DNA CONSTRUCT FOR EFFECTING HOMOLOGOUS RECOMBINATION AND USES THEREOF

Abstract: The invention relates to constructs comprising: a) a targeting sequence; b) a regulatory sequence; c) an exon; and d) an unpaired splice-donor site. The invention further relates to a method of producing protein in vitro or in vivo comprising the homologous recombination of a construct as described above within a cell. The homologously recombinant cell is then maintained under conditions which will permit transcription and translation, resulting in protein expression. The present invention further relates to homologously recombinant cells, including primary, secondary, or immortalized vertebrate cells, methods of making the cells, methods of homologous recombination to produce fusion genes, methods of altering gene expression in the cells, and methods of making a protein in a cell employing the constructs of the invention.

Description: DNA construct for effecting homologous recombination and uses thereof Background of the Invention Current approaches to treating disease by administer¬ ing therapeutic proteins include .in vitro production of therapeutic proteins for conventional pharmaceutical delivery (e.g. intravenous, subcutaneous, or intramuscular injection) and, more recently, gene therapy. Proteins of therapeutic interest are generally pro¬ duced by introducing exogenous DNA encoding the protein of therapeutic interest into appropriate cells. For example, exogenous DNA encoding a desired therapeutic protein is introduced into cells, such as immortalized cells in a vector, such as a plasmid, from which the encoded protein is expressed. Further, it has been suggested that endoge¬ nous cellular genes and their expression may be modified by gene targeting. See for example, U.S. Patent No. 5,272,071, WO 91/06666, WO 91/06667 and WO 90/11354. Presently-available approaches to gene therapy make use of infectious vectors, such as retroviral vectors, which include the genetic material to be expressed. Such approaches have limitations, such as the potential of generating replication-competent virus during vector production; recombination between the therapeutic virus and endogenous retroviral genomes, potentially generating infectious agents with novel cell specificities, host ranges, or increased virulence and cytotoxicity; indepen- dent integration into large numbers of cells, increasing the risk of a tumorigenic insertional event; limited cloning capacity in the retrovirus (which restricts thera¬ peutic applicability) and short-lived .in vivo expression of the product of interest. A better approach to provid- ing gene products, particularly one which avoids the limitations and risks associated with presently available methods, would be valuable. Summary of the Invention The present invention relates to improved methods for both the in vitro production of therapeutic proteins and for the production and delivery of therapeutic proteins by gene therapy. In the present method, expression of a desired targeted gene in a cell (i.e., a desired endoge¬ nous cellular gene) is altered by the introduction, by homologous recombination into the cellular genome at a preselected site, of DNA which includes at least a regula¬ tory sequence, an exon and a splice donor site. These components are introduced into the chromosomal (genomic) DNA in such a manner that this, in effect, results in production of a new transcription unit (in which the regulatory sequence, the exon and the splice donor site present in the DNA construct are operatively linked to the endogenous gene) . As a result of introduction of these components into the chromosomal DNA, the expression of the desired endogenous gene is altered. Altered gene expression, as used herein, encompasses activating (or causing to be expressed) a gene which is normally silent (unexpressed) in the cell as obtained, increasing expression of a gene which is not expressed at physiologically significant levels in the cell as obtained, changing the pattern of regulation or induction such that it is different than occurs in the cell as obtained, and reducing (including eliminating) expression of a gene which is expressed in the cell as obtained. The present invention further relates to DNA con¬ structs useful in the method of altering expression of a target gene. The DNA constructs comprise: (a) a targeting sequence; (b) a regulatory sequence; (c) an exon; and (d) an unpaired splice-donor site. The targeting sequence in the DNA construct directs the integration of elements (a) - (d) into a target gene in a cell such that the elements (b) - (d) are operatively linked to sequences of the endogenous target gene. In another embodiment, the DNA constructs comprise: (a) a targeting sequence, (b) a regulatory sequence, (c) an exon, (d) a splice-donor site, (e) an intron, and (f) a splice-acceptor site, wherein the targeting sequence directs the integration of elements (a) - (f) such that the elements of (b) - (f) are opera¬ tively linked to the endogenous gene. The targeting sequence is homologous to the preselected site in the cellular chromosomal DNA with which homologous recombina¬ tion is to occur. In the construct, the exon is generally 3' of the regulatory sequence and the splice-donor site is 3' of the exon. The following serves to illustrate two embodiments of the present invention, in which the sequences upstream of the human erythropoietin h(EPO) gene are altered to allow expression of hEPO in primary, secondary, or immortalized cells which do not express EPO in detectable quantities in their untransfected state as obtained. In embodiment 1, the targeting construct contains two targeting sequences. The first targeting sequence is homologous to sequences 5' of the second targeting sequence, and both sequences are upstream of the hEPO coding region. The targeting con- struct also contains a regulatory region (the mMT-1 pro¬ moter) an exon (human growth hormone (hGH) ) exon 1) and an unpaired splice-donor site. The product of homologous recombination with this targeting construct is shown in Figure 1. In embodiment 2, the targeting construct also con¬ tains two targeting sequences. The first targeting se¬ quence is homologous to sequences within the endogenous hEPO regulatory region, and the second targeting sequence is homologous to hEPO intron 1. The targeting construct also contains a regulatory region (the mMT-1 promotor) , an exon (hGH exon 1) and an unpaired splice-donor site. The product of homologous recombination with this targeting construct is shown in Figure 2. In these two embodiments, the products of the target¬ ing events are chimeric transcription units which generate a mature mRNA in which the first exon of the hGH gene is positioned upstream of hEPO exons 2-5. The product of transcription, splicing, and translation is a protein in which amino acids 1-4 of the hEPO signal peptide are replaced with amino acid residues 1-3 of hGH. The two embodiments differ with respect to both the relative positions of the regulatory sequences of the targeting construct that are inserted and the specific pattern of splicing that needs to occur to produce the final, pro¬ cessed transcript. The invention further relates to a method of pro¬ ducing protein in vitro or in vivo through introduction of a construct as described above into host cell chromosomal DNA by homologous recombination to produce a homologously recombinant cell . The homologously recombinant cell is then maintained under conditions which will permit tran¬ scription, translation and secretion, resulting in produc- tion of the protein of interest. The present invention relates to transfected cells, such as transfected primary or secondary cells (i.e., non- immortalized cells) and transfected immortalized cells, useful for producing proteins, particularly therapeutic proteins, methods of making such cells, methods of using the cells for .in vitro protein production, and methods of gene therapy. Cells of the present invention are of vertebrate origin, particularly of mammalian origin, and even more particularly of human origin. Cells produced by the method of the present invention contain DNA which encodes a therapeutic product, DNA which is itself a therapeutic product and/or DNA which causes the transfected cells to express a gene at a higher level or with a pattern of regulation or induction that is differ- ent than occurs in the corresponding nontransfected cell. The present invention also relates to methods by which cells, such as primary, secondary, and immortalized cells, are transfected to include exogenous genetic mate¬ rial, methods of producing clonal cell strains or heterog- enous cell strains, and methods of immunizing animals or producing antibodies in immunized animals, using the transfected primary, secondary, or immortalized cells. The present invention relates particularly to a method of gene targeting or homologous recombination in eukaryotic cells, such as cells of fungal, plant or ani¬ mal, e.g., vertebrate, particularly mammalian, and even more particularly, human origin. That is, it relates to a method of introducing DNA into primary, secondary, or immortalized cells of vertebrate origin through homologous recombination, such that the DNA is introduced into genom- ic DNA of the primary, secondary, or immortalized cells at a preselected site. The targeting sequences used are selected with reference to the site into which the DNA in the targeting DNA construct is to be inserted. The pres- ent invention further relates to homologously recombinant primary, secondary, or immortalized cells, referred to as homologously recombinant (HR) primary, secondary or immor¬ talized cells, produced by the present method and to uses of the HR primary, secondary, or immortalized cells. In one embodiment of the present invention in which expression of a gene is altered, the gene is activated. That is, a gene present in primary, secondary, or immor¬ talized cells of vertebrate origin, which is normally not expressed in the cells as obtained, is activated and, as a result, the encoded protein is expressed. In this embodi¬ ment, homologous recombination is used to replace, dis¬ able, or disrupt the regulatory region normally associated with the gene in cells as obtained through the insertion of a regulatory sequence which causes the gene to be expressed at levels higher than evident in the correspond¬ ing nontransfected cell. In one embodiment, the activated gene can be further amplified by the inclusion of an amplifiable selectable marker gene which has the property that cells containing amplified copies of the selectable marker gene can be selected for by culturing the cells in the presence of the appropriate selectable agent. The activated endogenous gene is amplified in tandem with the amplifiable select¬ able marker gene. Cells containing many copies of the activated endogenous gene are useful for .in vitro protein production and gene therapy. Gene targeting and amplification as disclosed in the present invention are particularly useful for activating the expression of genes which form transcription units which are sufficiently large that they are difficult to isolate and express, or for activating genes for which the entire protein coding region is unavailable or has not been cloned. In a further embodiment, expression of a gene which is expressed in a cell as obtained is enhanced or caused to display a pattern of regulation or induction that is different than evident in the corresponding nontransfected cell. In another embodiment, expression of a gene which is expressed in a cell as obtained is reduced (i.e., lessened or eliminated) . The present invention also de¬ scribes a method by which homologous recombination is used to convert a gene into a cDNA copy, devoid of introns, for transfer into yeast or bacteria for in vitro protein production. Transfected cells of the present invention are useful in a number of applications in humans and animals. In one embodiment, the cells can be implanted into a human or an animal for protein delivery in the human or animal. For example, hGH, hEPO, human insulinotropin, and other pro- teins can be delivered systemically or locally in humans for therapeutic benefits. In addition, transfected non- human cells producing growth hormone, erythropoietin, insulinotropin and other proteins of non-human origin may be produced. Barrier devices, which contain transfected cells which express a therapeutic product and through which the therapeutic product is freely permeable, can be used to retain cells in a fixed position .in vivo or to protect and isolate the cells from the host's immune system. Barrier devices are particularly useful and allow transfected immortalized cells, transfected xenogeneic cells, or transfected allogeneic cells to be implanted for treatment of human or animal conditions or for agricultural uses (e.g., bovine growth hormone for dairy production). Barrier devices also allow convenient short-ter (i.e., transient) therapy by providing ready access to the cells for removal when the treatment regimen is to be halted for any reason. In addition, transfected xenogeneic and allogeneic cells may be used in the absence of barrier devices for short-term gene therapy, such that the gene product produced by the cells will be delivered jLn vivo until the cells are rejected by the host's immune system. Transfected cells of the present invention are also useful for eliciting antibody production or for immunizing humans and animals against pathogenic agents. Implanted transfected cells can be used to deliver immunizing anti¬ gens that result in stimulation of the host's cellular and humoral immune responses. These immune responses can be designed for protection of the host from future infectious agents (i.e., for vaccination), to stimulate and augment the disease-fighting capabilities directed against an ongoing infection, or to produce antibodies directed against the antigen produced jLn vivo by the transfected cells that can be useful for therapeutic or diagnostic purposes. Removable barrier devices containing the cells can be used to allow a simple means of terminating expo¬ sure to the antigen. Alternatively, the use of cells that will ultimately be rejected (xenogeneic or allogeneic transfected cells) can be used to limit exposure to the antigen, since antigen production will cease when the cells have been rejected. The methods of the present invention can be used to produce primary, secondary, or immortalized cells produc¬ ing a wide variety of therapeutically useful products, including (but not limited to) : hormones, cytokines, antigens, antibodies, enzymes, clotting factors, transport proteins, receptors, regulatory proteins, structural proteins, transcription factors, ribozymes or anti-sense RNA. Additionally, the methods of the present invention can be used to produce cells which produce non-naturally occurring ribozymes, proteins, or nucleic acids which are useful for .in vitro production of a therapeutic product or for gene therapy. Brief Description of the Drawings Figure 1 is a schematic diagram of a strategy for transcriptionally activating the hEPO gene; thick lines, mouse metallothionein I promoter; stippled box, 5' un¬ translated region of hGH; solid box, hGH exon 1; striped box, 10 bp splice-donor sequence from hEPO intron 1; cross-hatched box, 5' untranslated region of hEPO; open numbered boxes, hEPO coding sequences; diagonally-stripped box, hEPO 3' untranslated sequences; HIII, Hindlll site. Figure 2 is a schematic diagram of a strategy for transcriptionally activating the hEPO gene; thick lines, mouse metallothionein I promoter; stippled box, 5' un¬ translated region of hGH; solid box, hGH exon 1; open numbered boxes, hEPO coding sequences; diagonally-stripped box, hEPO 3' untranslated sequences; HIII, Hindlll site. Figure 3 is a schematic representation of plasmid pXGH5, which includes the hGH gene under the control of the mouse metallothionein promoter. Figure 4 is a schematic representation of plasmid pE3neoEPO. The positions of the human erythropoietin gene and the neomycin phosphotranferase gene (neo) and ampicillin (amp) resistance genes are indicated. Arrows indicate the directions of transcription of the various genes. pmMTl denotes the mouse metallothionein promoter (driving hEPO expression) and pTK denotes the Herpes Simplex Virus thymidine kinase promoter (driving neo expression) . The dotted regions of the map mark the positions of sequences derived from the human hypoxan- thine-guanine phosphoribosyl transferase (HPRT) locus. The relative positions of restriction endonuclease recog¬ nition sites are indicated. Figure 5 is a schematic representation of plasmid pcDNEO, which includes the neo coding region (BamHI-Bglll fragment) from plasmid pSV2neo inserted into the BamHI site of plasmid pcD; the Amp-R and pBR3220ri sequences from pBR322; and the polyA, 16S splice junctions and early promoter regions from SV40. Figure 6 is a schematic representation of plasmid pREP04. Figure 7 is a graphic representation of erythropoie- tin expression in a targeted human cell line subjected to stepwise selection in 0.02, 0.05, 0.1, 0.2 and 0.4 μM methotrexate. Figure 8 is a schematic representation of plasmid pREP015. Fragments derived from genomic hEPO sequences are indicated by filled boxes. The region between BamHI (3537) and Bglll'/Hindlll' corresponds to sequences at positions 1-4008 in Genbank entry HUMERPALU. The region between BglII'/Hindlll' (11463) corresponds to DNA se¬ quences at positions 4009-5169 in Genbank entry HUMERPALU. The region between Hindlll (11463) and Xhol (624) contains sequence corresponding to positions 7-624 of Genbank entry HUMERPA. CMV promoter sequences are shown as an open box and contains sequence from nucleotides 546-2105 of Genbank sequence HS5MIEP. The neo gene is shown as an open box with an arrow. The thymidine kinase (tk) promoter driving the neo gene is shown as a hatched box. pBSIISK+ sequenc¬ es including the amp gene are indicated by a thin line. Figure 9A presents restriction enzyme maps and sche¬ matic representations of the products observed upon digestion of the endogenous hEPO gene (top) and the acti¬ vated hEPO gene after homologous recombination with the targeting fragment from pREP015 (bottom) . Figure 9B presents the results of restriction enzyme digestion and Southern hybridization analysis of untreated (HF) and targeted (Tl) human fibroblast clone HF342-15 (see Example 7) . Figure 10 is a schematic representation of plasmid pREPOlδ . Fragments derived from genomic hEPO sequences are indicated by filled boxes. The region between BamHI (3537) and Clal (7554) corresponds to sequences at posi¬ tions 1-4008 in Genbank entry HUMERPALU. The region between ATG (12246) and Hindlll (13426) corresponds to DNA sequence at positions 4009-5169 in Genbank entry HUMERPLAU. The region between Hindlll (13426) and Xhol (624) contains sequence corresponding to positions 7-624 of Genbank entry HUMERPA. CMV promoter sequences are shown as an open box and contains sequence from nucleo¬ tides 546-2015 of Genbank sequence HS5MIEP. The dihydrofolate reductase (dhfr) transcription unit is shown as a stippled box with an arrow. The neo gene is shown as an open box with an arrow. The tk promoter driving the neo gene is shown as a hatched box. pBSIISK+ sequences including the amp gene are indicated by a thin line. Figure 11 is a schematic illustration of a construct of the invention for activating and amplifying an intronless gene, the a-interferon gene, where the con¬ struct comprises a first targeting sequence (1) , an ampli¬ fiable marker gene (AM) , a selectable marker gene (SM) , a regulatory sequence, a CAP site, a splice-donor site (SD) , an intron (thin lines) , a splice-acceptor site (SA) and a second targeting sequence (2) . The black box represents coding DNA and the stippled boxes represent untranslated sequences. Figure 12 is a schematic illustration of a construct of the invention for activating and amplifying an endoge¬ nous gene wherein the first exon contributes to the signal peptide, the human GM-CSF gene, where the construct com¬ prises a first targeting sequence (1) , an amplifiable marker gene (AM) , a selectable marker gene (SM) , a regula- tory sequence, a CAP site, a splice-donor site (SD) , and a second targeting sequence (2) . The black boxes represent coding DNA and the stippled boxes represent untranslated sequences. Figure 13 is a schematic illustration of a construct of the invention for activating and amplifying an endoge¬ nous gene wherein the first exon contributes to the signal peptide, the human G-CSF gene, where the construct com¬ prises a first targeting sequence (1) , an amplifiable marker gene (AM) , a selectable marker gene (SM) , a regula¬ tory sequence, a CAP site, a splice-donor site (SD) , and a second targeting sequence (2) . The black boxes represent coding DNA and the stippled boxes represent untranslated sequences. Figure 14 is a schematic illustration of a construct of the invention for activating and amplifying an endoge¬ nous gene wherein the first exon is non-coding, the human FSH3 gene, where the construct comprises a first targeting sequence (1) , an amplifiable marker gene (AM) , a selecta- ble marker gene (SM) , a regulatory sequence, a CAP site, a splice-donor site (SD) , and a second targeting sequence (2) . The black boxes represent coding DNA and the stippled boxes represent untranslated sequences. Detailed Description of the Invention The invention is based upon the discovery that the regulation or activity of endogenous genes of interest in a cell can be altered by inserting into the cell genome, at a preselected site, through homologous recombination, DNA constructs comprising: (a) a targeting sequence; (b) a regulatory sequence; (c) an exon and (d) an unpaired splice-donor site, wherein the targeting sequence directs the integration of elements (a) - (d) such that the ele¬ ments (b) - (d) are operatively linked to the endogenous gene. In another embodiment, the DNA constructs comprise: (a) a targeting sequence, (b) a regulatory sequence, (c) an exon, (d) a splice-donor site, (e) an intron, and (f) a splice-acceptor site, wherein the targeting sequence directs the integration of elements (a) - (f) such that the elements of (b) - (f) are operatively linked to the firεt exon of the endogenous gene. The targeting sequen¬ ces used are selected with reference to the site into which the DNA is to be inserted. In both embodiments the targeting event is used to create a new transcription unit, which is a fusion product of sequences introduced by the targeting DNA constructs and the endogenous cellular gene. As discussed herein, for example, the formation of the new transcription unit allows transcriptionally silent genes (genes not expressed in a cell prior to transfec- tion) to be activated in host cells by introducing into the host cell's genome DNA constructs of the present invention. As also discussed herein, the expression of an endogenous gene which is expressed in a cell as obtained can be altered in that it is increased, reduced, including eliminated, or the pattern of regulation or induction may be changed through use of the method and DNA constructs of the present invention. The present invention as set forth above, relates to a method of gene or DNA targeting in cells of eukaryotic origin, such as of fungal, plant or animal, such as, vertebrate, particularly