient" BAG, and the recipient BAG would be in bacterial strain modified to be permissive for bacterial homologous recombination. The final steps in creating the human variable/mouse constant monoclonal antibody produring-mouse will be performing the equivalent variable region substitutions on the lambda and kappa light chain loci and breeding all three hybrid loci to homozygocity together in the same mouse. The resultant trans genie mouse will have a genome comprising entirely human heavy and light chain variable gene loci operably linked to entirely endogenous mouse constant region such that the mouse produces a serum containing an antibody comprising a human variable region and a mouse constant region in response to antigenic stimulation. Such a mouse may then be used as a source of DNA encoding the variable regions of human antibodies. Using standard recombinant technology, DNA encoding the variable regions of the heavy and light chains of the antibody is operably linked to DNA encoding the human heavy and light chain constant regions in cells, such as a CHO cells, which are capable of expressing active antibodies. The cells are grown under the appropriate conditions to express the fully human antibodies, which are then recovered. Variable region encoding sequences may be isolated, for example, by PCR amplification or cDNA cloning. In a preferred embodiment, hybridomas made from transgenic mice comprising some or all of the human variable region imrnunoglobulin loci (Kohler and Milstein, Eur. J. Immunol., 6:511-519 (1976) are used as a source of DNA encoding the human variable regions. In summary, the approach of creating LTVECs and directly using them as targeting vectors combined with MOA screening for homologous recombination events in ES cells creates a novel method for engineering genetically modified loci that is rapid, inexpensive and represents a significant improvement over the tedious, time-consuming methods previously in use. It thus opens the possibility of a rapid large scale in vivo functional genomics analysis of essentially any and all genes in an organism's genome in a fraction of the time and cost necessitated by previous methodologies. Although the foregoing iavention has been described in some detail by way of illustration and examples, it will be readily apparent to those of ordinary skill in the art that certain changes and modifications may be made to the teachings of the invention without departing from the spirit or scope of the appended claims. WE CLAIM: 1. An antibody comprising a human variable region encoded by the genetically modified variable gene generated by an in vitro method of replacing, in whole or in part, in a non-human eukaryotic cell, an endogenous immunoglobulin variable region gene locus with an homologous or orthologous human gene locus comprising: b) obtaining a large cloned genomic fragment containing a first part of the homologous or orthologous human gene locus; b) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (a) to create a large targeting vector for use in the, eukaryotic cells (LTVEC), which comprises a site-specific recombination site, a downstream homology arm containing the region immediately adjacent to, but not including, the J segments of the immunoglobulin variable gene locus region and an upstream homology arm within the variable gene locus; c) introducing the LTVEC of (b) into the eukaryotic cells to replace, in whole or in part, the endogenous immunoglobulin variable gene locus; d) using a quantitative assay to detect modification of allele (MOA) in the eukaryotic cells of (c) to identify those eukaryotic cells in which the endogenous immunoglobulin variable region gene locus has been replaced in whole or in part, with the first part of the homologous or orthologous human gene locus; e) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (a) to create a second LTVEC, which comprises a site-specific recombination site, an upstream homology arm containing the region adjacent to the most distal V gene segment, but not containing any V gene segments of the immunoglobulin variable gene locus region and a downstream homology arm; f) introducing the vector of (e) into the eukaryotic cells identified in step (d) such that, through recombination, the endogenous immunoglobulin variable region gene locus is replaced, in whole or in part, with the homologous or orthologous gene locus; and, (g) using a quantitative assay to detect MOA in the eukaryotic cells of (f) to identify those eukaryotic cells in which the endogenous immunogloblulin variable region gene locus has been replaced, in whole or in part, with the homologous or orthologous human gene locus. 2. An antibody as claimed in claim 1, wherein said non-human eukaryotic cell is a mouse embryonic stem cell. 3. An antibody as claimed in claim 1 wherein the immunoglobulin variable gene locus is a locus selected from the group consisting of: a) a variable gene locus of the kappa light chain; b) a variable gene locus of the lambda light chain; and c) a variable gene locus of the heavy chain. 4. An antibody as claimed in claim 3 wherein the quantitative assay comprises quantitative PCR, FISH, comparative genomic hybridization, isothermic DNA amplification, or quantitative hybridization to an immobilized probe. 5. An antibody as claimed in claim 4 wherein the quantitative PCR comprises 3'exonuclease assay technology or quantitative PCR using molecular beacons. 6. An antibody as claimed in claim 1, optionally comprising a non-human constant region; or further comprising a human constant region.

Documents:

01283-delnp-2003-abstract.pdf

01283-delnp-2003-claims.pdf

01283-delnp-2003-correspondence-others.pdf

01283-delnp-2003-description (complete).pdf

01283-delnp-2003-drawings.pdf

01283-delnp-2003-form-1.pdf

01283-delnp-2003-form-18.pdf

01283-delnp-2003-form-2.pdf

01283-delnp-2003-form-3.pdf

01283-delnp-2003-form-5.pdf

01283-delnp-2003-gpa.pdf

01283-delnp-2003-pct-101.pdf

01283-delnp-2003-pct-210.pdf

01283-delnp-2003-pct-304.pdf

01283-delnp-2003-pct-332.pdf

01283-delnp-2003-pct-402.pdf

01283-delnp-2003-pct-409.pdf

01283-delnp-2003-pct-416.pdf

1283-DELNP-2003-Abstract-(24-04-2009).pdf

1283-DELNP-2003-Abstract-04-12-2008.pdf

1283-DELNP-2003-Claims-(19-05-2009).pdf

1283-DELNP-2003-Claims-(24-04-2009).pdf

1283-DELNP-2003-Claims-04-12-2008.pdf

1283-DELNP-2003-Correspondence-Others-(24-04-2009).pdf

1283-DELNP-2003-Correspondence-Others-04-12-2008.pdf

1283-DELNP-2003-Correspondence-Others-08-12-2008.pdf

1283-DELNP-2003-Drawings-04-12-2008.pdf

1283-DELNP-2003-Form-2-04-12-2008.pdf

1283-DELNP-2003-Form-3-04-12-2008.pdf

1283-DELNP-2003-GPA-04-12-2008.pdf

1283-DELNP-2003-Others-Document-04-12-2008.pdf

1283-DELNP-2003-Petition-138-04-12-2008.pdf