mammalian, and even more particu¬ larly human origin. That is, it relates to a method of introducing DNA into a cell, such as primary, secondary, or immortalized cells of vertebrate origin, through homol- ogous recombination or targeting of the DNA, which is introduced into genomic DNA of the cells at a preselected site. It is particularly related to homologous recombina¬ tion in which the transcription and/or translation prod¬ ucts of endogenous genes are modified through the use of DNA constructs comprising a targeting sequence, a regula- tory sequence, an exon and a splice-donor site. The pres¬ ent invention further relates to homologously recombinant cells produced by the present method and to uses of the homologously recombinant cells. The present invention also relates to a method of activating a gene which is present in primary cells, secondary cells or immortalized cells of vertebrate ori¬ gin, but is normally not expressed in the cells. Homolo¬ gous recombination or targeting is used to introduce into the cell's genome sequences which causes the gene to be expressed in the recipient cell. In a further embodiment, expression of a gene in a cell is enhanced or the pattern of regulation or induction of a gene is altered, through introduction of the DNA construct. As a result, the encoded product is expressed at levels higher than evident in the corresponding nontransfected cell. The present method and DNA constructs are also useful to produce cells in which expression of a desired product is less in the transfected cell than in the corresponding nontransfected cell. That is, in the transfected cell, less protein (including no protein) is produced than in the cells as obtained. In another embodiment, a normally silent gene encod¬ ing a desired product is activated in a transfected, primary, secondary, or immortalized cell and amplified. This embodiment is a method of introducing, by homologous recombination with genomic DNA, DNA sequences which are not normally functionally linked to the endogenous gene and (1) which, when inserted into the host genome at or near the endogenous gene, serve to alter (e.g., activate) the expression of the endogenous gene, and further (2) allow for selection of cells in which the activated endog¬ enous gene is amplified. Alternatively, expression of a gene normally expressed in the cell as obtained is en- hanced and the gene is amplified. The following is a description of the DNA constructs of the present invention, methods in which they are used to produce transfected cellε, transfected cells and uses of these cells. The DNA Construct The DNA construct of the present invention includes at least the following components: a targeting sequence; a regulatory sequence; an exon and an unpaired splice- donor site. In the construct, the exon is 3' of the regulatory sequence and the unpaired splice-donor site is 3' of the exon. In addition, there can be multiple exons and/or introns preceding (5' to) the exon flanked by the unpaired splice-donor site. As described herein, there frequently are additional construct components, such as a selectable markers or amplifiable markers. The DNA in the construct may be referred to as exoge¬ nous. The term "exogenous" is defined herein as DNA which is introduced into a cell by the method of the present invention, such as with the DNA constructs defined herein. Exogenous DNA can possess sequences identical to or dif¬ ferent from the endogenous DNA present in the cell prior to transfection. The Targeting Sequence or Sequences The targeting sequence or sequences are DNA sequences which permit legitimate homologous recombination into the genome of the selected cell containing the gene of inter¬ est. Targeting sequences are, generally, DNA sequences which are homologous to (i.e., identical or sufficiently similar to cellular DNA such that the targeting sequence and cellular DNA can undergo homologous recombination) DNA sequences normally present in the genome of the cells as obtained (e.g., coding or noncoding DNA, lying upstream of the transcriptional start site, within, or downstream of the transcriptional stop site of a gene of interest, or sequences present in the genome through a previous modifi¬ cation) . The targeting sequence or sequences used are selected with reference to the site into which the DNA in the DNA construct is to be inserted. One or more targeting sequences can be employed. For example, a circular plasmid or DNA fragment preferably employs a single targeting sequence. A linear plasmid or DNA fragment preferably employs two targeting sequences. The targeting sequence or sequences can, independently, be within the gene of interest (such as, the sequences of an exon and/or intron) , immediately adjacent to the gene of interest (i.e., with no additional nucleotides between the targeting sequence and the coding region of the gene of interest) , upstream gene of interest (such as the sequenc¬ es of the upstream non-coding region or endogenous promot¬ er sequences) , or upstream of and at a distance from the gene (such as, sequences upstream of the endogenous pro¬ moter) . The targeting sequence or sequences can include those regions of the targeted gene presently known or sequenced and/or regions further upstream which are struc¬ turally uncharacterized but can be mapped using restric¬ tion enzymes and determined by one skilled in the art. As taught herein, gene targeting can be used to insert a regulatory sequence isolated from a different gene, assembled from components isolated from difference cellular and/or viral sources, or synthesized as a novel regulatory sequence by genetic engineering methods within, immediately adjacent to, upstream, or at a substantial distance from an endogenous cellular gene. Alternatively or additionally, sequences which affect the structure or stability of the RNA or protein produced can be replaced, removed, added, or otherwise modified by targeting. For example, RNA stability elements, splice sites, and/or leader sequences of RNA molecules can be modified to improve or alter the function, stability, and/or translat- ability of an RNA molecule. Protein sequences may also be altered, such as signal sequences, propeptide sequences, active sites, and/or structural sequences for enhancing or modifying transport, secretion, or functional properties of a protein. According to this method, introduction of the exogenous DNA results in the alteration of the normal expression properties of a gene and/or the structural properties of a protein or RNA. The Regulatory Sequence The regulatory sequence of the DNA construct can be comprised of one or more promoters (such as a constitutive or inducible promoter) , enhancers, scaffold-attachment regions or matrix attachment sites, negative regulatory elements, tranεcription factor binding sites, or combina¬ tions of said sequences. The regulatory sequence can contain an inducible promoter, with the result that cells as produced or as introduced into an individual do not express the product but can be induced to do so (i.e., expression is induced after the transfected cells are produced but before im¬ plantation or after implantation) . DNA encoding the desired product can, of course, be introduced into cellε in such a manner that it iε expressed upon introduction (e.g., under a constitutive promoter) . The regulatory sequence can be isolated from cellular or viral genomes, (such regulatory sequences include those that regulate the expression of SV40 early or late genes, adenovirus major late genes, the mouse metallothionein-I gene, the elonga- tion factor-lα gene, cytomegalovirus genes, collagen genes, actin genes, immunoglobulin genes or the HMG-CoA reductase gene) . The regulatory sequence preferably con¬ tains transcription factor binding sites, such as a TATA Box, CCAAT Box, API, Spl or NF-KB binding siteε. Additional DNA Construct Elements The DNA construct further comprises one or more exonε. An exon is defined herein as a DNA sequence which is copied into RNA and is present in a mature mRNA mole- cule. The exons can, optionally, contain DNA which encodes one or more amino acids and/or partially encodes an amino acid (i.e., one or two bases of a codon) . Alternatively, the exon contains DNA which corresponds to a 5' non-coding region. Where the exogenous exon or exons encode one or more amino acids and/or a portion of an amino acid, the DNA construct is designed such that, upon transcription and splicing, the reading frame is in-frame with the second exon or coding region of the targeted gene. As used herein, in-frame means that the encoding sequences of a first exon and a second exon, when fused, join together nucleotides in a manner that does not change the appropri¬ ate reading frame of the portion of the mRNA derived from the second exon. Where the first exon of the targeted gene contains the sequence ATG to initiate translation, the exogenous exon of the construct preferably contains an ATG and, if required, one or more nucleotides such that the resulting coding region of the mRNA including the second and subse¬ quent exons of the targeted gene is in-frame. Examples of such targeted genes in which the first exon contains an ATG include the genes encoding hEPO, hGH, human colony stimulating factor-granulocyte/macrophage (hGM-CSF) , and human colony stimulating factor-granulocyte (hG-CSF) . A splice-donor site is a sequence which directs the splicing of one exon to another exon. Typically, the first exon lies 5' of the second exon, and the splice- donor site overlapping and flanking the first exon on its 3' side recognizes a splice-acceptor site flanking the . second exon on the 5' side of the second exon. Splice- donor sites have a characteristic consensuε εequence represented as: (A/OAG GURAGU (where R denotes a purine nucleotide) with the GU in the fourth and fifth positions, being required (Jackson, I.J., Nucleic Acids Research 19 : 3715-3798 (1991)) . The first three baseε of the splice- donor consensus site are the last three bases of the exon. Splice-donor sites are functionally defined by their ability to effect the appropriate reaction within the mRNA splicing pathway. An unpaired splice-donor site is defined herein as a splice-donor site which is present in a targeting con¬ struct and is not accompanied in the construct by a splice-acceptor site positioned 3' to the unpaired splice- donor site. The unpaired εplice-donor site results in splicing to an endogenous splice-acceptor site. A splice-acceptor site in a sequence which, like a splice-donor site, directs the splicing of one exon to another exon. Acting in conjunction with a splice-donor site, the splicing apparatus uses a splice-acceptor site to effect the removal of an intron. Splice-acceptor siteε have a characteristic sequence repreεented as: YYYYYYYYYY- NYAG, where Y denotes any pyrimidine and N denotes any nucleotide (Jackson, I.J., Nucleic Acids Research 19: 3715- 3798 (1991) ) . An intron is defined as a sequence of one or more nucleotides lying between two exons and which is removed, by splicing, from a precursor RNA molecule in the forma¬ tion of an mRNA molecule. The regulatory sequence is, for example, operatively linked to an ATG start codon, which initiateε tranεlation. Optionally, a CAP site (a specific mRNA initiation site which is asεociated with and utilized by the regulatory region) is operatively linked to the regulatory sequence and the ATG start codon. Alternatively, the CAP site associated with and utilized by the regulatory sequence is not included in the targeting construct, and the trans- criptional apparatus will define a new CAP site. For moεt genes, a CAP εite is usually found approximately 25 nucle¬ otides 3' of the TATA box. In one embodiment, the splice- donor site is placed immediately adjacent to the ATG, for example, where the presence of one or more nucleotides is not required for the exogenous exon to be in-frame with the second exon of the targeted gene. Preferably, DNA encoding one or more amino acids or portionε of an amino acid in-frame with the coding sequence of the targeted gene, is placed immediately adjacent to the ATG on its 3' side. In such an embodiment, the splice-donor site is placed immediately adjacent to the encoding DNA on its 3' side. Operatively linked or functionally placed is defined as a configuration in which the exogenous regulatory sequence, exon, splice-donor site and, optionally, a se¬ quence and splice-acceptor site are appropriately targeted at a position relative to an endogenous gene such that the regulatory element directs the production of a primary RNA transcript which initiates at a CAP site (optionally included in the targeting construct) and includes sequen¬ ceε corresponding to the exon and splice-donor site of the targeting construct, DNA lying upstream of the endogenous gene's regulatory region (if present), the endogenous gene's regulatory region (if present), the endogenous genes 5' nontranscribed region (if present), and exons and introns (if present) of the endogenous gene. In an opera¬ tively linked configuration the splice-donor site of the targeting construct directs a splicing event to a splice- acceptor site flanking one of the exons of the endogenous gene, such that a desired protein can be produced from the fully spliced mature transcript. In one embodiment, the splice-acceptor site is endogenous, such that the splicing event is directed to an endogenous exon, for example, of the endogenous gene. In another embodiment where the εplice-acceptor site is included in the targeting con¬ struct, the splicing event removes the intron introduced by the targeting construct. The encoding DNA (e.g., in exon 1 of the targeting construct) employed can optionally encode one or more amino acids, and/or a portion of an amino acid, which are the same as those of the endogenous protein. The enco¬ ding DNA sequence employed herein can, for example, corre¬ spond to the first exon of the gene of interest. The encoding DNA can alternatively encode one or more amino acids or a portion of an amino acid different from the first exon of the protein of interest. Such an embodiment is of particular interest where the amino acids of the first exon of the protein of interest are not critical to the activity or activities of the protein. For example, when fusions to the endogenous hEPO gene are constructed, sequences encoding the first exon of hGH can be employed. In this example, fusion of hGH exon 1 to hEPO exon 2 results in the formation of a hybrid signal peptide which is functional. In related constructs, any exon of human or non-human origin in which the encoded amino acids do not prevent the function of the hybrid signal peptide can be used. In a related embodiment, this technique can also be employed to correct a mutation found in a target gene. Where the desired product is a fusion protein of the endogenous protein and encoding sequences in the targeting construct, the exogenous encoding DNA incorporated into the cells by the present method includeε DNA which encodeε one or more exonε or a sequence of cDNA corresponding to a translation or transcription product which is to be fused to the product of the endogenous targeted gene. In this embodiment, targeting is used to prepare chimeric or multifunctional proteins which combine structural, enzy¬ matic, or ligand or receptor binding properties from two or more proteins into one polypeptide. For example, the exogenous DNA can encode an anchor to the membrane for the targeted protein or a εignal peptide to provide or improve cellular εecretion, leader εequences, enzymatic regionε, tranεmembrane domain regionε, co-factor binding regionε or other functional regionε. Examples of proteins which are not normally secreted, but which could be fused to a signal protein to provide secretion include dopa-decarbox- ylaεe, tranεcriptional regulatory proteinε, α-galactosi- daεe and tyroεine hydroxylaεe. Where the firεt exon of the targeted gene correεponds to a non-coding region (for example, the first exon of the follicle-stimulating hormone beta (FSKβ) gene , an exoge¬ nous ATG is not required and, preferably, is omitted. The DNA of the construct can be obtained from sources in which it occurs in nature or can be produced, using genetic engineering techniques or synthetic processes. The Targeted Gene and Resulting Product The DNA construct, when transfected into cells, such as primary, εecondary or immortalized cells, can control the expression of a desired product for example, the active or, functional portion of the protein or RNA. The product can be, for example, a hormone, a cytokine, an antigen, an antibody, an enzyme, a clotting factor, a transport protein, a receptor, a regulatory protein, a structural protein, a transcription factor, an anti-sense RNA, or a ribozyme. Additionally, the product can be a protein or a nucleic acid which does not occur in nature (i.e., a fusion protein or nucleic acid) . The method as described herein can produce one or more therapeutic products, such as erythropoietin, calci- tonin, growth hormone, insulin, insulinotropin, insulin¬ like growth factors, parathyroid hormone, interferon β, and interferon β, nerve growth factors, FSH / 3, TGF-/3, tumor necrosis factor, glucagon, bone growth factor-2, bone - 23 - growth factor-7, TSH-?, interleukin 1, interleukin 2, interleukin 3, interleukin 6, interleukin 11, interleukin 12, CSF-granulocyte, CSF-macrophage, CSF-granulocyte/ macrophage, immunoglobulins, catalytic antibodies, protein kinase C, glucocerebrosidase, superoxide diεmutase, tissue plasminogen activator, urokinase, antithrombin III, DNAse, cv-galactosidase, tyrosine hydroxylase, blood clotting factors V, blood clotting factor VII, blood clotting factor VIII, blood clotting factor IX, blood clotting factor X, blood clotting factor XIII, apolipoprotein E or apolipoprotein A-I, globins, low denεity lipoprotein receptor, IL-2 receptor, IL-2 antagoniεtε, alpha-1 anti- trypsin, immune responεe modifierε, and soluble CD4. Selectable Markers and Amplification The identification of the targeting event can be facilitated by the use of one or more selectable marker geneε. Theεe markers can be included in the targting construct or be present on different constructs. Select¬ able markers can be divided into two categories: posi- tively selectable and negatively selectable (in other words, markers for either positive selection or negative selection) . In positive selection, cells expressing the positively selectable marker are capable of surviving treatment with a selective agent (such as neo, xanthine- guanine phosphoribosyl transferase (gpt) , dhfr, adenosine deaminase (ada) , puromycin (pac) , hygromycin (hyg) , CAD which encodeε carbamyl phoεphate synthase, aspartate transcarbamylase, and dihydro-orotase glutamine εynthetase (GS) , multidrug resistance 1 (mdrl) and histidine D (hisD) , allowing for the selection of cells in which the targeting construct integrated into the host cell genome. In negative selection, cells expressing the negatively selectable marker are destroyed in the presence of the selective agent. The identification of the targeting event can be facilitated by the use of one or more marker genes exhibiting the property of negative selection, such that the negatively εelectable marker is linked to the exogenouε DNA, but configured such that the negatively selectable marker flanks the targeting sequence, and such that a correct homologous recombination event with se¬ quences in the host cell genome does not result in the stable integration of the negatively selectable marker (Mansour, S.L. et. al. , Nature 336:348-352 (1988)) . Mark- erε uεeful for thiε purpoεe include the Herpes Simplex Virus thymidine kinase (TK) gene or the bacterial gpt gene. A variety of selectable markerε can be incorporated into primary, εecondary or immortalized cells. For exam- pie, a selectable marker which confers a selectable pheno¬ type such as drug reεistance, nutritional auxotrophy, resistance to a cytotoxic agent or expression of a surface protein, can be used. Selectable marker genes which can be used include neo, gpt, dhfr, ada, pac, hyg, CAD, GS, mdrl and hisD. The selectable phenotype conferred makes it possible to identify and isolate recipient cells. Amplifiable genes encoding selectable markers (e.g., ada, GS, dhfr and the multifunctional CAD gene) have the added characteristic that they enable the selection of cells containing amplified copies of the selectable marker inserted into the genome. This feature provides a mecha¬ nism for significantly increasing the copy number of an adjacent or linked gene for which amplification is desir¬ able. Mutated versions of these sequences showing im- proved selection properties and other amplifiable sequenc¬ es can also be used. The order of components in the DNA construct can vary. Where the construct is a circular plasmid, the order of elements in the resulting structure can be: targeting sequence - plasmid DNA (comprised of sequences uεed for the selection and/or replication of the targeting plasmid in a microbial or other suitable host) - select¬ able marker(ε) - regulatory sequence - exon - splice-donor site. Preferably, the plasmid containing the targeting sequence and exogenous DNA elements is cleaved with a restriction enzyme that cuts one or more times within the targeting sequence to create a linear or gapped molecule prior to introduction into a recipient cell, such that the free DNA ends increase the frequency of the deεired homol- ogouε recombination event as described herein. In addi¬ tion, the free DNA ends may be treated with an exonuclease to create protruding 5' or 3' overhanging single-stranded DNA ends to increase the frequency of the desired homolo¬ gous recombination event. In this embodiment, homologous recombination between the targeting sequence and the cellular target will result in two copies of the targeting sequences, flanking the elements contained within the introduced plasmid. Where the construct is linear, the order can be, for example: a first targeting sequence - selectable marker - regulatory sequence - an exon - a splice-donor site - a second targeting sequence or, in the alternative, a first targeting sequence - regulatory sequence - an exon - a splice-donor site - DNA encoding a εelectable marker - a second targeting sequence. Cells that stably integrate the construct will survive treatment with the selective agent; a subset of the stably transfected cells will be homologously recombinant cells. The homologously recombi¬ nant cells can be identified by a variety of techniques, including PCR, Southern hybridization and phenotypic screening. In another embodiment, the order of the construct can be: a first targeting sequence - selectable marker - regulatory sequence - an exon - a splice-donor site - an intron - a εplice-acceptor εite - a εecond targeting sequence. Alternatively, the order of components in the DNA construct can be, for example: a firεt targeting sequence - selectable marker 1 - regulatory sequence - an exon - a splice-donor site - a second targeting εequence - selec - able marker 2, or, alternatively, a first targeting se¬ quence - regulatory sequence - an exon - a splice-donor site - selectable marker 1 - a second targeting sequence - εelectable marker 2. In this embodiment selectable marker 2 displays the property of negative selection. That is, the gene product of selectable marker 2 can be selected against by growth in an appropriate media formulation containing an agent (typically a drug or metabolite ana¬ log) which kills cells expressing selectable marker 2. Recombination between the targeting sequences flanking selectable marker 1 with homologous sequences in the hoεt cell genome results in the targeted integration of select¬ able marker 1, while selectable marker 2 is not integrat¬ ed. Such recombination eventε generate cellε which are stably transfected with selectable marker 1 but not stably transfected with selectable marker 2, and such cells can be selected for by growth in the media containing the selective agent which selects for selectable marker 1 and the selective agent which selects against selectable marker 2. The DNA construct also can include a positively selectable marker that allows for the selection of cells containing amplified copies of that marker. The amplifi¬ cation of such a marker resultε in the co-amplification of flanking DNA sequences. In this embodiment, the order of conεtruct componentε iε, for example: a first targeting sequence - an amplifiable poεitively selectable marker - a second selectable marker (optional) - regulatory sequence - an exon - a splice-donor site - a second tar¬ geting DNA sequence. In this embodiment, the activated gene can be further amplified by the inclusion of a εelectable marker gene which has the property that cells containing amplified copies of the selectable marker gene can be selected for by culturing the cells in the presence of the appropriate selectable agent. The activated endogenous gene will be amplified in tandem with the amplified selectable marker gene. Cells containing many copies of the activated endogenous gene may produce very high levels of the de- sired protein and are useful for in vitro protein produc¬ tion and gene therapy. In any embodiment, the selectable and amplifiable marker genes do not have to lie immediately adjacent to each other. Optionally, the DNA construct can include a bacterial origin of replication and bacterial antibiotic resistance markers or other selectable markers, which allow for large-scale plasmid propagation in bacteria or any other suitable cloning/host syεtem. A DNA construct which includes DNA encoding a selectable marker, along with additional εequences, εuch aε a promoter, and splice junctionε, can be used to confer a εelectable phenotype upon tranεfected cells (e.g., plasmid pcDNEO, schematical¬ ly represented in Figure 4) . Such a DNA construct can be co-transfected into primary or secondary cells, along with a targeting DNA sequence, using methods described herein. Transfection and Homologous Recombination According to the preεent method, the construct is introduced into the cell, such as a primary, secondary, or immortalized cell, as a single DNA construct, or as sepa- rate DNA εequences which become incorporated into the chromoεomal or nuclear DNA of a transfected cell. The targeting DNA conεtruct, including the targeting εequences, regulatory sequence, an exon, a splice-donor site and εelectable marker gene(ε), can be introduced into cells on a single DNA construct or on separate constructs. The total length of the DNA construct will vary according to the number of components (targeting sequences, regula- tory sequences, exons, selectable marker gene, and other elements, for example) and the length of each. The entire construct length will generally be at least about 200 nucleotides. Further, the DNA can be introduced as lin¬ ear, double-stranded (with or without single-stranded regions at one or both ends) , εingle-εtranded, or circular. Any of the conεtruct typeε of the diεcloεed invention is then introduced into the cell to obtain a tranεfected cell. The tranεfected cell iε maintained under conditionε which permit homologouε recombination, as is known in the art (Capecchi, M.R., Science 244:1288-1292 (1989)) . When the homologously recombinant cell is maintained under conditions sufficient for transcription of the DNA, the regulatory region introduced by the targeting construct, as in the case of a promoter, will activate transcription. The DNA constructs may be introduced into cells by a variety of physical or chemical methods, including elec¬ troporation, microinjection, microprojectile bombardment, calcium phosphate precipitation, and liposome-, poly- brene-, or DEAE dextran-mediated transfection. Alter¬ natively, infectious vectors, εuch aε retroviral, herpes, adenovirus, adenovirus-associated, mumps and poliovirus vectors, can be uεed to introduce the DNA. Optionally, the targeting DNA can be introduced into a cell in two or more εeparate DNA fragments. In the event two fragments are used, the two fragments εhare DNA sequence homology (overlap) at the 3' end of one fragment and the 5' end of the other, while one carries a first targeting sequence and the other carries a second target- ing sequence. Upon introduction into a cell, the two fragments can undergo homologous recombination to form a single fragment with the first and second targeting se¬ quences flanking the region of overlap between the two original fragments. The product fragment is then in a form suitable for homologous recombination with the cellular target sequenceε. More than two fragments can be used, designed such that they will undergo homologouε recombination with each other to ultimately form a product suitable for homologous recombination with the cellular target sequences as described above. The Homologously Recombinant Cells The targeting event resultε in the inεertion of the regulatory εequence of the targeting conεtruct, placing the endogenous gene under their control (for example, by insertion of either a promoter or an enhancer, or both, upstream of the endogenous gene or regulatory region) . Optionally, the targeting event can simultaneously result in the deletion of the endogenous regulatory element, such aε the deletion of a tissue-specific negative regulatory element. The targeting event can replace an existing element; for example, a tissue- specific enhancer can be replaced by an enhancer that has broader or different cell-type specificity than the naturally-occurring ele¬ ments, or displayε a pattern of regulation or induction that iε different from the corresponding nontransfected cell. In this embodiment the naturally occurring sequenc¬ eε are deleted and new εequences are added. Alternative¬ ly, the endogenous regulatory elementε are not removed or replaced but are diεrupted of disabled by the targeting event, such as by targeting the exogenous sequences within the endogenous regulatory elements. After the DNA is introduced into the cell, the cell is maintained under conditions appropriate for homologous recombination to occur between the genomic DNA and a portion of the introduced DNA, as is known in the art (Capecchi, M.R. , Science 244:1288-1292 (1989)) . Homologous recombination between the genomic DNA and the introduced DNA results in a homologously recombinant cell, such aε a fungal, plant or animal, and particularly, primary, εecondary, or immortalized human or other mamma¬ lian cell in which sequences which alter the expression of an endogenous gene are operatively linked to an endogenous gene encoding a product, producing a new transcription unit with expression and/or coding potential that is different from that of the endogenous gene. Particularly, the invention includeε a homologouεly recombinant cell comprising regulatory sequences and an exon, flanked by a splice-donor site, which are introduced at a predetermined site by a targeting DNA construct, and are operatively linked to the second exon of an endogenous gene. Option¬ ally, there may be multiple exogenous exons (coding or non-coding) and introns operatively linked to any exon of the endogenous gene. The resulting homologously recombi- nant cells are cultured under conditions which εelect for amplification, if appropriate, of the DNA encoding the amplifiable marker and the novel tranεcriptional unit. With or without amplification, cellε produced by thiε method can be cultured under conditionε, aε are known in the art, εuitable for the expreεsion of the protein, thereby producing the protein in vitro, or the cells can be used for in vivo delivery of a therapeutic protein (i.e., gene therapy) . As used herein, the term primary cell includes cellε preεent in a εuεpenεion of cellε iεolated from a verte¬ brate tissue source (prior to their being plated, i.e., attached to a tissue culture substrate such as a dish or flask) , cells present in an explant derived from tisεue, both of the previouε types of cells plated for the first time, and cell suspensionε derived from theεe plated cells. The term secondary cell or cell strain refers to cells at all subsequent steps in culturing. That is, the first time a plated primary cell is removed from the culture εubstrate and replated (pasεaged) , it iε referred to herein as a secondary cell, as are all cells in subεe- quent paεsageε. Secondary cells are cell strainε which conεiεt of secondary cells which have been passaged one or more times. A cell strain consistε of secondary cells that: 1) have been passaged one or more times; 2) exhibit a finite number of mean population doublings in culture; 3) exhibit the properties of contact-inhibited, anchorage dependent growth (anchorage-dependence does not apply to cells that are propagated in suεpension culture) ; and 4) are not immortalized. Immortalized cells are cell lines (as opposed to cell strains with the designation "strain" reserved for primary and secondary cells) , a critical feature of which is that they exhibit an apparently unlimited lifeεpan in culture. Cells selected for the subject method can fall into four types or categories: 1) cells which do not, as ob¬ tained, make or contain the protein or product (such as a protein that is not normally expresεed by the cell or a fusion protein not normally found in nature) , 2) cells which make or contain the protein or product but in quan- tities other than that deεired (such as, in quantities less than the physiologically normal lower level for the cell as it is obtained) , 3) cells which make the protein or product at physiologically normal levels for the cell as it is obtained, but are to be augmented or enhanced in their content or production, and 4) cells in which it is desirable to change the pattern of regulation or induction of a gene encoding a protein. Primary, εecondary and immortalized cellε to be tranεfected by the present method can be obtained from a variety of tissues and include all cell types which can be maintained in culture. For example, primary and secondary cells which can be tranεfected by the preεent method include fibroblaεts, keratinocytes, epithelial cells (e.g., mammary epithelial cellε, intestinal epithelial cells) , endothelial cells, glial cells, neural cellε, formed elementε of the blood (e.g., lymphocyteε, bone marrow cellε) , muεcle cellε and precurεorε of theεe εomat- ic cell typeε. Where the homologously recombinant cells are to be used in gene therapy, primary cells are prefera- bly obtained from the individual to whom the transfected primary or εecondary cellε are administered. However, primary cells can be obtained from a donor (other than the recipient) of the same species. Homologously recombinant immortalized cells can alεo be produced by the present method and used for either protein production or gene therapy. Exampleε of immortal¬ ized human cell lineε uεeful for protein production or gene therapy by the present method include, but are not limited to, HT1080 cells (ATCC CCL 121) , HeLa cells and derivatives of HeLa cells (ATCC CCL 2, 2.1 and 2.2), MCF-7 breast cancer cellε (ATCC BTH 22) , K-562 leukemia cells (ATCC CCL 243), KB carcinoma cells (ATCC CCL 17), 2780AD ovarian carcinoma cells (Van der Blick, A.M. et. al. , Cancer Res. 48:5927-5932 (1988), Raji cells (ATCC CCL 86), Jurkat cellε (ATCC TIB 152), Namalwa cellε (ATCC CRL 1432), HL-60 cellε (ATCC CCL 240), Daudi cells (ATCC CCL 213), RPMI 8226 cells (ATCC CCL 155), U-937 cells (ATCC CRL 1593), Bowes Melanoma cells (ATCC CRL 9607), WI-38VA13 subline 2R4 cellε (ATCC CLL 75.1), and MOLT-4 cells (ATCC CRL 1582) , as well as heterohybridoma cells produced by fusion of human cellε and cells of another species. Secondary human fibroblast εtrainε, εuch aε WI-38 (ATCC CCL 75) and MRC-5 (ATCC CCL 171) may be uεed. In addi¬ tion, primary, εecondary, or immortalized human cellε, aε well aε primary, secondary, or immortalized cells from other species which display the properties of gene ampli¬ fication in vitro can be used for iri vitro protein produc¬ tion or gene therapy. Method of Converting a Gene into a cDNA Copy The present invention also relates to a method by which homologous recombination is used to convert a gene into a cDNA copy (a gene copy devoid of intronε) . The cDNA copy can be tranεferred into yeaεt or bacteria for in vitro protein production, or the cDNA copy can be inserted into a mammalian cell for in vitro or in vivo protein production. If the cDNA is to be transferred to microbial cells, two DNA constructs containing targeting sequences are introduced by homologous recombination, one construct upstream of and one construct downstream of a human gene encoding a therapeutic protein. For example, the sequenc¬ eε introduced upεtream include DNA sequences homologous to genomic DNA sequences at or upstream of the DNA encoding the first amino acid of a mature, processed therapeutic protein; a retroviral long term repeat (LTR) ; sequenceε encoding a marker for selection in microbial cells; a regulatory element that functions in microbial cells,* and DNA encoding a leader peptide that promotes secretion from microbial cells with a splice-donor site. The sequences introduced upstream are introduced near to and upstream of genomic DNA encoding the first amino acid of a mature, processed therapeutic protein. The sequenceε introduced downstream include DNA sequenceε homologous to genomic DNA sequenceε at or downstream of the DNA encoding the last amino acid of a mature, processed protein; a microbial transcriptional termination sequence; sequenceε capable of directing DNA replication in microbial cells; and a retro¬ viral LTR. The sequences introduced downstream are intro¬ duced adjacent to and downstream of the DNA encoding the stop codon of the mature, proceεsed therapeutic protein. After the two DNA constructε are introduced into cellε, the reεulting cells are maintained under conditions appro¬ priate for homologous recombination between the introduced DNA and genomic DNA, thereby producing homologously recom- binant cells. Optionally, one or both of the DNA con¬ structs can encode one or more markers for either positive or negative εelection of cellε containing the DNA con¬ struct, and a selection step can be added to the method after one or both of the DNA constructs have been intro- duced into the cells. Alternatively, the sequences encod¬ ing the marker for selection in microbial cells and the sequences capable of directing DNA replication in microbi¬ al cells can both be present in either the upstream or the downstream targeting conεtruct, or the marker for selec- tion in microbial cellε can be present in the downstream targeting construct and the sequences capable of directing DNA replication in microbial cells can be present in the upstream targeting construct. The homologously recombi¬ nant cells are then cultured under conditions appropriate for LTR directed transcription, processing and reverse transcription of the RNA product of the gene encoding the therapeutic protein. The product of reverse transcription is a DNA construct comprising an intronlesε DNA copy encoding the therapeutic protein, operatively linked to DNA εequences comprising the two exogenous DNA constructε described above. The intronlesε DNA conεtruct produced by the preεent method is then introduced into a microbial cell. The microbial cell is then cultured under condi¬ tions appropriate for expression and secretion of the therapeutic protein. In Vivo Protein Production Homologously recombinant cells of the present inven¬ tion are useful, as populations of homologously recombi¬ nant cell lines, aε populations of homologously recombi- nant primary or secondary cells, homologously recombinant clonal cell strainε or lines, homologously recombinant heterogenous cell εtrains or lines, and aε cell mixtures in which at least one representative cell of one of the four preceding categories of homologously recombinant cells is present. Such cells may be used in a delivery system for treating an individual with an abnormal or undesirable condition which respondε to delivery of a therapeutic product, which iε either: 1) a therapeutic protein (e.g., a protein which iε absent, underproduced relative to the individual's physiologic needs, defective or inefficiently or inappropriately utilized in the indi¬ vidual; a protein with novel functions, such aε enzymatic or tranεport functionε) or 2) a therapeutic nucleic acid (e.g., RNA which inhibitε gene expression or has intrinsic enzymatic activity) . In the method of the present inven¬ tion of providing a therapeutic protein or nucleic acid, homologously recombinant primary cells, clonal cell strainε or heterogenouε cell strains are administered to an individual in whom the abnormal or undesirable condi¬ tion is to be treated or prevented, in sufficient quantity and by an appropriate route, to express or make available the protein or exogenous DNA at physiologically relevant levels. A physiologically relevant level is one which either approximates the level at which the product is normally produced in the body or results in improvement of the abnormal or undesirable condition. According to an embodiment of the invention described herein, the homo¬ logously recombinant immortalized cell lines to be admin- istered can be enclosed in one or more semipermeable barrier devices. The permeability properties of the device are such that the cells are prevented from leaving the device upon implantation into an animal, but the therapeutic product is freely permeable and can leave the barrier device and enter the local space surrounding the implant or enter the εystemic circulation. For example, hGH, hEPO, human insulinotropin, hGM-CSF, hG-CSF, human a- interferon, or human FSH/3 can be delivered systemically in humans for therapeutic benefits. Barrier devices are particularly useful and allow homologously recombinant immortalized cells, homologously recombinant cells from another species (homologously recombinant xenogeneic cells) , or cells from a nonhisto- compatibility-matched donor (homologously recombinant allogeneic cells) to be implanted for treatment of human or animal conditions or for agricultural uses (i.e., meat and dairy production) . Barrier devices alεo allow conve¬ nient εhort-term (i.e., transient) therapy by providing ready access to the cells for removal when the treatment regimen is to be halted for any reason. A number of synthetic, semisynthetic, or natural filtration membranes can be used for this purpose, includ¬ ing, but not limited to, cellulose, cellulose acetate, nitrocellulose, polysulfone, polyvinylidene difluoride, polyvinyl chloride polymerε and polymers of polyvinyl chloride derivatives. Barrier devices can be utilized to allow primary, secondary, or immortalized cells from another species to be used for gene therapy in humans. In Vitro Protein Production Homologously recombinant cells from human or non- human species according to this invention can also be used for in vitro protein production. The cells are maintained under conditions, as are known in the art, which result in expression of the protein. Proteins expressed using the methods described may be purified from cell lysateε or cell εupernatants in order to purify the desired protein. Proteins made according to this method include therapeutic proteins which can be delivered to a human or non-human animal by conventional pharmaceutical routes as is known in the art (e.g., oral, intravenous, intramuscular, intra¬ nasal or subcutaneouε) . Such proteins include hGH, hEPO, and human insulinotropin, hGM-CSF, hG-CSF, FSH/3 or a- interferon. These cells can be immortalized, primary, or secondary cells. The use of cellε from other species may be desirable in cases where the non-human cells are advan¬ tageous for protein production purposes where the non- human protein is therapeutically or commercially useful, for example, the use of cells derived from salmon for the production of salmon calcitonin, the use of cells derived from pigs for the production of porcine insulin, and the uεe of bovine cellε for the production of bovine growth hormone. Advantageε The methodologieε, DNA conεtructs, cells, and resul¬ ting proteinε of the invention herein possess versatility and many other advantages over processes currently em¬ ployed within the art in gene targeting. The ability to activate an endogenouε gene by positioning an exogenous regulatory sequence at various positionε ranging from immediately adjacent to the gene of interest (directly fused to the normal gene's transcribed region) to 30 kilobaεe pairε or further upstream of the tranεcribed region of an endogenouε gene, or within an intron of an endogenous gene, is advantageous for gene expression in cells. For example, it can be employed to position the regulatory element upstream or downstream of regions that normally silence or negatively regulate a gene. The positioning of a regulatory element upstream or downstream of such a region can override εuch dominant negative effectε that normally inhibit tranεcription. In addition, regions of DNA that normally inhibit transcription or have an otherwise detrimental effect on the expresεion of a gene may be deleted uεing the targeting constructs, des¬ cribed herein. Additionally, since promoter function is known to depend strongly on the local environment, a wide range of positionε may be explored in order to find thoεe local environments optimal for function. However, since, ATG start codons are found frequently within mammalian DNA (approximately one occurrence per 48 base pairs) , tran¬ scription cannot simply initiate at any position upstream of a gene and produce a transcript containing a long leader sequence preceding the correct ATG start codon, since the frequent occurrence of ATG codons in such a leader sequence will prevent translation of the correct gene product and render the message useless. Thus, the incorporation of an exogenous exon, a splice-donor site, and, optionally, an intron and a splice-acceptor site into targeting conεtructε compriεing a regulatory region allows gene expression to be optimized by identifying the optimal site for regulatory region function, without the limita- tion imposed by needing to avoid inappropriate ATG start codons in the mRNA produced. This provideε εignificantly increased flexibility in the placement of the construct and makes it possible to activate a wider range of genes. The DNA constructε of the present invention are also useful, for example, in processes for making fusion pro¬ teins encoded by recombinant, or exogenous, sequenceε and endogenouε sequences. Gene targeting and amplification as disclosed above are particularly useful for altering on the expression of genes which form transcription units which are εufficient- ly large that they are difficult to isolate and express, or for turning on genes for which the entire protein coding region is unavailable or has not been cloned. Thus, the DNA constructs described above are useful for operatively linking exogenous regulatory elementε to endogenous genes in a way that precisely defines the tranεcriptional unit, provides flexibility in the relative positioning of exogeneous regulatory elements and endoge¬ nous genes ultimately, enables a highly controlled system for obtaining and regulating expresεion of geneε of thera¬ peutic interest. Explanation of the Exampleε As described herein, Applicants have demonstrated that DNA can be introduced into cells, such as primary, secondary or immortalized vertebrate cells and integrated into the genome of the transfected cells by homologous recombination. They have further demonstrated that the exogenous DNA has the desired function in the homologously recombinant (HR) cells and that correctly targeted cells can be identified on the basis of a detectable phenotype conferred by the properly targeted DNA. Applicants describe construction of a plasmid useful for targeting to a particular locus (the HPRT locus) in the human genome and selection based upon a drug resistant phenotype (Example la) . This plasmid is designated pE3Neo and its integration into the cellular genome at the HPRT locuε produces cells which have an hprt " , 6-TG resistant phenotype and are also G418 resistant. As described, they have shown that pE3Neo functions properly in gene target- ing in an establiεhed human fibroblaεt cell line (Example lb) , by demonstrating localization of the DNA introduced into established cells within exon 3 of the HPRT gene. In addition, Applicants demonstrate gene targeting in primary and secondary human skin fibroblasts using pE3Neo (Example lc) . The subject application further demon¬ strates that modification of DNA termini enhances target¬ ing of DNA into genomic DNA (Examples lc and le) . Applicants also describe methods by which a gene can be inserted at a preselected site in the genome of a cell, such as a primary, secondary, or immortalized cell by gene targeting (Example Id) . In addition, the preεent invention relates to a method of protein production using transfected cells. The method involves transfecting cells, such as primary cells, secondary cellε or immortalized cells, with exogenous DNA which encodes a therapeutic product or with DNA which is sufficient to target to an endogenous gene which encodes a therapeutic product. For example, Examples lg, lh, 1j , Ik, 2, 3, 4 and 6 - 9 describe protein production by targeting of a selected endogenous gene with DNA sequence elements which will alter the expresεion of the endogenouε gene. Applicantε alεo deεcribe DNA conεtructs and methods for amplifying an endogenous cellular gene that has been activated by gene targeting (Examples 3, 6, 8 and 9) . Examples lf-lh, 2, 4 and 6 illustrate embodiments in which the normal regulatory sequenceε upεtream of the human EPO gene are altered to allow expression of hEPO in primary or secondary fibroblaεt strains which do not express EPO in detectable quantities in their untrans- fected state. In one embodiment the product of targeting leaves the normal EPO protein intact, but under the con¬ trol of the mouse metallothionein promoter. Examples li and lj demonstrate the use of similar targeting constructs to activate the endogenous growth hormone gene in primary or secondary human fibroblasts. In other embodiments described for activating EPO expression in human fibro¬ blasts, the products of targeting events are chimeric transcription units, in which the first exon of the human growth hormone gene is positioned upstream of EPO exons 2- 5. The product of transcription (controlled by the mouse metallothionein promoter) , splicing, and translation is a protein in which amino acids 1-4 of the hEPO signal pep- tide are replaced with amino acid residues 1-3 of hGH. The chimeric portion of this protein, the signal peptide, is removed prior to secretion from cells. Example 5 describeε targeting conεtructs and methods for producing cells which will convert a gene (with introns) into an expressible cDNA copy of that gene (without introns) and the recovery of such expressible cDNA molecules in micro¬ bial (e.g., yeast or bacterial) cells. Example 6 de¬ scribes construction of a targeting vector, designated pREP04 for dual selection and selection of cells in which the dhfr gene iε amplified. Plasmid pREP04 has been used to amplify the human EPO (hEPO) locus in HT1080 cells (an immortalized human cell line) after activation of the endogenous hEPO gene by homologous recombination. As described, stepwiεe selection in methotrexate-containing media resulted in a 70-fold increase in hEPO production in cells resiεtant to 0.4 μM methotrexate. Examples 7 and 8 describe methods for inserting a regulatory sequence upstream of the normal EPO promoter and methods for EPO production using such a construct. In addition, Example 8 describeε the amplification of a targeted EPO gene produced by the method of Example 7. Example 9 describes methods for targeting the human c_- interferon, GM-CSF, G-CSF, and FSH/3 genes to create cells useful for in protein production. The Examples provide methods for activating or for activating and amplifying endogenous genes by gene target¬ ing which do not require manipulation or other uses of the target genes' protein coding regions. Using the methods and DNA constructs or plasmids taught herein or modifica- tions thereof which are apparent to one of ordinary skill in the art, gene expresεion can be altered in cellε that have propertieε desirable for in vitro protein production (e.g., pharmaceutics) or in vivo protein delivery methods (e.g. gene therapy) . Figures 5 and 6 illustrate two strategies for transcriptionally activating the hEPO gene. Using the methods and DNA constructs or plasmidε taught herein or modifications thereof which are apparent to one of ordinary εkill in the art, exogenouε DNA which encodes a therapeutic product (e.g., protein, ribozyme, anti-εenεe RNA) can be inεerted at preεelected sites in the genome of vertebrate (e.g., mammalian, both human and nonhuman) primary or secondary cells. The present invention will now be illustrated by the following examples, which are not intended to be limiting in any way. EXAMPLES EXAMPLE 1. PRODUCTION OF TRANSFECTED CELL STRAINS BY GENE TARGETING Gene targeting occurs when transfecting DNA either integrates into or partially replaces chromosomal DNA sequences through a homologous recombinant event. While εuch eventε can occur in the course of any given transfec¬ tion experiment, they are usually masked by a vast excess of eventε in which plasmid DNA integrateε by nonhomolo- gous, or illegitimate, recombination. a. GENERATION OF A CONSTRUCT USEFUL FOR SELECTION OF GENE TARGETING EVENTS IN HUMAN CELLS One approach to selecting the targeted events is by genetic selection for the losε of a gene function due to the integration of tranεfecting DNA. The human HPRT locuε encodeε the enzyme hypoxanthine-phoεphoribosyl tranεfer¬ ase, hprt " cells can be selected for by growth in medium containing the nucleoside analog 6-thioguanine (6-TG) : cells with the wild-type (HPRT+) allele are killed by 6-TG, while cells with mutant (hprf) alleles can survive. Cells harboring targeted events which disrupt HPRT gene function are therefore selectable in 6-TG medium. To construct a plasmid for targeting to the HPRT locus, the 6.9 kb Hindlll fragment extending from posi- tions 11,960-18,869 in the HPRT sequence (Genebank name HUMHPRTB; Edwards, A. et al. , Genomics 6:593-608 (1990)) and including exons 2 and 3 of the HPRT gene, is subcloned into the Hindlll site of pUC12. The resulting clone is cleaved at the unique Xhol site in exon 3 of the HPRT gene fragment and the 1.1 kb Sall-Xhol fragment containing the neo gene from pMClNeo (Stratagene) is inserted, disrupting the coding sequence of exon 3. One orientation, with the direction of neo transcription opposite that of HPRT transcription was chosen and designated pE3Neo. The replacement of the normal HPRT exon 3 with the neo-disrup- ted version will result in an hprt " , 6-TG resistant pheno¬ type. Such cells will also be G418 resiεtant. b. GENE TARGETING IN AN ESTABLISHED HUMAN FIBROBLAST CELL LINE As a demonstration of targeting in immortalized cell lineε, and to eεtablish that pE3Neo functions properly in gene targeting, the human fibrosarcoma cell line HT1080 (ATCC CCL 121) waε transfected with pE3Neo by electropora- tion. HT1080 cells were maintained in HAT (hypoxanthine/ aminopterin/xanthine) supplemented DMEM with 15% calf serum (Hyclone) prior to electroporation. Two days before electroporation, the cells are switched to the same medium without aminopterin. Exponentially growing cellε were trypεinized and diluted in DMEM/15% calf εerum, centri¬ fuged, and reεuεpended in PBS (phoεphate buffered εaline) at a final cell volume of 13.3 million cellε per ml. pE3Neo iε digeεted with Hindlll, separating the 8 kb HPRT-neo fragment from the pUC12 backbone, purified by phenol extraction and ethanol precipitation, and resus¬ pended at a concentration of 600 μg/ml. 50 μl (30 μg) was added to the electroporation cuvette (0.4 cm electrode gap; Bio-Rad Laboratories) , along with 750 μl of the cell suεpension (10 million cells) . Electroporation was at 450 volts, 250 μFarads (Bio-Rad Gene Pulser; Bio-Rad Laborato- rieε) . The contentε of the cuvette were immediately added to DMEM with 15% calf serum to yield a cell suεpenεion of 1 million cells per 25 ml media. 25 ml of the treated cell suεpenεion was plated onto 150 mm diameter tissue culture disheε and incubated at 37°C, 5% C0 2 . 24 hrs later, a G418 solution was added directly to the plates to yield a final concentration of 800 μg/ml G418. Five days later the media was replaced with DMEM/15% calf serum/ 800 μg/ml G418. Nine days after electroporation, the media was replaced with DMEM/15% calf serum/800 μg/ml G418 and 10 μM 6-thioguanine. Colonies resiεtant to G418 and 6-TG were picked uεing cloning cylinderε 14-16 dayε after the dual εelection was initiated. The resultε of five repreεentative targeting experi¬ ments in HT1080 cells are shown in Table 1. TABLE 1 Number of Number of G418 r Transfection Treated Cells 6-TG r Clones 1 1 x 10 7 32 2 1 x 10 7 28 3 1 x 10 7 24 4 1 x 10 7 32 5 1 x 10 7 66 For transfection 5, control plates designed to deter¬ mine the overall yield of G418 r colonies indicated that 33,700 G418 r colonieε could be generated from the initial 1 x 10 7 treated cells. Thus, the ratio of targeted to non-targeted events is 66/33,700, or 1 to 510. In the five experiments combined, targeted events arise at a frequency of 3.6 x 10 6 , or 0.00036% of treated cells. Restriction enzyme and Southern hybridization experi¬ ments using probes derived from the neo and HPRT geneε localized the neo gene to the HPRT locuε at the predicted εite within HPRT exon 3. c. GENE TARGETING IN PRIMARY AND SECONDARY HUMAN SKIN FIBROBLASTS pE3Neo is digeεted with Hindlll, εeparating the 8 kb HPRT-neo fragment from the pUC12 backbone, and purified by phenol extraction and ethanol precipitation. DNA waε resuspended at 2 mg/ml. Three million secondary human foreskin fibroblasts cells in a volume of 0.5 ml were electroporated at 250 volts and 960 μFarads, with 100 μg of Hindlll pE3Neo (50 μl) . Three εeparate transfectionε were performed, for a total of 9 million treated cellε. Cellε are proceεεed and selected for G418 resiεtance. 500,000 cellε per 150 mm culture diεh were plated for G418 selection. After 10 days under selection, the culture medium is replaced with human fibroblast nutrient medium containing 400 μg/ml G418 and 10 μM 6-TG. Selection with the two drug combination is continued for 10 additional days. Plates are scanned microscopically to localize human fibroblast colonies resistant to both drugs. The fraction of G418 r t-TG r colonies is 4 per 9 million treat- ed cells. These colonies constitute 0.0001% (or 1 in a million) of all cells capable of forming colonies. Con¬ trol plates designed to determine the overall yield of G418 r colonies indicated that 2,850 G418 r colonies could be generated from the initial 9 x 10 6 treated cells. Thus, the ratio of targeted to non-targeted events is 4/2,850, or 1 to 712. Restriction enzyme and Southern hybridization experiments using probeε derived from the neo and HPRT geneε were used to localize the neo gene to the HPRT locus at the predicted site within HPRT exon 3 and demonstrate that targeting had occurred in these four clonal cell strains. Colonies resistant to both drugs have alεo been iεolated by tranεfecting primary cells (1/3.0 x 10 7 ) . The results of several pE3Neo targeting experiments are εummarized in Table 2. Hindlll digeεted pE3Neo waε either transfected directly or treated with exonuclease III to generate 5' single-εtranded overhangs prior to tranεfection (εee Example lc) . DNA preparationε with εingle-stranded regions ranging from 175 to 930 base pairs in length were tested. Using pE3neo digested with Hindlll alone, 1/799 G418-resiεtant colonieε were identified by reεtriction enzyme and Southern hybridization analysis as having a targeted insertion of the neo gene at the HPRT locus (a total of 24 targeted clones were isolated) . Targeting was maximally stimulated (approximately 10-fold stimulation) when overhangs of 175 bp were used, with 1/80 G418 r colonies displaying restriction fragments that are diagnostic for targeting at HPRT (a total of 9 targeted clones were isolated) . Thus, using the conditions and recombinant DNA constructs described here, targeting is readily observed in normal human fibroblasts and the overall targeting frequency (the number of targeted clones divided by the total number of clones stably transfected to G418-resistance) can be stimulated by transfection with targeting constructε containing εingle-stranded overhang¬ ing tails, by the method as described in Example le. TABLE 2 TARGETING TO THE HPRT LOCUS IN HUMAN FIBROBLASTS pE3neo Number of Number Targeted Total Number of Treatment Experiments Per G418 r Colony Targeted Clone Hindlll digest 6 1/799 24 175 bp overhang 1 1/80 9 350 bp overhang 3 1/117 20 930 bp overhang 1 1/144 1 d. GENERATION OF A CONSTRUCT FOR TARGETED INSERTION OF A GENE OF THERAPEUTIC INTEREST INTO THE HUMAN GENOME AND ITS USE IN GENE TARGETING A variant of pE3Neo, in which a gene of therapeutic interest is inserted within the HPRT coding region, adja¬ cent to or near the neo gene, can be used to target a gene of therapeutic interest to a specific position in a recip¬ ient primary or secondary cell genome. Such a variant of pE3Neo can be constructed for targeting the hGH gene to the HPRT locus. pXGH5 (εchematically preεented in Figure 3) is di¬ geεted with EcoRI and the 4.1 kb fragment containing the hGH gene and linked mouse metallothionein (mMT) promoter is isolated. The EcoRI overhangs are filled in with the Klenow fragment from E. coli DNA polymerase. Separately, pE3Neo is digested with Xhol, which cuts at the junction of the neo fragment and HPRT exon 3 (the 3' junction of the insertion into exon 3) . The Xhol overhanging ends of the linearized plasmid are filled in with the Klenow fragment from E. coli DNA polymerase, and the resulting fragment is ligated to the 4.1 kb blunt-ended hGH-mMT fragment. Bacterial colonies derived from the ligation mixture are screened by restriction enzyme analysis for a single copy insertion of the hGH-mMT fragment and one orientation, the hGH gene transcribed in the same direc¬ tion as the neo gene, is chosen and designated pE3Neo/hGH. pE3Neo/hGH is digested with Hindlll, releasing the 12.1 kb fragment containing HPRT, neo and mMT-hGH sequences. Digested DNA is treated and transfected into primary or secondary human fibroblaεtε as described in Example lc. G418 r TG r colonies are selected and analyzed for targeted insertion of the mMT-hGH and neo sequences into the HPRT gene as described in Example lc. Individual colonies are aεεayed for hGH expression using a commercially available immunoassay (Nichols Institute) . Secondary human fibroblasts were tranεfected with pE3Neo/hGH and thioguanine-reεistant colonies were ana¬ lyzed for stable hGH expression and by restriction enzyme and Southern hybridization analysis. Of thirteen TG r colonies analyzed, eight colonies were identified with an insertion of the hGH gene into the endogenouε HPRT locuε. All eight strains stably expressed significant quantities of hGH, with an average expression level of 22.7 μg/10 6 cells/24 hourε. Alternatively, plasmid pE3neoEP0, Figure 4, may be used to target EPO to the human HPRT locus. The use of homologouε recombination to target a gene of therapeutic intereεt to a specific position in a cell'ε genomic DNA can be expanded upon and made more uεeful for producing productε for therapeutic purposes (e.g., pharma- ceutics, gene therapy) by the insertion of a gene through which cells containing amplified copieε of the gene can be εelected for by expoεure of the cells to an appropriate drug selection regimen. For example, pE3neo/hGH (Example Id) can be modified by inserting the dhfr, ada, or CAD gene at a position immediately adjacent to the hGH or neo genes in pE3neo/hGH. Primary, secondary, or immortalized cellε are tranεfected with such a plaεmid and correctly targeted eventε are identified. These cells are further treated with increasing concentrations of drugε appropri- ate for the selection of cells containing amplified genes (for dhfr, the selective agent iε methotrexate, for CAD the εelective agent is N- (phosphonacetyl) -L-aspartate (PALA) , and for ada the εelective agent iε an adenine nucleoεide (e.g., alanoεine) . In thiε manner the integra- tion of the gene of therapeutic intereεt will be coampli- fied along with the gene for which amplified copieε are εelected. Thus, the genetic engineering of cells to produce genes for therapeutic uses can be readily con¬ trolled by preselecting the site at which the targeting construct integrates and at which the amplified copies reside in the amplified cells. e. MODIFICATION OF DNA TERMINI TO ENHANCE TARGETING Several lines of evidence suggest that 3' -overhanging ends are involved in certain homologous recombination pathways of E. coli, bacteriophage, S. cerevisiae and Xenopus laevis. In Xenopus laevis oocytes, molecules with 3' -overhanging endε of εeveral hundred baεe pairε in length underwent recombination with εimilarly treated moleculeε much more rapidly after microinjection than molecules with very short overhangε (4 bp) generated by restriction enzyme digestion. In yeast, the generation of 3' -overhanging ends several hundred base pairs in length appears to be a rate limiting εtep in meiotic recombinati- on. No evidence for an involvement of 3' -overhanging ends in recombination in human cells has been reported, and in no case have modified DNA substrates of any sort been shown to promote targeting (one form of homologous recom¬ bination) in any species. The experiment described in the following example and Example lc suggeεtε that 5'-over¬ hanging endε are effective for stimulating targeting in primary, secondary and immortalized human fibroblasts. There have been no reportε on the enhancement of targeting by modifying the ends of the transfecting DNA molecules. This example serves to illustrate that modifi¬ cation of the ends of linear DNA molecules, by conversion of the molecules' termini from a double-stranded form to a single-stranded form, can stimulate targeting into the genome of primary and secondary human fibroblasts. 1100 μg of plasmid pE3Neo (Example la) is digested with Hindlll. This DNA can be used directly after phenol extraction and ethanol precipitation, or the 8 kb Hindlll fragment containing only HPRT and the neo gene can be separated away from the pUC12 vector seguenceε by gel electrophoresis. ExoIII digestion of the Hindlll digested DNA results in extensive exonucleolytic digeεtion at each end, initiating at each free 3' end, and leaving 5'- overhanging ends. The extent of exonucleolytic action and, hence, the length of the resulting 5' -overhangs, can be controlled by varying the time of ExoIII digestion. ExoIII digestion of 100 μg of Hindlll digested pE3Neo is carried out according to the supplier's recommended condi¬ tions, for times of 30 sec, 1 min, 1.5 min, 2 min, 2.5 min, 3 min, 3.5 min, 4 min, 4.5 min, and 5 min. To moni¬ tor the extent of digestion an aliquot from each time point, containing 1 μg of ExoIII treated DNA, is treated with mung bean nuclease (Promega) , under conditionε recom¬ mended by the εupplier, and the sampleε fractionated by gel electrophoresis. The difference in size between non-treated, Hindlll digested pE3Neo and the same mole¬ cules treated with ExoIII and mung bean nuclease is mea¬ sured. This size difference divided by two gives the average length of the 5'-overhang at each end of the molecule. Using the time points described above and digestion at 30°, the 5'-overhangs produced should range from 100 to 1,000 bases. 60 μg of ExoIII treated DNA (total Hindlll digest of pE3Neo) from each time point is purified and electropor- ated into primary, secondary, or immortalized human fibro¬ blasts under the conditions described in Example lc. The degree to which targeting is enhanced by each ExoIII treated preparation iε quantified by counting the number of G418 r 6-TG r colonieε and comparing these numbers to targeting with Hindlll digested pE3Neo that was not treat¬ ed with ExoIII. The effect of 3' -overhanging ends can also be quanti¬ fied using an analogous syεtem. In thiε caεe Hindlll digeεted pE3Neo iε treated with bacteriophage T7 gene 6 exonuclease (United Stateε Biochemicals) for varying time intervals under the supplier'ε recommended conditionε. Determination of the extent of digestion (average length of 3'-overhang produced per end) and electroporation conditions are as described for ExoIII treated DNA. The degree to which targeting is enhanced by each T7 gene 6 exonuclease treated preparation is quantified by counting the number of G418 r 6-TG r colonies and comparing these numbers to targeting with Hindlll digested pE3Neo that was not treated with T7 gene 6 exonuclease. Other methods for generating 5' and 3' overhanging ends are posεible, for example, denaturation and annealing of two linear moleculeε that partially overlap with each other will generate a mixture of molecules, each molecule having 3' -overhangs at both endε or 5'-overhangε at both endε, aε well as reannealed fragments indiεtinguiεhable from the εtarting linear molecules. The length of the overhangε is determined by the length of DNA that is not in common between the two DNA fragments. f. CONSTRUCTION OF TARGETING PLASMIDS FOR PLACING THE HUMAN ERYTHROPOIETIN GENE UNDER THE CONTROL OF THE MOUSE METALLOTHIONEIN PROMOTER IN PRIMARY. SECONDARY AND IMMORTALIZED HUMAN FIBROBLASTS The following serveε to illuεtrate one embodiment of the preεent invention, in which the normal poεitive and negative regulatory sequences upstream of the human eryth- ropoietin (hEPO) gene are altered to allow expression of human erythropoietin in primary, secondary or immortalized human fibroblasts, which do not express hEPO in signifi¬ cant quantitieε aε obtained. A region lying exclusively upstream of the human EPO coding region can be amplified by PCR. Three sets of primers useful for this purpose were designed after analy- εiε of the published human EPO sequence [Genbank designa- tion HUMERPA; Lin, F-K. , et al. , Proc. Natl. Acad. Sci.. USA 12:7580-7584 (1985)] . These primer pairs can amplify fragments of 609, 603, or 590 bp. TABLE 3 HUMERPA Primer Coordinate Sequence Fragment Size FI 2 → 20 5' AGCTTCTGGGCTTCCAGAC (SEQ ID NO 1) R2 610 → 595 5' GGGGTCCCTCAGCGAC 609 bp (SEQ ID NO 2) F2 8 → 24 5' TGGGCTTCCAGACCCAG (SEQ ID NO 3) R2 610 -. 595 5' GGGGTCCCTCAGCGAC 603 bp F3 21 → 40 5' CCAGCTACTTTGCGGAACTC (SEQ ID NO 4) R2 610 *. 595 5' GGGGTCCCTCAGCGAC 590 bp The three fragments overlap subεtantially and are interchangeable for the preεent purposes. The 609 bp fragment, extending from -623 to -14 relative to the translation start site (HUMERPA nucleotide positionε 2 to 610), iε ligated at both endε with Clal linkers. The resulting Clal-linked fragment is digested with Clal and inserted into the Clal site of pBluescriptIISK/+ (Strata¬ gene) , with the orientation εuch that HUMERPA nucleotide position 610 is adjacent to the Sail site in the plasmid polylinker) . This plasmid, p5'EPO, can be cleaved, sepa¬ rately, at the unique Fspl or Sfil sites in the human EPO upstream fragment (HUMERPA nucleotide positions 150 and 405, reεpectively) and ligated to the mouse metallothion¬ ein promoter. Typically, the 1.8 kb EcoRI-Bglll from the mMT-I gene [containing no mMT coding sequences; Hamer, D.H. and Walling M. , J. Mol. Appl. Gen. 1:273 288 (1982); this fragment can also be isolated by known methods from mouse genomic DNA using PCR primers designed from analysiε of mMT εequenceε available from Genbank; i.e., MUSMTI, MUSMTIP, MUSMTIPRM] iε made blunt-ended by known methods and ligated with Sfil digested (also made blunt-ended) or Fspl digested p5'EPO. The orientations of resulting clones are analyzed and those in which the former mMT Bglll site is proximal to the Sail site in the plasmid polylinker are used for targeting primary and secondary human fibro¬ blasts. This orientation directs mMT transcription to- wards HUMERPA nucleotide poεition 610 in the final con¬ struct. The resulting plasmids are designated p5'EPO-mMTF and p5'EP0-mMTS for the mMT insertionε in the Fspl and Sfil sites, respectively. Additional upstream sequences are useful in cases where it is desirable to modify, delete and/or replace negative regulatory elementε or enhancerε that lie up- εtream of the initial target sequence. In the caεe of EPO, a negative regulatory element that inhibitε EPO expression in extrahepatic and extrarenal tissues [Semen- za, G.L. et. al. , Mol. Cell. Biol. 10:930-938 (1990)] can be deleted. A series of deletions within the 6 kb frag¬ ment are prepared. The deleted regions can be replaced with an enhancer with broad host-cell activity [e.g. an enhancer from the Cytomegalovirus (CMV)] . The orientation of the 609 bp 5'EPO fragment in the pBluescriptIISK/+ vector was chosen since the HUMERPA sequences are preceded on their 5' end by a BamHI (distal) and Hindlll site (proximal) . Thus, a 6 kb BamHI-Hindll.I fragment normally lying upstream of the 609 bp fragment [Semenza, G. L. et . al .. Mol. Cell. Biol. 10:930-938 (1990)] can be isolated from genomic DNA by known methods. For example, a bacteriophage, cosmid, or yeast artificial chromosome library could be screened with the 609 bp PCR amplified fragment as a probe. The desired clone will have a 6 kb BamHI-Hindlll fragment and its identity can be confirmed by comparing its reεtriction map from a reεtric¬ tion map around the human EPO gene determined by known methodε. Alternatively, constructing a restriction map of the human genome upstream of the EPO gene using the 609 bp fragment as a probe can identify enzymes which generate a fragment originating between HUMERPA coordinates 2 and 609 and extending past the upstream BamHI site; this fragment can be isolated by gel electrophoresiε from the appropri¬ ate digest of human genomic DNA and ligated into a bacte- rial or yeast cloning vector. The correct clone will hybridize to the 609 bp 5'EPO probe and contain a 6 kb BamHI-Hindlll fragment. The isolated 6 kb fragment is inserted in the proper orientation into p5'EP0, pδ'EPO- mMTF, or p5'EPO-mMTS (such that the Hindlll site is adja- cent to HUMERPA nucleotide position 2) . Additional up¬ stream sequences can be isolated by known methods, using chromosome walking techniques or by isolation of yeast artificial chromosomes hybridizing to the 609 bp 5'EPO probe. The cloning strategies described above allow εequenc¬ es upstream of EPO to be modified in vitro for subsequent targeted transfection of primary, secondary or immortal¬ ized human fibroblasts. The strategies describe simple insertions of the mMT promoter, as well as deletion of the negative regulatory region, and deletion of the negative regulatory region and replacement with an enhancer with broad host-cell activity. g. ACTIVATING THE HUMAN EPO GENE AND ISOLATION OF TAR¬ GETED PRIMARY. SECONDARY AND IMMORTALIZED HUMAN FIBROBLASTS BY SCREENING For targeting, the plasmids are cut with restriction enzymes that free the insert away from the plasmid back¬ bone. In the case of p5'EPO-mMTS, Hindlll and Sail digeε¬ tion releaεeε a targeting fragment of 2.4 kb, comprised of the 1.8 kb mMT promoter flanked on the 5' and 3' εides by 405 bp and 204 base pairs, respectively, of DNA for tar- geting this construct to the regulatory region of the human EPO gene. This DNA or the 2.4 kb targeting fragment alone is purified by phenol extraction and ethanol precip¬ itation and transfected into primary or secondary human fibroblaεtε under the conditions described in Example lc. Tranεfected cellε are plated onto 150 mm disheε in human fibroblaεt nutrient medium. 48 hourε later the cells are plated into 24 well dishes at a density of 10,000 cells/cm 2 [approximately 20,000 cells per well; if target¬ ing occurs at a rate of 1 event per 10 6 clonable cells (Example lc, then about 50 wells would need to be assayed to isolate a single expressing colony] . Cells in which the transfecting DNA has targeted to the homologous region upstream of the human EPO gene will expresε hEPO under the control of the mMT promoter. After 10 days, whole well supernatants are assayed for EPO expression using a com¬ mercially available immunoassay kit (Amgen) . Cloneε from wellε displaying hEPO syntheεis are isolated using known methods, typically by assaying fractions of the heteroge- nouε populationε of cellε εeparated into individual wells or plates, assaying fractions of these positive wells, and repeating as needed, ultimately isolating the targeted colony by screening 96-well microtiter plates seeded at one cell per well. DNA from entire plate lysates can also be analyzed by PCR for amplification of a fragment using a mMT specific primer in conjunction with a primer lying upεtream of HUMERPA nucleotide poεition 1. Thiε primer pair should amplify a DNA fragment of a size precisely predicted based on the DNA sequence. Positive plates are trypsinized and replated at succeεsively lower dilutions, and the DNA preparation and PCR εteps repeated as needed to isolate targeted cells. The targeting schemes herein described can also be used to activate hGH expression in immortalized human cells (for example, HT1080 cells (ATCC CCL 121) , HeLa cells and derivatives of HeLa cellε (ATCC CCL2, 2.1 and 2.2), MCF-7 breaεt cancer cells (ATCC HBT 22), K-562 leukemia cells (ATCC CCL 232) , KB carcinoma cells (ATCC CCL 17) , 2780AD ovarian carcinoma cells (Van der Blick, A.M. e al . , Cancer Res, 48:5927-5932 (1988), Raji cellε (ATCC CCL 86), Jurkat cellε (ATCC TIB 152), Namalwa cellε (ATCC CRL 1432), HL-60 cellε (ATCC CCL 240), Daudi cells (ATCC CCL 213), RPMI 8226 cells (ATCC CCL 155), U-937 cells (ATCC CRL 1593) , Bowes Melanoma cells (ATCC CRL 9607), WI-38VA13 subline 2R4 cellε (ATCC CLL 75.1), MOLT-4 cellε (ATCC CRL 1582) , and varous heterohybridoma cells) for the purposeε of producing hGH for conventional pharma- ceutic delivery. h. ACTIVATING THE HUMAN EPO GENE AND ISOLATION OF TAR¬ GETED PRIMARY. SECONDARY AND IMMORTALIZED HUMAN FIBROBLASTS BY A POSITIVE OR A COMBINED POSITIVE/ NEGATIVE SELECTION SYSTEM The εtrategy for conεtructing p5'EPO-mMTF, p5'EPO- mMTS, and derivativeε of εuch with the additional upεtream 6 kb BamHI-Hindlll fragment can be followed with the addi- tional εtep of inεerting the neo gene adjacent to the mMT promoter. In addition, a negative εelection marker, for example, gpt [from pMSG (Pharmacia) or another suitable source] , can be inserted adjacent to the HUMERPA sequences in the pBluescriptIISK/+ polylinker. In the former case, G418 r colonieε are iεolated and εcreened by PCR amplifica¬ tion or reεtriction enzyme and Southern hybridization analysis o£ DNA prepared from pools of colonies to identi¬ fy targeted colonies. In the latter case, G418 r colonies are placed in medium containing 6-thioxanthine to select against the integration of the gpt gene [Besnard, C. et al., Mol. Cell. " Biol. 7:4139-4141 (1987)] . In addition, the HSV-TK gene can be placed on the oppoεite εide of the inεert aε gpt, allowing selection for neo and against both gpt and TK by growing cells in human fibroblast nutrient medium containing 400 μg/ml G418, 100 μM 6-thioxanthine, and 25 μg/ml gancyclovir. The double negative selection should provide a nearly absolute selection for true tar¬ geted events and Southern blot analysiε provideε an ulti- mate confirmation. The targeting schemes herein described can also be used to activate hEPO expression in immortalized human cells (for example, HT1080 cells (ATCC CCL 121) , HeLa cells and derivatives of HeLa cells (ATCC CCL2, 2.1 and 2.2), MCF-7 breast cancer cells (ATCC HBT 22), K-562 leukemia cells (ATCC CCL 232) , KB carcinoma cellε (ATCC CCL 17) , 2780AD ovarian carcinoma cellε (Van der Blick, A.M. et al., Cancer Res. 48:5927-5932 (1988), Raji cellε (ATCC CCL 86) , Jurkat cellε (ATCC TIB 152) , Namalwa cellε (ATCC CRL 1432), HL-60 cells (ATCC CCL 240), Daudi cellε (ATCC CCL 213), RPMI 8226 cells (ATCC CCL 155), U-937 cells (ATCC CRL 1593), Bowes Melanoma cells (ATCC CRL 9607), WI-38VA13 subline 2R4 cells (ATCC CLL 75.1), MOLT-4 cells (ATCC CRL 1582) , and various heterohybridoma cells) for the purposeε of producing hEPO for conventional phar- maceutic delivery. i. CONSTRUCTION OF TARGETING PLASMIDS FOR PLACING THE HUMAN GROWTH HORMONE GENE UNDER THE CONTROL OF THE MOUSE METALLOTHIONEIN PROMOTER IN PRIMARY, SECONDARY OR IMMORTALIZED HUMAN FIBROBLASTS The following example serves to illustrate one em¬ bodiment of the present invention, in which the normal regulatory sequenceε upstream of the human growth hormone gene are altered to allow expression of human growth hormone in primary, secondary or immortalized human fibro- blasts. Targeting molecules similar to those described in Example If for targeting to the EPO gene regulatory region are generated uεing cloned DNA fragmentε derived from the 5' end of the human growth hormone N gene. An approxi- mately 1.8 kb fragment εpanning HUMGHCSA (Genbank Entry) nucleotide poεitionε 3787-5432 (the poεitionε of two EcoNI sites which generate a convenient sized fragment for cloning or for diagnostic digestion of subcloneε involving thiε fragment) is amplified by PCR primers deεigned by analyεiε of the HUMGHCSA sequence in this region. This region extends from the middle of hGH gene N intron 1 to an upstream position approximately 1.4 kb 5' to the trans¬ lational start site. pUC12 is digested with EcoRI and BamHI, treated with Klenow to generate blunt ends, and recircularized under dilute conditions, resulting in plasmidε which have loεt the EcoRI and BamHI εites. This plasmid is designated pUC12XEB. Hindlll linkers are ligated onto the amplified hGH fragment and the resulting fragment is digested with Hindlll and ligated to Hindlll digested pUC12XEB. The resulting plasmid, pUC12XEB-5'hGH, is digeεted with EcoRI and BamHI, to remove a 0.5 kb fragment lying immediately upεtream of the hGH transcrip- tional initiation site. The digested DNA is ligated to the 1.8 kb EcoRI-Bglll from the mMT-I gene [containing no mMT coding sequences; Hamer, D.H. and Walling, M. , J. Mol. Appl . Gen. 1:273-288 (1982); the fragment can also be isolated by known methods from mouse genomic DNA using PCR primers designed from analyεiε of mMT εequences available from Genbank; i.e., MUSMTI, MUSMTIP, MUSMTIPRM] . This plasmid p5'hGH-mMT has the mMT promoter flanked on both sides by upstream hGH sequences. The cloning strategies described above allow sequenc¬ es upstream of hGH to be modified in vitro for subsequent targeted transfection of primary, secondary or immortal- ized human fibroblasts. The strategy described a simple insertion of the mMT promoter. Other strategieε can be enviεioned, for example, in which an enhancer with broad hoεt-cell specificity is inserted upstream of the inserted mMT sequence. j . ACTIVATING THE HUMAN hGH GENE AND ISOLATION OF TAR¬ GETED PRIMARY, SECONDARY AND IMMORTALIZED HUMAN FIBROBLASTS BY SCREENING For targeting, the plasmids are cut with restriction enzymes that free the insert away from the plasmid back- bone. In the case of p5'hGH-mMT, Hindlll digestion re- leaεeε a targeting fragment of 2.9 kb, comprised of the 1.8 kb mMT promoter flanked on the 5' end 3' sides by DNA for targeting this construct to the regulatory region of the hGH gene. This DNA or the 2.9 kb targeting fragment alone is purified by phenol extraction and ethanol precip¬ itation and transfected into primary or secondary human fibroblastε under the conditionε deεcribed in Example 11. Transfected cells are plated onto 150 mm dishes in human fibroblast nutrient medium. 48 hourε later the cellε are plated into 24 well diεhes at a density of 10,000 cells/cm 2 [approximately 20,000 cells per well; if target¬ ing occurs at a rate of 1 event per 10 6 clonable cells (Example lc) , then about 50 wellε would need to be aεεayed to isolate a single expressing colony] . Cells in which the transfecting DNA has targeted to the homologous region upstream of hGH will express hGH under the control of the mMT promoter. After 10 days, whole well supernatants are assayed for hGH expression using a commercially available immunoassay kit (Nicholε) . Clones from wells displaying hGH εyntheεiε are isolated using known methods, typically by assaying fractions of the heterogenous populations of cells separated into individual wells or plates, assaying fractions of these positive wells, and repeating as need- ed, ultimately isolated the targeted colony by screening 96-well microtiter plates seeded at one cell per well. DNA from entire plate lysateε can also be analyzed by PCR for amplification of a fragment using a mMT specific primer in conjunction with a primer lying downstream of HUMGHCSA nucleotide position 5,432. This primer pair should amplify a DNA fragment of a εize precisely predict¬ ed based on the DNA sequence. Positive plateε are tryp- εinized and replated at εuccessively lower dilutions, and the DNA preparation and PCR steps repeated as needed to isolate targeted cells. The targeting schemes herein described can also be used to activate hGH expression in immortalized human cells (for example, HT1080 cells (ATCC CCL 121), HeLa cells and derivatives of HeLa cells (ATCC CCL2, 2.1 and 2.2), MCF-7 breast cancer cells (ATCC HBT 22), K-562 leukemia cells (ATCC CCL 232) , KB carcinoma cells (ATCC CCL 17) , 2780AD ovarian carcinoma cells (Van der Blick, A.M. et al. , Cancer Res. 48:5927-5932 (1988), Raji cellε (ATCC CCL 86) , Jurkat cells (ATCC TIB 152) , Namalwa cells (ATCC CRL 1432), HL-60 cells (ATCC CCL 240), Daudi cells (ATCC CCL 213), RPMI 8226 cells (ATCC CCL 155), U-937 cells (ATCC CRL 1593) , Bowes Melanoma cells (ATCC CRL 9607), WI-38VA13 subline 2R4 cells (ATCC CLL 75.1), MOLT-4 cells (ATCC CRL 1582) , and various heterohybridoma cells) for the purposes of producing hGH for conventional pharma- ceutic delivery. k. ACTIVATING THE HUMAN hGH GENE AND ISOLATION OF TAR¬ GETED PRIMARY. SECONDARY AND IMMORTALIZED HUMAN FIBROBLASTS BY A POSITIVE OR A COMBINED POSITIVE/ NEGATIVE SELECTION SYSTEM The εtrategy for conεtructing p5'hGH-mMT can be followed with the additional εtep of inserting the neo gene adjacent to the mMT promoter. In addition, a nega- tive selection marker, for example, gpt [from pMSG (Phar¬ macia) or another suitable source] , can be inserted adja¬ cent to the HUMGHCSA sequenceε in the pUC12 poly-linker. In the former case, G418 r colonies are isolated and screened by PCR amplification or restriction enzyme and Southern hybridization analysis of DNA prepared from pools of colonieε to identify targeted colonieε. In the latter case, G418 r colonieε are placed in medium containing thioxanthine to εelect againεt the integration of the gpt gene (Besnard, C. e_t al. , Mol. Cell. Biol. 2: 4139-4141 (1987)] . In addition, the HSV-TK gene can be placed on the opposite side of the insert as gpt, allowing selection for neo and against both gpt and TK by growing cells in human fibroblast nutrient medium containing 400 μg/ml G418, 100 μM 6-thioxanthine, and 25 μg/ml gancyclovir. The double negative selection should provide a nearly absolute selection for true targeted events. Southern hybridization analysis is confirmatory. The targeting schemeε herein described can also be used to activate hGH expression in immortalized human cells (for example, HT1080 cells (ATCC CCL 121), HeLa cells and derivatives of HeLa cells (ATCC CCL2, 2.1 and 2.2), MCF-7 breast cancer cells (ATCC HBT 22), K-562 leukemia cells (ATCC CCL 232) , KB carcinoma cells (ATCC CCL 17) , 2780AD ovarian carcinoma cells (Van der Blick, A.M. et al., Cancer Res, 48:5927-5932 (1988), Raji cellε (ATCC CCL 86), Jurkat cellε (ATCC TIB 152), Namalwa cells (ATCC CRL 1432), HL-60 cells (ATCC CCL 240), Daudi cells (ATCC CCL 213), RPMI 8226 cells (ATCC CCL 155), U-937 cells (ATCC CRL 1593), Bowes Melanoma cells (ATCC CRL 9607), WI-38VA13 εubline 2R4 cellε (ATCC CLL 75.1), MOLT-4 cells (ATCC CRL 1582) , and various heterohybridoma cells) for the purposes of producing hGH for conventional pharma- ceutic delivery. The targeting constructs described in Examples If and li, and used in Examples lg, lh, lj and Ik can be modified to include an amplifiable selectable marker (e.g., ada, dhfr, or CAD) which iε useful for selecting cells in which the activated endogenous gene, and the amplifiable select- able marker, are amplified. Such cells, expresεing or capable of expreεsing the endogenous gene encoding a therapeutic product can be used to produce proteins (e.g., hGH and hEPO) for conventional pharmaceutic delivery or for gene therapy. 1. TRANSFECTION OF PRIMARY AND SECONDARY FIBROBLASTS WITH EXOGENOUS DNA AND A SELECTABLE MARKER GENE BY ELECTROPORATION Exponentially growing or early stationary phase fibroblasts are trypsinized and rinsed from the plastic surface with nutrient medium. An aliquot of the cell suspension is removed for counting, and the remaining cells are subjected to centrifugation. The supernatant is aεpirated and the pellet iε reεuεpended in 5 ml of elec¬ troporation buffer (20 mM HEPES pH 7.3, 137 mM NaCl, 5 mM KCI, 0.7 mM Na 2 HP0 4 , 6 mM dextrose) . The cells are recen- trifuged, the supernatant aspirated, and the cells resus¬ pended in electroporation buffer containing 1 mg/ml acety- lated bovine serum albumin. The final cell suεpenεion contains approximately 3 x 10 6 cells/ml. Electroporation should be performed immediately following resuεpenεion. Supercoiled plasmid DNA is added to a sterile cuvette with a 0.4 cm electrode gap (Bio-Rad.) The final DNA concentration is generally at least 120 μg/ml. 0.5 ml of the cell suεpenεion (containing approximately 1.5 x 10 6 cellε) is then added to the cuvette, and the cell suspen¬ sion and DNA solutions are gently mixed. Electroporation is performed with a Gene-Pulser apparatus (Bio-Rad) . Capacitance and voltage are set at 960 μF and 250-300 V, respectively. As voltage increaεeε, cell εurvival de¬ creases, but the percentage of surviving cells that stably incorporate the introduced DNA into their genome increases dramatically. Given these parameters, a pulse time of approximately 14-20 mSec should be observed. Electroporated cells are maintained at room tempera¬ ture for approximately 5 min, and the contents of the cuvette are then gently removed with a sterile transfer pipette. The cells are added directly to 10 ml of pre- warmed nutrient media (as above with 15% calf serum) in a 10 cm dish and incubated aε described above. The follow¬ ing day, the media is aspirated and replaced with 10 ml of fresh media and incubated for a further 16-24 hours. Subculture of cells to determine cloning efficiency and to select for G418-resistant colonies is performed the fol¬ lowing day. Cells are trypsinized, counted and plated; typically, fibroblasts are plated at 10 3 cells/10 cm dish for the determination of cloning efficiency and at 1-2 x 10 4 cells/10 cm dish for G418 selection. Human fibroblastε are selected for G418 resistance in medium consiεting of 300-400 μg/ml G418 (Geneticin, diεul- fate εalt with a potency of approximately 50%; Gibco) in fibroblasts nutrient media (with 15% calf serum) . Cloning efficiency is determined in the absence of G418. The plated cells are incubated for 12-14 dayε, at which time colonies are fixed with formalin, stained with crystal violet and counted (for cloning efficiency plated) or isolated using cloning cylinders (for G418 plates) . Electroporation and selection of rabbit fibroblasts is performed essentially as described for human fibroblasts, with the exception of the selection conditions used. Rabbit fibroblastε are selected for G418 resiεtance in medium containing 1 g /ml G418. Fibroblasts were isolated from freshly excised human foreskins. Cultures were seeded at 50,000 cells/cm in DMEM + 10% calf serum. When cultures became confluent, fibroblastε were harvested by trypεinization and tranε¬ fected by electroporation. Electroporation conditionε were evaluated by transfection with the plasmid pcDNEO (Figure 5) . A repreεentative electroporation experiment uεing near optimal conditions (60 μg of plasmid pcDNEO at an electroporation voltage of 250 volts and a capacitance setting of 960 μFarads) resulted in one G418 colony per 588 treated cells (0.17% of all cells treated), or one G418 colony per 71 clonable cells(1.4%). When nine separate electroporation experiments at near optimal conditions (60 μg of plasmid pcDNEO at an electroporation voltage of 300 voltε and a capacitance εetting of 960 μFarads) were performed, an average of one G418 colony per 1,899 treated cells (0.05%) was observed, with a range of 1/882 to 1/7,500 treated cells. This corresponds to an average of one G418 colony per 38 clon¬ able cells (2.6%) . Low pasεage primary human fibroblaεts were converted to hGH expressing cells by co-transfection with plasmids; pcDNEO and pXGH5. Typically, 60 μg of an equimolar mix¬ ture of the two plaεmids were tranεfected at near optimal conditions (electroporation voltage of 300 volts and a capacitance setting of 960 μFarads) . The results of such an experiment resulted in one G418 colony per 14,705 treated cells. hGH expression data for these and other cells isolat¬ ed under identical transfection conditions are summarized below. Ultimately, 98% of all G418 r colonies could be expanded to generate mass cultures. Number of G418 r Clones Analyzed 154 Number of G418 r /hGH Expressing Clones 65 Average hGH Expresεion Level 2.3 μg hGH/10 6 Cellε/24 hr Maximum hGH Expreεεion Level 23.0 μg hGH/10 6 Cellε/24 hr Stable transfectants alεo have been generated by electroporation of primary or εecondary human fibroblastε with pXGH301, a DNA construct in which the neo and hGH genes are present on the same plasmid molecule. pXGH301 was conεtructed by a two-step procedure. The Sall-Clal fragment from pBR322 (positionε 23-651 in pBR322) waε isolated and inserted into Sall-Clal digested pcDNEO, introducing a BamHI site upstream of the SV40 early pro¬ moter region of pcDNEO. This plasmid, pBNEO was digested with BamHi and the 2.1 kb fragment containing the neo gene under the control of the SV40 early promoter, was isolated and inserted into BamHI digested pXGH5. A plasmid with a single insertion of the 2.1 kb BamHI fragment was isolated in which neo and hGH are transcribed in the same direction relative to each other. This plasmid was designated pXGH301. For example, 1.5 x 10 6 cells were electroporated with 60 μg pXGH301 at 300 volts and 960 μFarads. G418 resiεtant colonies were isolated from transfected second¬ ary fibroblaεtε at a frequency of 652 G418 resistant colonieε per 1.5 x 10 treated cells (1 per 2299 treated cells) . Approximately 59% of these colonies express hGH. EXAMPLE 2. CONSTRUCTION OF TARGETING PLASMIDS WHICH RE¬ SULT IN CHIMERIC TRANSCRIPTION UNITS IN WHICH HUMAN GROWTH HORMONE AND ERYTHROPOIETIN SE¬ QUENCES ARE FUSED The following serves to illustrate two further em¬ bodiments of the present invention, in which the normal regulatory εequences upstream of the human EPO gene are altered to allow expreεεion of hEPO in primary or εecond¬ ary fibroblast strains which do not expresε hEPO in de¬ tectable quantitieε in their untranεfected εtate aε ob¬ tained. In these embodiments, the products of the target¬ ing events are chimeric transcription units in which the first exon of the human growth hormone gene is positioned upstream of hEPO exons 2-5. The product of transcription, splicing and translation is a protein in which amino acids 1-4 of the hEPO signal peptide are replaced with amino acid reεidueε 1-3 of hGH. The two embodiments differ with respect to both the relative positions of the foreign regulatory sequences that are inserted and the specific pattern of splicing that needs to occur to produce the final, processed transcript. Plasmid pXEPO-10 is designed to replace exon 1 of hEPO with exon 1 of hGH by gene targeting to the endoge¬ nous hEPO gene on human chromoεome 7. Plasmid pXEPO-10 is constructed as follows. First, the intermediate plasmid pT163 is constructed by inserting the 6 kb Hindlll-BamHI fragment (see Example If) lying upstream of the hEPO coding region into Hindlll-BamHI digested pBluescriptll SK+ (Stratagene, LaJolla, CA) . The product of this liga¬ tion is digested with Xhol and Hindlll and ligated to the 1.1 kb Hindlll-Xhol fragment from pMClneoPolyA [Thomas, K. R. and Capecchi, M. R. Cell 5_1: 503-512 (1987) available from Strategene, LaJolla, CA] to create pT163. Oligo- nucleotideε 13.1 - 13.4 are utilized in polymeraεe chain reactions to generate a fusion fragment in which the mouse metallothionein 1 (mMT-I) promoter - hGH exon 1 sequenceε are additionally fused to hEPO intron 1 sequences. First, oligonucleotides 13.1 and 13.3 are used to amplify the approximately 0.73 kb mMT-I promoter - hGH exon 1 fragment from pXGH5 (Figure 5) . Next, oligonucleotides 13.2 and 13.4 are used to amplify the approximately 0.57 kb frag- ment comprised predominantly of hEPO intron 1 from human genomic DNA. Finally, the two amplified fragments are mixed and further amplified with oligonucleotides 13.1 and 13.4 to generate the final fuεion fragment (fuεion frag¬ ment 3) flanked by a Sail site at the 5' side of the mMT-I moiety and an Xhol site at the 3' side of the hEPO intron 1 sequence. Fusion fragment 3 is digested with Xhol and Sail and ligated to Xhol digested pT163. The ligation mixture is tranεformed into E. coli and a clone containing a single insert of fusion fragment 3 in which the Xhol site is regenerated at the 3' εide of hEPO intron 1 se¬ quences is identified and designated pXEPO-10. 13.1 5' TTTTGTCGAC GGTACCTTGG TTTTTAAAAC C Sail Kpnl (SEQ ID NO 5) 13.2 5' CCTAGCGGCA ATGGCTACAG GTGAGTACTC GCGGGCTGGG CG (SEQ ID NO 6) 13.3 5' CGCCCAGCCC GCGAGTACTC ACCTGTAGCC ATTGCCGCTA GG (SEQ ID NO 7) 13.4 5' TTTTCTCGAG CTAGAACAGA TAGCCAGGCT G Xhol (SEQ ID NO 8) The non-boldface region of oligo 13.1 is identi¬ cal to the mMT-I promoter, with the natural Kpnl site as its 5' boundary. The boldface type denoteε a Sail site tail to convert the 5' boun¬ dary to a Sail site. The boldface region of oligos 13.2 and 13.3 denote hGH sequences, while 5 the non-boldface regions are intron 1 sequences from the hEPO gene. The non-boldface region of oligo 13.4 is identical to the last 25 baseε of hEPO intron 1. The boldface region includeε an Xhol site tail to convert the 3' boundary of the 10. amplified fragment to an Xhol site. Plasmid pXEPO-11 is designed to place, by gene tar¬ geting, the mMT-I promoter and exon 1 of hGH upstream of the hEPO structural gene and promoter region at the endog¬ enous hEPO locus on human chromosome 7. Plasmid pXEPO-11 5 is constructed as follows. Oligonucleotides 13.1 and 13.5 - 13.7 are utilized in polymerase chain reactions to generate a fusion fragment in which the mouse metallo¬ thionein I (mMT-I) promoter - hGH exon 1 sequences are additionally fused to hEPO sequences from -1 to -630 0 relative to the hEPO coding region. First, oligonucleo¬ tides 13.1 and 13.6 are used to amplify the approximately 0.75 kb mMT-I promoter - hGH exon 1 fragment from pXGH5 (Figure 5) . Next, oligonucleotides 13.5 and 13.7 are used to amplify, from human genomic DNA, the approximately 5 0.65 kb fragment comprised predominantly of hEPO sequences from -1 to -620 relative to the hEPO coding region. Both oligos 13.5 and 13.6 contain a 10 bp linker sequence located at the hGH intron 1 - hEPO promoter region, which correspondε to the natural hEPO intron 1 εplice-donor 0 εite. Finally, the two amplified fragmentε are mixed and further amplified with oligonucleotideε 13.1 and 13.7 to generate the final fuεion fragment (fuεion fragment 6) flanked by a Sail site at the 5' side of the mMT-I moiety and an Xhol site at the 3' side of the hEPO promoter region. Fusion fragment 6 is digested with Xhol and Sail and ligated to Xhol digested pT163. The ligation mixture is transformed into E. coli and a clone containing a single insert of fusion fragment 6 in which the Xhol site is regenerated at the 3' side of hEPO promoter sequenceε is identified and designated pXEPO-11. 13.5 5' GACAGCTCAC CTAGCGGCAA TGGCTACAGG TGAGTACTC AAGCTTCTGG GCTTCCAGAC CCAG (SEQ ID NO 9) Hindlll 13.6 5' CTGGGTCTGG AAGCCCAGAA GCTTGAGTAC TCACCTGTAG Hindlll CCATTGCCGC TAGGTGAGCT GTC (SEQ ID NO 10) 13.7 5' TTTTCTCGAG CTCCGCGCCT GGCCGGGGTC CCTC Xhol (SEQ ID NO 11) The boldface regionε of oligos 13.5 and 13.6 denote hGH sequences. The italicized regions correεpond to the first 10 base pairs of hEPO intron 1. The remainder of the oligos corre¬ spond to hEPO sequences from -620 to -597 rela¬ tive to the hEPO coding region. The non-bold¬ face region of oligo 13.7 is identical to bas¬ es -1 to -24 relative to the hEPO coding region. The boldface region includes an Xhol site tail to convert the 3' boundary of the amplified fragment to an Xhol site. Plasmid pXEPO-10 can be used for gene targeting by digestion with BamHI and Xhol to release the 7.3 kb frag- ment containing the mMT-I/hGH fusion flanked on both sides by hEPO sequences. Thiε fragment (targeting fragment 1) contains no hEPO coding sequences, having only sequences lying between -620 and approximately -6620 upstream of the hEPO coding region and hEPO intron 1 εequenceε to direct targeting to the human EPO locuε. Targeting fragment 1 iε transfected into primary or secondary human skin fibro- blaεtε using conditions similar to those described in Example lc. G418-resiεtant colonies are picked into individual wells of 96-well plates and screened for EPO expression by an ELISA assay (R&D Systems, Minneapolis MN) . Cells in which the transfecting DNA integrates randomly into the human genome cannot produce EPO. Cells in which the transfecting DNA has undergone homologous recombination with the endogenous hEPO intron 1 and hEPO upstream sequences contain a chimeric gene in which the mMT-I promoter and non-transcribed sequences and the hGH 5' untranslated εequenceε and hGH exon 1 replace the normal hEPO promoter and hEPO exon 1 (see Figure 1) . Non- hEPO sequences in targeting fragment 1 are joined to hEPO sequences down-stream of hEPO intron 1. The replacement of the normal hEPO regulatory region with the mMT-I pro¬ moter will activate the EPO gene in fibroblasts, which do not normally express hEPO. The replacement of hEPO exon 1 with hGH exon 1 results in a protein in which the first 4 amino acids of the hEPO signal peptide are replaced with amino acids 1-3 of hGH, creating a functional, chimeric signal peptide which is removed by post-tranεlation pro¬ cessing from the mature protein and is secreted from the expresεing cells. Plasmid pXEPO-11 can be used for gene targeting by digestion with BamHI and Xhol to release the 7.4 kb frag¬ ment containing the mMT-I/hGH fusion flanked on both sides by hEPO sequences. This fragment (targeting fragment 2) contains no hEPO coding εequenceε, having only sequences lying between -1 and approximately -6620 upstream of the hEPO coding region to direct targeting to the human EPO locus. Targeting fragment 2 is transfected into primary or secondary human skin fibroblasts using conditions similar to those described in Example lg. G418-resistant colonies are picked into individual wells of 96-well plates and screened for EPO expression by an ELISA assay (R&D Systems, Minneapoliε, MN) . Cellε in which the trans- fecting DNA integrates randomly into the human genome cannot produce EPO. Cells in which the transfecting DNA has undergone homologous recombination with the endogenous hEPO promoter and upstream sequences contain a chimeric gene in which the mMT-I promoter and non-transcribed sequences, hGH 5' untranslated sequences and hGh exon 1, and a 10 base pair linker comprised of the first 10 baεeε of hEPO intron 1 are inεerted at the Hindlll εite lying at poεition -620 relative to the hEPO coding region (see Figure 2) . The localization of the mMT-I promoter up¬ stream of the normally silent hEPO promoter will direct the synthesis, in primary or secondary skin fibroblasts, of a message reading (5' to 3') non-translated metallo- thionein and hGH sequences, hGH exon 1, 10 bases of DNA identical to the first 10 base pairs of hEPO intron 1, and the normal hEPO promoter and hEPO exon 1 (-620 to +13 relative to the hEPO coding sequence) . The 10 baεe pair linker εequence from hEPO intron 1 acts as a splice-donor site to fuse hGH exon 1 to the next downstream splice acceptor site, that lying immediately upstream of hEPO exon 2. Processing of the resulting transcript will therefore splice out the hEPO promoter, exon 1, and intron 1 sequences. The replacement of hEPO exon 1 with hGH exon 1 results in a protein in which the first 4 amino acids of the hEPO signal peptide are replaced with amino acids 1-3 of hGH, creating a functional, chimeric signal peptide which is removed by post-tranεlation processing from the mature protein and is secreted from the expressing cells. - 73 - A series of constructs related to pXEPO-10 and pXEPO- 11 can be constructed, using known methods. In these constructs, the relative positions of the mMT-I promoter and hGH sequences, as well as the position at which the mMT-I/hGH sequenceε are inserted into hEPO upstream se¬ quenceε, are varied to create alternative chimeric tran¬ εcription unitε that facilitate gene targeting, reεult in more efficient expreεεion of the fusion transcripts, or have other desirable properties. Such constructε will give εimilar reεultε, such that an hGH-hEPO fuεion gene is placed under the control of an exogenous promoter by gene targeting to the normal hEPO locus. For example, the 6 kb Hindlll-BamHI fragment upstream of the hEPO gene (See Example If) has numerous restriction enzyme recognition sequences that can be utilized as sites for insertion of the neo gene and the mMT-I promoter/hGH fusion fragment. One such site, a Bglll site lying approximately 1.3 kb upstream of the Hindlll site, is unique in this region and can be used for insertion of one or more selectable mark- ers and a regulatory region derived from another gene that will εerve to activate hEPO expreεsion in primary, second¬ ary, or immortalized human cells. Firεt, the intermediate plasmid pT164 is constructed by inserting the 6 kb Hindlll-BamHI fragment (Example If) lying upstream of the hEPO coding region into Hindlll- BamHI digested pBluescriptII SK+ (Stratagene, LaJolla, CA) . Plasmid pMClneoPolyA [Thomas, K.R. and Capecchi, M.R. Cell 51:503-512 (1987); available from Stratagene, LaJolla, CA] is digested with BamHI and Xhol, made blunt- ended by treatment with the Klenow fragment of E. coli DNA polymerase, and the resulting 1.1 kb fragment is purified. pT164 is digested with Bglll and made blunt-ended by treatment with the Klenow fragment of E. coli DNA polymer¬ ase. The two preceding blunt-ended fragments are ligated together and transformed into competent E. coli . Clones with a single insert of the 1.1 kb neo .fragment are iso¬ lated and analyzed by restriction enzyme analysis to identify thoεe in which the Bglll εite recreated by the fusion of the blunt Xhol and Bglll siteε iε localized 1.3 kb away from the unique Hindlll εite preεent in plaε¬ mid pT164. The resulting plasmid, pT165, can now be cleaved at the unique Bglll site flanking the 5' side of the neo transcription unit. Oligonucleotides 13.8 and 13.9 are utilized in poly- merase chain reactions to generate a fragment in which the mouse metallothionein I (mMT-I) promoter - hGH exon 1 sequences are additionally fused to a 10 base pair frag¬ ment comprising a splice-donor εite. The splice-donor site chosen corresponds to the natural hEPO intron 1 splice-donor site, although a larger number of splice- donor sites or consensus splice-donor siteε can be used. The oligonucleotides (13.8 and 13.9) are used to amplify the approximately 0.73 kb mMT-I promoter - hGH exon 1 fragment from pXGH5 (Figure 5) . The amplified fragment (fragment 7) is digested with Bglll and ligated to Bglll digested pT165. The ligation mixture is transformed into E. coli and a clone, containing a single insert of frag¬ ment 7 in which the Kpnl site in the mMT-I promoter is adjacent to the 5' end of the neo gene and the mMT-I promoter is oriented such that transcription is directed towards the unique Hindlll site, is identified and desig¬ nated pXEPO-12. 13.8 5' AAAAAGATCT GGTACCTTGG TTTTTAAAAC CAGCCTGGAG Bglll Kpnl (SEQ ID NO 12) The non-boldface region of oligo 13.8 is identi¬ cal to the mMT-I promoter, with the natural Kpnl site as its 5' boundary. The boldface type denotes a Bglll site tail to convert the 5' boundary to a Bglll site. 13.9 5' TTTTAGATCT GAGTACTCAC CTGTAGCCAT TGCCGCTAGG Bglll (SEQ ID NO 13) The boldface region of oligoε 13.9 denote hGH sequences. The italicized region corresponds to the first 10 base pairs of hEPO intron 1. The underlined Bglll site is added for plasmid con- εtruction purposes. Plasmid pXEPO-12 can be used for gene targeting by digestion with BamHI and Hindlll to release the 7.9 kb fragment containing the neo gene and the mMT-I/hGH fusion flanked on both sided by hEPO sequences. This fragment (targeting fragment 3) contains no hEPO coding sequenceε, having only εequenceε lying between approximately -620 and approximately -6620 upstream of the hEPO coding region to direct targeting upεtream of the human EPO locus. Target¬ ing fragment 3 is tranεfected into primary, εecondary, or immortalized human skin fibroblasts using conditions similar to those described in Examples lb and lc. G418- resistant colonies are picked into individual wells of 96- well plates and screened for EPO expression by an ELISA asεay (R&D Systems, Minneapolis MN) . Cells in which the tranεfecting DNA integrates randomly into the human genome cannot produce hEPO. Cells in which the tranεfecting DNA has undergone homologous recombination with the endogenouε hEPO promoter and upεtream εequences contain a chimeric gene in which the mMT-I promoter and non-transcribed sequenceε, hGH 5' untranslated sequences, and hGH exon 1, and a 10 base pair linker comprised of the first 10 bases of hEPO intron 1 are inserted at the Bglll site lying at position approximately -1920 relative to the hEPO coding region. The localization of the mMT-I promoter upstream of the normally silent hEPO promoter will direct the syntheεis, in primary, secondary, or immortalized human fibroblasts (or other human cells) , of a message reading: (5' to 3') nontranslated metallothionein and hGH sequenc¬ es, hGH exon 1, 10 bases of DNA identical to the first 10 base pairs of hEPO intron 1, and hEPO upstream region and hEPO exon 1 (from approximately -1920 to +13 relative to the EPO coding εequence) . The 10 base pair linker se¬ quence from hEPO intron 1 acts as a splice-donor site to fuse hGH exon 1 to a downstream splice acceptor site, that lying immediately upstream of hEPO exon 2. Processing of the resulting transcript will therefore splice out the hEPO upstream sequenceε, promoter region, exon 1, and intron 1 sequences. When using pXEPO-10, -11 and -12, post-transcriptional processing of the message can be improved by using in vitro mutagenesiε to eliminate εplice acceptor εites lying in hEPO upstream sequenceε between the mMT-I promoter and hEPO exon 1, which reduce level of productive splicing events needed create the desired message. The replacement of hEPO exon 1 with hGH exon 1 resultε in a protein in which the first 4 amino acids of the hEPO signal peptide are replaced with amino acids 1-3 of hGH, creating a functional, chimeric signal peptide which is removed by post-translation processing from the mature protein and is secreted from the expressing cells. EXAMPLE 3. TARGETED MODIFICATION OF SEQUENCES UPSTREAM AND AMPLIFICATION OF THE TARGETED GENE Human cells in which the hEPO gene has been activated by the methods previously described can be induced to amplify the neo/mMT-l/EPO transcription unit if the tar¬ geting plasmid contains a marker gene that can confer resistance to a high level of a cytotoxic agent by the phenomenon of gene amplification. Selectable marker genes such as dihydrofolate reductase (dhfr, selective agent is methotrexate) , the multifunctional CAD gene [encoding carbamyl phosphate synthase, aspartate transcarbamylase, and dihydro-orotase; selective agent is N- (phosphono- acetyl) -L-aεpartate (PALA)], glutamine synthetase; selec¬ tive agent is methionine εulphoximine (MSX) , and adenosine deaminase (ada; selective agent is an adenine nucleoside) , have been documented, among other genes, to be amplifiable in immortalized human cell lines (Wright, J.A. et. al. Proc. Natl. Acad. Sci. USA 87:1791-1795 (1990); Cockett, M.I. et. al . . Bio/Technology 8:662-667 (1990)) . In these studies, gene amplification has been documented to occur in a number of immortalized human cell lines. HT1080, HeLa, MCF-7 breast cancer cells, K-562 leukemia cells, KB carcinoma cells, or 2780AD ovarian carcinoma cells, among other cells, display amplification under appropriate selection conditions. Plasmids pXEPO-10 and pXEPO-11 can be modified by the insertion of a normal or mutant dhfr gene into the unique Hindlll siteε of theεe plasmids. After transfection of HT1080 cells with the appropriate DNA, selection for G418- reεiεtance (conferred by the neo gene) , and identification of cells in which the hEPO gene has been activated by gene targeting of the neo, dhfr, and mMT-1 sequences to the correct position upstream of the hEPO gene, these cells can be exposed to stepwise selection in methotrexate (MTX) in order to select for amplification of dh.fr and co-ampli¬ fication of the linked neo, mMT-1, and hEPO sequences (Kaufman, R.J. Technique 2:221-236 (1990)) . A stepwise selection εcheme in which cells are first exposed to low levels of MTX (0.01 to 0.08 μM) , followed by successive exposure to incremental increases in MTX concentrations up to 250 μM MTX or higher is employed. Linear incremental steps of 0.04 to 0.08 μM MTX and successive 2-fold in- creases in MTX concentration will be effective in select¬ ing for amplified transfected cell lines, although a variety of relatively shallow increments will also be effective. Amplification is monitored by increases in dhfr gene copy number and confirmed by measuring in vitro hEPO expression. By this strategy, εubεtantial over- expression of hEPO can be attained by targeted modifica¬ tion of sequences lying completely outside of the hEPO coding region. Constructε similar to those described (Examples If, lh, li, Ik, 2 and 7) to activate hGH expresεion in human cellε can alεo be further modified to include the dhfr gene for the purpoεe of obtaining cellε that overexpreεε the hGH gene by gene targeting to non-coding εequences and subsequent amplification. EXAMPLE 4. TARGETING AND ACTIVATION OF THE HUMAN EPO LOCUS IN AN IMMORTALIZED HUMAN FIBROBLAST LINE The targeting construct pXEPO-13 was made to test the hypothesis that the endogenous hEPO gene could be activat- ed in a human fibroblast cell. First, plaεmid pT22.1 was constructed, containing 63 bp of genomic hEPO sequence upstream of the first codon of the hEPO gene fused to the mouse metallothionein-1 promoter (mMT-I) . Oligonucleo¬ tides 22.1 to 22.4 were used in PCR to fuse mMT-I and hEPO sequences. The properties of these primers are as fol¬ lows: 22.1 is a 21 base oligonucleotide homologous to a segment of the mMT-I promoter beginning 28 bp upstream of the mMT-I Kpnl site; 22.2 and 22.3 are 58 nucleotide complementary primers which define the fuεion of hEPO and mMT-I sequences such that the fusion contains 28 bp of hEPO sequence beginning 35 bases upstream of the first codon of the hEPO gene, and mMT-I sequences beginning at base 29 of oligonucleotide 22.2, comprising the natural Bglll site of mMT-I and extending 30 bases into mMT-I sequence; 22.4 is 21 nucleotides in length and is homolo¬ gous to hEPO sequences beginning 725 bp downstream of the first codon of the hEPO gene. These primers were used to amplify a 1.4 kb DNA fragment comprising a fuεion of mMT-I and hEPO εequences as deεcribed above. The reεulting fragment was digested with Kpnl (the PCR fragment con¬ tained two Kpnl siteε: a εingle natural Kpnl εite in the mMT-I promoter region and a εingle natural Kpnl εite in the hEPO sequence) , and purified. The plasmid pXEPOl was also digested with Kpnl, releasing a 1.4 kb fragment and a 6.4 kb fragment. The 6.4 kb fragment was purified and ligated to the 1.4 kb Kpnl PCR fusion fragment. The resulting construct was called pT22.1. A second interme¬ diate, pT22.2, was constructed by ligating the approxi- mately 6 kb Hindlll-BamHI fragment lying upstream of the hEPO structural gene (see Example If) to BamHI and Hindlll digested pBSIISK+ (Stratagene, LaJolla, CA) . A third intermediate, pT22.3, was constructed by first excising a 1.1 kb XhoI/BamHI fragment from pMCINEOpolyA (Stratagene,, LaJolla, CA) containing the neomycin phosphotranεferase gene. The fragment was then made blunt-ended with the Klenow fragment of DNA polymerase I (New England Biolabs) . This fragment was then ligated to the Hindi site of pBSIISK+ (similarly made blunt with DNA polymerase I) to produce pT22.3. A fourth intermediate, pT22.4, was made by purifying a 1.1 kb Xhol/Hindlll fragment comprising the neo gene from pT22.3 and ligating this fragment to Xhol and Hindlll digested pT22.2. pT22.4 thus contains the neo gene adjacent to the Hindlll εide of the BamHI-HindiII upstream hEPO fragment. Finally, pXEPO-13 was generated by first excising a 2.0 kb EcoRI/AccI fragment from pT22.- 1. The EcoRI site of this fragment defines the 5' bound¬ ary of the mMT-I promoter, while the Accl εite of this fragment lies within hEPO exon 5. Thus, the AccI/EcoRI fragment contains a nearly complete hEPO expression unit, missing only a part of exon 5 and the natural polyadenyla¬ tion site. This 2.0 kb EcoRI/AccI fragment was purified, made blunt-ended by treatment with the Klenow fragment of DNA polymerase I, and ligated to Xhol digested, blunt- ended, pT22.4. HT1080 cells were transfected with PvuI-BamHI digest¬ ed pXEPO-13. pXEPO-13 digested in this way generates three fragments; a 1 kb vector fragment including a por¬ tion of the amp gene, a 1.7 kb fragment of remaining vector sequences and an approximately 9 kb fragment con¬ taining hEPO, neo and mMT-I sequences. This approximately 9 kb BamHI/PvuI fragment contained the following sequences in order from the BamHI site: an approximately 5.2 kb of upstream hEPO genomic sequence, the 1.1 kb neo transcrip- tion unit, the 0.7 kb mMT-I promoter and the 2.0 kb frag¬ ment containing hEPO coding sequence truncated within exon 5. 45μg of pEXPO-13 digested in this way was used in an electroporation of 12 million cells (electroporation conditions were described in Example lb) . This electro- poration was repeated a total of eight times, resulting in electroporation of a total of 96 million cells. Cells were mixed with media to provide a cell density of 1 million cells per ml and 1 ml aliquots were dispensed into a total of 96, 150mm tissue culture plates (Falcon) each containing a minimum of 35 ml of DMEM/15% calf serum. The following day, the media was aspirated and replaced with fresh medium containing 0.8 mg/ml G418 (Gibco) . After 10 days of incubation, the media of each plate was sampled for hEPO by ELISA analysiε (R & D Systems) . Six of the 96 plates contained at least 10 mU/ml hEPO. One of these plates, number 18, was selected for purification of hEPO expressing colonies. Each of the 96, 150 mm plates con¬ tained approximately 600 G418 resiεtant colonieε (an eεtimated total of 57,600 G418 resiεtant colonies on all 96 plates) . The approximately 600 colonies on plate number 18 were trypsinized and replated at 50 cells/ml into 364 well plates (Sterilin) . After one week of incu¬ bation, εingle colonies were visible at approximately 10 colonies per large well of the 364 well plates (these plates are comprised of 16 small wells within each of the 24 large wells) . Each well was screened for hEPO expres¬ εion at this time. Two of the large wells contained media with at least 20 mU/ml hEPO. Well number A2 was found to contain 15 colonies distributed among the 16 small wells. The contents of each of these small wells were trypsinized and tranεferred to 16 individual wellε of a 96 well plate, following 7 dayε of incubation the media from each of these wells was sampled for hEPO ELISA analysiε. Only a single well, well number 10, contained hEPO. This cell strain was designated HT165-18A2-10 and was expanded in culture for quantitative hEPO analysis, RNA isolation and DNA isolation. Quantitative measurement of hEPO produc¬ tion reεulted in a value of 2,500 milliunits/million cells/24 hours. A 0.2 kb DNA probe extending from the Accl site in hEPO exon 5 to the Bglll site in the 3' untranslated region was used to probe RNA isolated from HT165-18A2-10 cells. The targeting construct, pXEPO-13, truncated at the Accl site in exon 5 does not contain these Accl/Bglll sequences and, therefore, is diagnostic for targeting at the hEPO locuε. Only cell strainε that have recombined in a homologouε manner with natural hEPO sequences would produce an hEPO mRNA containing sequence homologouε to the Accl/Bglll sequences. HT165-18A2-10 was found to express an mRNA of the predicted size hybridizing with the 32-P labeled Accl/Bglll hEPO probe on Northern blots. Restric¬ tion enzyme and Southern blot analysiε confirmed that the neo gene and mMT-I promoter were targeted to one of the two hEPO alleles in HT165-18A2-10 cells. Theεe reεults demonεtrate that homologouε recombina¬ tion can be uεed to target a regulatory region to a gene that iε normally εilent in human fibroblasts, resulting in the functional activation of that gene. 22.1 5' CACCTAAAAT GATCTCTCTG G (SEQ ID NO 14) 22.2 5' CGCGCCGGGT GACCACACCG GGGGCCCTAG ATCTGGTGAA GCTGGAGCTA CGGAGTAA (SEQ ID NO 15) 22.3 5' TTACTCCGTA GCTCCAGCTT CACCAGATCT AGGGCCCCCG GTGTGGTCAC CCGGCGCG (SEQ ID NO 16) 22.4 5' GTCTCACCGT GATATTCTCG G (SEQ ID NO 17) EXAMPLE 5. PRODUCTION OF INTRONLESS GENES Gene targeting can also be used to produce a pro¬ cessed gene, devoid of introns, for transfer into yeast or bacteria for gene expresεion and in vitro protein produc- tion. For example, hGH can by produced in yeaεt by the approach described below. Two εeparate targeting constructε are generated. Targeting construct 1 (TCI) includes a retroviral LTR sequence, for example the LTR from the Moloney Murine Leukemia Virus (MoMLV) , a marker for selection in human cells (e.g., the neo gene from Tn5) , a marker for selec¬ tion in yeast (e.g., the yeast URA3 gene), a regulatory region capable of directing gene expresεion in yeaεt (e.g., the GAL4 promoter), and optionally, a sequence that, when fuεed to the hGH gene, will allow secretion of hGH from yeast cells (leader sequence) . The vector can also include a DNA sequence that permits retroviral pack¬ aging in human cells. The construct is organized such that the above εequenceε are flanked, on both εides, by hGH genomic sequenceε which, upon homologous recombination with genomic hGH gene N sequences, will integrate the exogenous sequences in TCI immediately upstream of hGH gene N codon 1 (corresponding to amino acid position 1 in the mature, proceεεed protein) . The order of DNA sequenc¬ es upon integration is: hGH upstream and regulatory se¬ quences, neo gene, LTR, URA3 gene, GAL4 promoter, yeast leader sequence, hGH sequenceε including and downεtream of amino acid 1 of the mature protein. Targeting Construct 2 (TC2) includes sequenceε εufficient for plasmid replica¬ tion in yeast (e.g., 2-micron circle or ARS sequences), a yeast transcriptional termination sequence, a viral LTR, and a marker gene for selection in human cells (e.g., the bacterial gpt gene) . The construct is organized such that the above sequenceε are flanked on both sides by hGH genomic sequences which, upon homologous recombination with genomic hGH gene N sequences, will integrate the exogenous sequences in TC2 immediately downstream of the hGH gene N stop codon. The order of DNA sequences upon integration is: hGH exon 5 sequences, yeast transcription termination sequences, yeast plasmid replication sequenc¬ eε, LTR, gpt gene, hGH 3' non- ranslated sequences. Linear fragments derived from TCI and TC2 are sequen¬ tially targeted to their respective positions flanking the hGH gene. After superinfection of these cells with helper retrovirus, LTR directed transcription through this region will result in an RNA with LTR sequences on both ends. Splicing of this RNA will generate a molecule in which the normal hGH introns are removed. Reverse transcription of the procesεed tranεcript will result in the accumulation of double-stranded DNA copies of the processed hGH fusion gene. DNA is isolated from the doubly-targeted, retro- virally-infected cells, and digested with an enzyme that cleaves the transcription unit once within the LTR. The digested material is ligated under conditions that promote circularization, introduced into yeast cells, and the cells are subsequently exposed to selection for the URA3 gene. Only cells which have taken up the URA3 gene (linked to the sequenceε introduced by TCI and TC2 and the proceεεed hGH gene) can grow. Theεe cells contain a plasmid which will expresε the hGH protein upon galactoεe induction and εecrete the hGH protein from cells by virtue of the fused yeast leader peptide εequence which iε cleaved away upon εecretion to produce the mature, biolog- ically active, hGH molecule. Expreεsion in bacterial cells is accomplished by simply replacing, in TCI and TC2, the ampicillin-resiε- tance gene from pBR322 for the yeaεt URA3 gene, the tac promoter (deBoer et al. , Proc. Natl. Acad. Sci. 80:21-25 (1983)) for the yeast GAL4 promoter, a bacterial leader sequence for the yeast leader sequence, the pBR322 origin of replication for the 2-micron circle or ARS sequence, and a bacterial transcriptional termination (e.g., trpA transcription terminator; Christie, G.E. et al., Proc. Natl. Acad. Sci. 78:4180-4184 (1981)) sequence for the yeast transcriptional termination sequence. Similarly, hEPO can be expressed in yeast and bacteria by simply replacing the hGH targeting sequenceε with hEPO targeting sequences, such that the yeast or bacterial leader se- quence is positioned immediately upstream of hEPO codon 1 (corresponding to amino acid poεition 1 in the mature proceεsed protein) . EXAMPLE 6. ACTIVATION AND AMPLIFICATION OF THE EPO GENE IN AN IMMORTALIZED HUMAN CELL LINE Incorporation of a dhfr expresεion unit into the unique Hindlll εite of pXEPO-13 (εee Example 4) results in a new targeting vector capable of dual selection and selection of cells in which the dhfr gene is amplified. The single Hindlll site in pXEPO-13 defines the junction of the neo gene and genomic sequence naturally residing upstream of the human EPO gene. Placement of a dhfr gene at thiε εite provideε a conεtruct with the neo and dhfr geneε εurrounded by DNA εequence derived from the natural hEPO locuε. Like pXEPO-13, derivativeε with the dhfr gene inserted are useful to target to the hEPO locus by homolo¬ gous recombination. Such a construct designated pREP04, is represented in Figure 6. The plasmid includes exons 1- 4 and part of exon 5 of the human EPO gene, as well as the HindiII-BamHI fragment lying upstream of the hEPO coding region. pSVe, pTK and pmMT-I correspond to the promoters from the SV40 early region, the Herpes Simplex Virus (HSV) thymidine kinase (TK) gene and the mouse metallothionein-I gene. It was produced as follows: Hindlll-digested pXEPO-13 was purified and made blunt with the Klenow fragment of DNA polymerase I. To obtain a dhfr expression unit, the plasmid construct pF8CIS9080 (Eaton et. al.. Biochemistry 2j5.8343-8347 (1986)) was digested with EcoRI and Sail. A 2 Kb fragment containing the dhfr expression unit was purified from this digest and made blunt with Klenow fragment of DNA polymerase I. Thiε dhfr-containing fragment was then ligated to the blunted Hindlll site of pXEPO-13. An aliquot of this ligation was transformed into Ej. coli and plated on ampicillin selection plates. Following an overnight incubation at 37°C, individual bacterial colonies were observed, picked and grown. Miniplasmid preparations were made from these cultures and the resulting DNA was then subjected to restriction enzyme digestion with the enzymeε Bgll+Hindlll, and Sfil in order to determine the orientation of the inεerted dhfr frag¬ mentε. Plasmid DNA from one of these preparations was found to contain such a 2 Kb insertion of the dhfr frag¬ ment. The transcription orientation of the dhfr expres¬ sion unit in this plaεmid was found to be opposite that of the adjacent neo gene. Thiε iε the construct designated pREP04. Plasmid pREP04 waε uεed to amplify the hEPO locus in cells subεequent to activation of the endogenouε hEPO gene by homologouε recombination. Gene activation with this construct allows selection for increased DHFR expression by the use of the drug methotrexate (MTX) . Typically, increased DHFR expreεεion would occur by an increase in copy number through DNA amplification. The net result would be co-amplification of the activated hEPO gene along with dhfr εequenceε. Co-amplification of the activated EPO locuε should result in increased EPO expression. Targeting experiments were performed in HT1080 cells with pREP04. hEPO expressing line HTREPO-52 was isolated. This line was analyzed quantitatively for EPO production and by Southern and Northern blot. This strain was found to be targeted with a εingle copy of dhfr/πeo/mMT-l se¬ quences. Expression levels obtained under 0.8 mg/ml G418 selection were approximately 1300 mU/million cells/day. Becauεe the targeted EPO locuε contained a dhfr expression unit, it was possible to select for increased expression of DHFR with the antifolate drug, MTX. This strain was therefore subjected to stepwise selection in 0.02, 0.05, 0.1, 0.2 and 0.4 μM MTX. Reεultε of initial selection of this strain are shown in Table 4 and Figure 7. TABLE 4 mU/ Million Cells/ Cell Line MTX(uM) 24h 52C20-5-0 0 1368 52C20-5-.01 0.01 1744 52C20-5-.02 0.02 11643 52C20-5-0.05 0.05 24449 52-3-5-0.10 0.1 37019 52-3-2-0.20 0.2 67867 52-3-2-0.4B 0.4 99919 Selection with elevated levels of MTX was successful in increasing hEPO expression in line HTREPO-52, with a 70- fold increase in EPO production seen in the cell line resistant to 0.4 μM MTX. Confirmation of amplification of the hEPO locus was accomplished by Southern blot analysis in MTX-resistant cell lineε, which revealed an approxi¬ mately 10-fold increase in the copy number of the activat¬ ed hEPO locus relative to the parental (untargeted) hEPO allele. EXAMPLE 7: PRODUCTION OF AN hEPO FUSION GENE BY INSERTION OF THE CMV PROMOTER 1.8 KB UPSTREAM OF THE GE¬ NOMIC hEPO CODING REGION Construction of targeting plasmid pREP015: pREP015 was constructed by first fusing the CMV promoter to hGH exon 1 by PCR amplification. A 1.6 kb fragment was amplified from hGH expresεion conεtruct pXGH308, which has the CMV promoter region beginning at nucleotide 546 and ending at nucleotide 2105 of Genbank sequence HS5MIEP fused to the hGH sequences beginning at nucleotide 5225 and ending at nucleotide 7322 of Genbank sequence HUMGHCSA, using oligonucleotides 20 and 35. Oligo 20 (35 bp, SEQ ID NO: 18), hybridized to the CMV promoter at -614 relative to the cap site (in Genbank sequence HEHCMVP1) , and included a Sail site at its 5' end. Oligo 35 (42 bp, SEQ ID NO: 19), annealed to the CMV promoter at +966 and the adjacent hGH exon 1, and included the first 10 base pairs of hEPO intron 1 (containing a portion of the splice-donor site) and a Hindlll site at its 5' end. The resulting PCR fragment was digested with Hindlll and Sail and gel-purified. Plasmid pT163 (Example 2) was digeεted with Xhol and Hindlll and the approxi¬ mately 1.1 kb fragment containing the neo expression unit waε gel-purified. The 1.6 kb CMV promoter/hGH exon 1/εplice-donor site fragment and the 1.2 kb neo fragment were ligated together and inserted into the Hindlll site of pBSIISK+ (Stratagene, Inc.) . The resulting intermedi¬ ate plasmid (deεignated pBNCHS) contained a neo expression unit in a transcriptional orientation opposite to that of the CMV promoter/hGH exon l/splice-donor site fragment) . A second intermediate, pREPOSΔHindlll, was constructed by first digesting pREP05 with Hindlll. Thiε released two fragmentε of 1.9 kb and 8.7 kb, and the 8.7 Kb fragment containing EPO targeting εequences was gel purified and circularized by self-ligation. The resulting plasmid, pREPOSΔHindlll, contained only non-coding genomic DNA sequenceε normally residing upstream of the hEPO gene. This included sequence from -5786 to -1 relative to EPO exon 1. The 2.8 kb fragment containing neo, the CMV promoter, hGH exon 1, and the splice-donor site was ex¬ cised from pBNCHS with Hindlll and gel-purified. This fragment was made blunt with the Klenow fragment of DNA polymeraεe I (New England Biolabs, Inc.) and ligated to Bglll-digested and blunt-ended pREP05ΔHindIII. Bglll cutε at a position -1779 bp upstream of hEPO exon 1 in pREP05ΔHindIII . The resulting construct, pREP015 (Figure 8) , contained EPO upstream sequences from -5786 to -1779 relative to the hEPO coding region, the neo expression unit, the CMV promoter, hGH exon 1, a splice-donor site, and sequences from -1778 to -1 bp upstream of the hEPO coding region, with the various elementε assembled, in the order listed, 5' to 3' relative to nucleotide sequence of the hEPO upstream region. For transfection of human cells, pREP015 was digested with Not I and Pvul to liber¬ ate an 8.6 kb targeting fragment. The targeting fragment contained first and second targeting sequences of 4.0 kb and 1.8 kb, respectively, with homology to DNA upstream of the hEPO gene. Cell culture, transfection, and identification of EPO expressing targeted clones: All cells were maintained at 37°C, 5% C0 2 and 98% humidity in DMEM containing 10% calf serum (DMEM/10, HyClone Laboratories) . Transfection of secondary human foreskin fibroblasts was performed by electroporating 12 x 10 6 cells in PBS (GIBCO) with 100 μg of DNA at 250 volts and 960 μF. The treated cells were seeded at 1 x 10 6 cells per 150 mm plate. The following day, the media was changed to DMEM/10 containing 0.8 mg/ml G418 (GIBCO) . Selection proceeded for 14 days, at which time the media was sampled for EPO production. All colonies on plates exhibiting significant hEPO levels (> 5 mU/ml) as deter- mined by an EPO ELISA (Genzyme Inc.) were isolated with εterile glaεs cloning cylinders (Bellco) and transferred to individual wells of a 96 well plate. Following incuba¬ tion for 1-2 days, these wells were sampled for hEPO pro¬ duction by ELISA. Resulting hEPO-producing cell strains were expanded in culture for freezing, nucleic acid isola¬ tion, and quantification of EPO production. Tranεfection of HT1080 cells (ATCC CCL 121) was performed by treating 12x 10 6 cells in PBS (GIBCO) with 45 μg of DNA at 450 volts and 250 μF. Growth and identifica¬ tion of clones occurred as for secondary human foreskin fibroblastε described above. Isolation of hEPO producing clonal cell lines occurred by limiting dilution. This was performed by first plating colonies harvested from the initial selection plates in pools of 10-15 colonies per well of a 24 well plate. hEPO producing pools were then plated at cell densities resulting in < 1 colony per well of a 96 well plate. Individual clones were expanded for further analysis as described for human foreskin fibro- blasts above. Characterization of EPO expressing clones: pREP015 is devoid of any hEPO coding sequence. Upon targeting of the neo/CMV promoter/hGH exon 1/splice-donor fragment upstream of hEPO exon 1, hEPO expression occurs by transcriptional initiation from the CMV promoter, producing a primary transcript that includes CMV sequenc¬ es, hGH exon 1 and the splice-donor site, 1.8 kb of up¬ stream hEPO sequenceε, and the normal hEPO exonε, intronε, and 3' untranεlated sequences. Splicing of this tran- script would occur from the splice-donor site adjacent to hGH exon 1 to the next downstream splice-acceptor site, which is located adjacent to hEPO exon 2. Effectively, thiε resultε in a new intron consisting of genomic se¬ quence upstream of the hEPO gene, the normal hEPO promot- er, hEPO exon 1, and hEPO intron 1. In the mature tran¬ script, hGH exon 1 would replace hEPO exon 1. hEPO exon 1 encodes only the first four and one-third amino acidε of the 26 amino acid signal peptide, which is cleaved off of the precurεor protein prior to secretion from the cell. hGH exon 1 encodes the first three and one-third amino acids of the hGH signal peptide, which also is cleaved off of the precursor protein prior to εecretion from the cell. Tranεlation of the message in which hGH exon 1 replaces hEPO exon 1 would therefore result in a protein in which the signal peptide is a chimera of hGH and hEPO sequence. Removal -of the signal peptide by the normal post-transla- tional cleavage event will produce a mature hEPO molecule whose primary sequence is indistinguishable from the normal product. Transfection of pREP015 into human fibroblasts re¬ sulted in EPO expression by these cells. Table 5 showε the reεultε of targeting experiments with pREP015 in human fibroblaεtε and HT1080 cells. The targeting frequency in normal human fibroblasts was found to be 1/264 G418 r colonies, and the targeting frequency with HT1080 cells was found to be 1/450 G418 r colonies. hEPO production levels from each of these cell strainε waε quantified. An hEPO producer obtained from tranεfection of human fibro- blasts was found to be secreting 7,679 mU/ 10 6 cells/ day (Table 5) . An activated hEPO cell line from HT1080 cells was producing 12,582 mU/10 6 cells/ day (Table 5) . These results indicated that activation of the hEPO locus waε efficient and cauεed hEPO to be produced constituitively at relatively high levels. Restriction enzyme and South¬ ern hybridization analysis was used to confirm that tar¬ geting events had occurred at the EPO locus. Southern blot analyεiε of the human fibroblast and HT1080 clones that were targeted with pREP015 was per- formed. Figure 9A shows the restriction map of the parental and targeted hEPO locus, and Figure 9B showε the results of restriction enzyme and Southern hybridization analysis of a targeted human fibroblast clone. Bglll/EcoRI and BamHI digests revealed 5.9 and 6.6 kb fragments, respectively, as a result of a targeting event at the hEPO locus (lanes Tl) . Both of these fragments resulted from the insertion of 2.7 kb of DNA containing the neo gene and CMV promoter sequences. Since only one of the two hEPO alleles were targeted, fragments of 4.3 kb (Bglll/EcoRI) or 10.6 kb (BamHI) reflecting the unaltered hEPO locus were seen in these strains and in parental DNA (lanes HF) . These resultε confirm that a homologouε recombination event had occurred at the hEPO locuε result¬ ing in the production of a novel tranεcription unit which directed the production of human erythropoietin. Oligonucleotide Sequence 20 5' TTTTCTCGAG TCGACGACAT TGATTATTGA CTAGT (SEQ ID NO: 18) 35 5'TTTTAAGCTT GAGTACTCAC CTGTAGCCAT GGTGGATCCC GT (SEQ ID NO: 19) Table 5. Transfection of pREP015 and Activation of hEPO Expresεion in Human Cells a estimated by counting colonies on 2 plates, averaging the results and extrapolating to the total number of plates medium from plates with G418 r colonieε waε sampled for EPO ELISA analyεiε and thoεe exhibiting hEPO levels greater than 5 mU/ml were counted as EPO activation events c quantitative hEPO production waε determined from human fibroblast strain, HF342-15 or HT1080 cell line, HTREP015-1-6-6 EXAMPLE 8: PRODUCTION AND AMPLIFICATION OF AN hEPO FUSION GENE BY INSERTION OF THE CMV PROMOTER 1.8 KB UPSTREAM OF THE GENOMIC hEPO CODING REGION Construction of targeting plasmid pREPQ18 : pREPOlδ (Figure 10) was constructed by insertion of a diifr expression unit at the Clal site located at the 5' end of the neo gene of pREP015. To obtain a dhfr ex¬ pression unit, the plasmid construct pF8CIS9080 [Eaton et al. , Biochemistry 25: 8343-8347 (1986)] was digested with EcoRI and Sail . A 2 kb fragment containing the dhfr expression unit was purified from this digest and made blunt by treatment with the Klenow fragment of DNA poly¬ merase I. A Clal linker (New England Biolabs) was then ligated to the blunted dhfr fragment. The products of this ligation were then digested with Clal ligated to Clal digested pREP015. An aliquot of this ligation was trans¬ formed into E. coli and plated on ampicillin selection plateε. Bacterial colonies were analyzed by restriction enzyme digestion to determine the orientation of the inserted dhfr fragment. One plasmid with dhfr in a transcriptional orientation opposite that of the neo gene was designated pREP018(-) . A second plasmid with dhfr in the same transcriptional orientation aε that of the neo gene waε designated pREP018(+) Cell culture, transfection, and identification of EPO expressing targeted clones: All cells were maintained at 37°C, 5% C0 2 , and 98% humidity in DMEM containing 10% calf serum (DMEM/10, HyClone Laboratories) . Transfection of HT1080 cells (ATCC, CCL 121) occurred by treating 12x 10 s cells in PBS (GIBCO) with 45 μg of DNA at 450 volts and 250 μF. The treated cellε were seeded at 1 x 10 6 cells per 150 mm plate. The following day, the media was changed to DMEM/10 containing 0.8 mg/ml G418 (GIBCO). Selection proceeded for 14 days, at which time the media was sampled for hEPO production. Plates exhibiting significant hEPO production levels (> 5 mU/ml) as determined by an hEPO ELISA (Genzyme Inc.) were trypsinized and the cells were re-plated for clone isolation. Isolation of hEPO produc¬ ing clonal cell lines occurred by limiting dilution, by firεt plating clones in pools of 10-15 colonies per well of a 24 well plate, and next plating cells from hEPO pro¬ ducing pools at cell densitieε resulting in less than 1 colony per well of a 96 well plate. Individual clones were expanded in culture for freezing, nucleic acid isola¬ tion and quantification of hEPO production. Iεolation of cellε containing amplified dhfr εequences bv methotrexate εelection: Targeted G418 r cell lines producing hEPO following transfection with pREPOlδ were plated at various cell densitieε for selection in methotrexate (MTX) . As new clones emerged following selection at one MTX concentra¬ tion, they were assayed for hEPO production and re-plated at various cell densities in a higher concentration of MTX (usually double the previous concentration) . This process was repeated until the desired hEPO production level was reached. At each step of MTX-resistance, DNA and RNA was isolated for respective southern and northern blot analy¬ siε. Characterization of EPO expressing clones: pREPOlδ, with two different orientations of dhfr, was transfected into HT1080 cells. Prior to transfection, pREP018(+) and pREP018(-) were digested with Xbal, releas¬ ing a 7.9 kb targeting fragment containing, in the follow¬ ing order, a 2.1 kb region of genomic DNA upstream of hEPO exon 1 (from -3891 to -1779 relative to the hEPO ATG start codon), a 2 kb region containing the dhfr gene, a 1.1 b region containing the neo gene, a 1.5 kb region contain¬ ing the CMV promoter fused to hGH exon 1, 10 bp of hEPO intron 1 (containing a splice-donor site) , followed by a 1.1 kb region of genomic DNA upstream of hEPO exon 1 (from -1778 to -678 relative to the EPO ATG start codon) . Transfection and targeting frequencies from two experi¬ ments are shown in Table 6. Five primary G418 r clones were isolated from these experiments. These were expanded in culture for quantitative analysis of hEPO expression (Table 7) . As pREPOlδ contained the dhfr gene, it is possible to select for cells containing amplified copieε of the targeting construct using MTX as deεcribed in Example 6. G418 r cloneε confirmed to be targeted to the hEPO locuε by reεtriction enzyme and Southern hybridiza¬ tion analyεis were εubjected to εtepwise selection in MTX as described. Table 6: Targeting of pREPOlβ in HT1080 cells hEPO Plates Expressors - Primary DNA Cells G418 1" With hEPO /G418 1" Clones Construct Digest Treated Colonies Expressors Colony Analyzed pREPOlβ Xba I 36 x 10" 16 , 980 39 1/435 1 ( -) PREP018 Xbal 36 x 10" 19 , 290 41 1/470 4 (+) Table 7. hEPO production in HT1080 Cell lines targeted with pREPOlδ Cell Line Construct hEPO mU/10 β Cells/24 hr 18B3 -147 pREPOlβ (+) 24759 18B3 -181 pREPOlδ (+) 20831 18B3 -145 pREPOlδ (+) 17586 18B3 -168 pREPOlδ (+) 5293 18A3 -119 pREPOlβ ( -) 2881 EXAMPLE 9: ACTIVATION AND AMPLIFICATION OF ENDOGENOUS -INTERFERON. GM-CSF. G-CSF AND FSHθ GENES IN IMMORTALIZED HUMAN CELLS A wide variety of endogenous cellular genes can be activated and amplified using the methods and DNA con¬ structs of the. invention. The following describes a general strategy for activating and amplifying the human cn-interferon (leukocyte interferon) , GM-CSF (colony stimu¬ lating factor-granulocyte/macrophage) , G-CSF (colony stimulating factor-granulocyte) and FSH/S (follicle stimu¬ lating hormone beta subunit) genes. o;-interferon The human α.-interferon gene (Genbank sequence HUMIFNAA) encodes a lδδ amino acid precursor protein containing a 23 amino acid signal peptide. The gene contains no introns. Figure 11 schematically illustrates one strategy for activating the α-interferon gene. The targeting construct is designed to include a first target¬ ing sequence homologous to sequences upstream of the gene, an amplifiable marker gene, a selectable marker gene, a regulatory region, a CAP site, a splice-donor site, an intron, a splice acceptor site, and a second targeting sequence corresponding to sequences downstream of the first targeting sequence. The second targeting sequence should not extend further upstream than to position -107 relative to the normal start codon in order to avoid undesired ATG start codons. In this strategy the first and second targeting εequenceε are immediately adjacent to each other in the normal target gene, but this is not required (see below) . Amplifiable marker genes and selectable marker genes suitable for selection are described herein. The amplifi¬ able marker gene and selectable marker gene may be the same gene, their positions may be reversed, and one or both may be situated in the intron of the targeting con¬ struct. A selectable marker gene is optional and the amplifiable marker gene is only required when amplifica¬ tion is desired. The incorporation of a specific CAP site is optional. Optionally, exon εequences from another gene can be included 3' to the splice-acceptor site and 5' to the second targeting sequence in the targeting construct. The regulatory region, CAP site, splice-donor site, intron, and splice acceptor site can be isolated as a complete unit from the human elongation factor-lα (EF-lα; Genbank sequence HUMEF1A) gene or the cytomegalovirus (CMV; Genbank εequence HEHCMVP1) immediate early region, or the components can be assembled from appropriate compo¬ nents isolated from different genes. Genomic DNA corresponding to the upstream region of the α-interferon gene for use as targeting sequences and assembly of the targeting construct can be performed using recombinant DNA methods known by those skilled in the art. As described herein, a number of selectable and amplifi- able markers can be used in the targeting constructs, and the activation and amplification can be effected in a large number of cell-types. Transfection of primary, secondary, or immortalized human cells and isolation of homologously recombinant cellε expreεεing α-interferon can be accompliεhed uεing the methodε deεcribed in Example 4, using an ELISA assay for human α-interferon (Biosource International, Camarillo, CA) . Alternatively, homo¬ logously recombinant cells may be identified by PCR screening as described in Example lg and lj . The isola- tion of cells containing amplified copies of the amplifi¬ able marker gene and the activated α-interferon locuε is performed as described in Example 6. In the homologously recombinant cells, an mRNA pre¬ cursor is produced which includes the exogenous exon, splice-donor site, intron, splice-acceptor site, second targeting sequence, and human α-interferon coding region and 3' untranslated sequences (Figure 11). Splicing of this meεsage will generate a functional mRNA which can be translated to produce human α-interferon. The size of the intron and thus the position of the regulatory region relative to the coding region of the gene may be varied to optimize the function of the regula¬ tory region. Multiple exonε may be present in the target¬ ing construct. In addition, the second targeting sequence does not need to lie immediately adjacent to or near the first targeting sequence in the normal gene, such that portionε of the gene'ε normal upεtream region are deleted upon homologouε recombination. GM-CSF The human GM-CSF gene (Genbank sequence HUMGMCSFG) encodes a 144 amino acid precursor protein containing a 17 amino acid signal peptide. The gene contains four exons and three introns, and the N-terminal 50 amino acids of the precursor are encoded in the first exon. Figure 12 schematically illustrates a strategy for activating the GM-CSF gene. In this strategy the targeting construct is designed to include a first targeting sequence homologous to sequenceε upstream of the gene, an amplifiable marker gene, a selectable marker gene, a regulatory region, a CAP site, an exon which encodes an amino acid sequence which is identical or functionally equivalent to that of the first 50 amino acids of GM-CSF, a splice-donor site, and a second targeting sequence corresponding to sequences downεtream of the first targeting sequence. By this strategy, homologously recombinant cells produce an mRNA precursor which corresponds to the exogenous exon and splice-donor site, the second targeting sequence, any sequenceε between the second targeting sequence and the start codon of the GM-CSF gene, and the exonε, introns, and 3' untranslated region of the GM-CSF gene (Figure 11) . Splicing of this message resultε in the fusion of the exogenous exon to exon 2 of the endogenous GM-CSF gene which, when translated, will produce GM-CSF. In this εtrategy the firεt and εecond targeting sequences are immediately adjacent in the normal target gene, but this is not required (see below) . Amplifiable marker genes and selectable marker geneε suitable for selection are described herein. The amplifiable marker gene and selectable marker gene can be the same gene or their positions can be reversed. A selectable marker gene is optional and the amplifiable marker gene is only re¬ quired when amplification is desired. The selectable marker and/or amplifiable marker can be positioned between the splice-donor site and the second targeting sequence in the targeting construct. The incorporation of a specific CAP site is optional. The regulatory region, CAP site, and splice-donor site can be isolated as a complete unit from the human elongation factor-lα (EF-lα; Genbank se- quence HUMEF1A) gene or the cytomegalovirus (CMV; Genbank sequence HEHCMVP1) immediate early region, or the compo¬ nents can be assembled from an appropriate component isolated from different geneε (such as the mMT-I promoter and CAP site, and exon 1 and a splice donor site from the hGH or hEPO genes. Other approaches can be employed, for example, the first and second targeting sequenceε can correεpond to εequences in the first intron of the GM-CSF gene. Alter¬ natively, a targeting construct similar to that described for the α-interferon can be used, in which the targeting construct is designed to include a firεt targeting se¬ quence homologous to sequenceε upεtream of the GM-CSF gene, an amplifiable marker gene, a εelectable marker gene, a regulatory region, a CAP site, a splice-donor site, an intron, a splice acceptor site, and a second targeting sequence corresponding to sequences downstream of the first targeting sequence. In any case the second targeting sequence does not need to lie immediately adjacent to or near the first targeting εequence in the normal gene, such that portions of the gene's normal upstream region are deleted upon homologous recombination. In addition, multiple non-coding or coding exons can be present in the targeting construct. Genomic DNA corresponding to the upεtream or intron regions of the human GM-CSF gene for use as target¬ ing sequences and assembly of the targeting construct can be performed using recombinant DNA methods known by those skilled in the art. As described herein, a number of selectable and amplifiable markers can be used in the targeting constructs, and the activation can be effected in a large number of cell-types. Transfection of primary, secondary, or immortalized human cells and iεolation of homologouεly recombinant cellε expressing GM-CSF can be accomplished using the methodε described in Example 4, using an ELISA assay for human GM-CSF (R&D Systemε, Minne¬ apolis, MN) . Alternatively, homologously recombinant cells may be identified by PCR screening as described above. The isolation of cells containing amplified copies of the amplifiable marker gene and the activated GM-CSF locus is performed as described above. G-CSF The human G-CSF gene (Genbank sequence HUMGCSFG) encodes 204-207 amino acid precursor protein containing a 30 amino acid signal peptide. The gene contains five exons and four introns. The first exon encodes 13 amino acids of the signal peptide. Figure 13 schematically illustrateε a strategy for activating the G-CSF gene. The targeting conεtruct iε deεigned to include a first target¬ ing sequence homologous to sequences upstream of the gene, an amplifiable marker gene, a selectable marker gene, a regulatory region, a CAP site, an exon which encodes an amino acid sequence which is identical or functionally equivalent to that of the first 13 amino acids of the G-CSF signal peptide, a splice- donor site, and a second targeting sequence corresponding to sequences downstream of the first targeting sequence. By this strategy, homo¬ logously recombinant cells produce an mRNA precursor which correspondε to the exogenous exon and splice-donor site, the second targeting sequence, any sequences between the second targeting sequence and the start codon of the G-CSF gene, and the exons, introns, and 3' untranslated region of the G-CSF gene (Figure 13) . Splicing of this message result in the fuεion of the exogenouε exon to exon 2 of the endogenouε G-CSF gene which, when translated, will produce G-CSF. The ability to functionally substitute the first 13 amino acids of the normal G-CSF signal peptide with those present in the exogenouε exon allows one to make modificationε in the signal peptide, and hence the secretory properties of the protein produced. In this strategy the first and second targeting sequences are immediately adjacent in the normal target gene, but this is not required. The second targeting sequence doeε not need to lie immediately adjacent to or near the firεt targeting sequence in the normal gene, such that portions of the gene's normal upεtream region are deleted upon homologouε recombination. The amplifiable marker gene and selectable marker gene can be the same gene or their poεitionε can be reversed. A εelectable marker gene iε optional and the amplifiable marker gene iε only required when amplification is desired. The select¬ able marker and/or amplifiable marker can be positioned between the splice-donor site and the second targeting sequence in the targeting construct. The incorporation of a specific CAP εite iε optional. The regulatory region, CAP site, and splice-donor site can be isolated as a complete unit from the human elongation factor-lα (EF-lα; Genbank sequence HUMEF1A) gene or the cytomegalovirus (CMV; Genbank sequence HEHCMVP1) immediate early region, or the components can be assembled from an appropriate component isolated from different genes (such aε the mMT-I promoter and CAP site, and exon 1 and a splice donor site from the hGH or EPO genes. Multiple exogenous exons, coding or non-coding, can be used in the targeting con- struct so long as an ATG start codon which, upon splicing, will be in-frame with the mature protein, is included in one of the exons. Other approaches may be employed, for example, the firεt and εecond targeting sequences can correspond to εequenceε in the firεt intron of the G-CSF gene. Alterna¬ tively, a targeting construct similar to that described for the α-interferon can be used, in which the targeting construct is designed to include a firεt targeting se¬ quence homologous to sequences upstream of the G-CSF gene, an amplifiable marker gene, a selectable marker gene, a regulatory region, a CAP site, a splice-donor site, an intron, a splice acceptor site, and a second targeting sequence corresponding to sequences downstream of the first targeting sequence. Genomic DNA corresponding to the upεtream or intron regionε of the human G-CSF gene for uεe as targeting sequences and assembly of the targeting construct can be performed using recombinant DNA methods known by those skilled in the art. As described herein, a number of selectable and amplifiable markerε can be used in the targeting constructs, and the activation can be effected in a large number of cell-types. Transfection of primary, secondary, or immortalized human cells and isolation of homologously recombinant cells expressing G-CSF can be accomplished using the methods described in Example 4, uεing an ELISA aεsay for human G-CSF (R&D Systems, Minne¬ apolis, MN) . Alternatively, homologously recombinant cellε may be identified by PCR εcreening aε deεcribed above. The iεolation of cellε containing amplified copieε of the amplifiable marker gene and the activated α-interferon locuε iε performed aε described above. FSH? The human FSH/3 gene (Genbank sequence HUMFSHl) en¬ codes a 129 amino acid precursor protein containing a 16 amino acid signal peptide. The gene contains three exons and two intronε, with the first exon being a non-coding exon. The activation of FSH / 8 can be accomplished by a number of strategieε. One εtrategy iε shown in Figure 14. In this strategy, a targeting construct is designed to include a first targeting sequence homologous to sequences upstream of the gene, an amplifiable marker gene, a selec¬ table marker gene, a regulatory region, a CAP site, an exon, a splice-donor site, and a second targeting sequence corresponding to sequences downstream of the first target- ing sequence. By this strategy, homologously recombinant cellε produce an mRNA precurεor which corresponds to the exogenous exon and splice-donor site, the second targeting sequence, any sequences between the second targeting sequence and the start codon of the FSH / S gene, and the exons, introns, and 3' untranslated regions of the FSH/S gene (Figure 14) . Splicing of this message results in the fusion of the exogenous exon to exon 2 of the endogenous FSH/S gene which, when translated, can produce FSH/S. In this strategy the first and second targeting sequences are immediately adjacent in the normal target gene, but this is not required (see below) . Other approaches can be employed, for example, the first and second targeting sequences can correspond to sequenceε in the first intron of the FSH / S gene. Alterna- tively, a targeting construct similar to that described for the α-interferon can be used. In this strategy, the targeting construct iε designed to include a first target¬ ing sequence homologous to sequences upstream of the FSH/S gene, an amplifiable marker gene, a selectable marker gene, a regulatory region, a CAP site, a splice-donor site, an intron, a splice acceptor site, and a second targeting sequence corresponding to sequences downstream of the first targeting sequence. The second targeting sequence should not extend further upstream than to posi¬ tion -40 relative to the normal FSH8 transcriptional start site in order to avoid undesired ATG start codons. In the homologously recombinant cells, an mRNA precursor is produced which includes the exogenous exon, splice-donor εite, intron, splice-acceptor site, second targeting sequence, and human FSH / 8 coding exons, intron and 3' un¬ translated sequences. Splicing of this message will generate a functional mRNA which can be translated to produce human FSH / S. The size of the intron and thus the position of the regulatory region relative to the coding region of the gene can be varied to optimize the function of the regulatory region. In any activation strategy, the second targeting sequence does not need to lie immediately adjacent to or near the first targeting sequence in the normal gene, such that portions of the gene's normal upstream region are deleted upon homologous recombination. Furthermore, one targeting sequence can be upstream of the gene and one may be within an exon or intron of the FSH / S gene. The amplifiable marker gene and selectable marker gene can be the same gene, their poεitionε can be reverεed, and one or both can be εituated in the intron of the targeting conεtruct. Amplifiable marker genes and selectable marker genes εuitable for εelection are de- scribed herein. A selectable marker gene is optional and the amplifiable marker gene is only required when amplifi¬ cation is desired. The incorporation of a specific CAP site is optional. Optionally, exon sequenceε from another gene can be included 3' to the splice-acceptor site and 5' to the second targeting sequence in the targeting con¬ struct. The regulatory region, CAP site, exon, splice-donor site, intron, and splice acceptor site can be iεolated as a complete unit from the human elongation factor-lα (EF-lα; Genbank sequence HUMEF1A) gene or the cytomegalovirus (CMV; Genbank sequence HEHCMVPl) immediate early region, or the componentε can be assembled from appropriate components isolated from different genes. In any case, the exogenous exon can be the same or different from the first exon of the normal FSH/S gene, and multiple exons can be present in the targeting construct. Genomic DNA corresponding to the upstream region of the FSH/S gene for use as targeting sequences and assembly of the targeting construct can be performed using recombi¬ nant DNA methods known by those skilled in the art. As described herein, a number of selectable and amplifiable markers can be used in the targeting constructs, and the activation can be effected in a large number of cell-types. If desirable, the product of the activated FSH/S gene can be produced in a cell type that expresses the human glycoprotein α-subunit, the product of which formε a heterodimer with the product of the FSH / 8 gene. Thiε may be a naturally occurring cell εtrain or line. Alternatively, the human glycoprotein α-εubunit gene (Genbank εequence HUMGLYCAl) can be co-expreεεed with the product of the FSH / 8 gene, with such co-expreεεion accom¬ plished by expression of the human glycoprotein α-subunit gene or cDNA under the control of a suitable promoter, or by activation of the human glycoprotein α-subunit gene through the methods described herein. Tranεfection of primary, secondary, or immortalized human cells and iεola- tion of homologously recombinant cells expressing FSH / 8 can be accomplished using the methods described above using an ELISA assay for human FSH / 8 (Accurate Chemical and Scien¬ tific, estbury, NY) . Alternatively, homologously recom- binant cells may be identified by PCR screening as de¬ scribed above. The isolation of cellε containing ampli¬ fied copies of the amplifiable marker gene and the acti¬ vated α-interferon locus is performed aε deεcribed above. Equivalents Those skilled in the art will recognize, or be able to ascertain using not more than routine experimentation, many equivalents to the specific embodiments of the inven¬ tion described herein. Such equivalents are intended to be encompasεed by the following claims.

Claims: CLAIMS 1. A DNA construct capable of altering the expression of a targeted gene when inserted into chromosonal DNA of a cell compriεing: (a) a targeting sequence; (b) a regulatory sequence; (c) an exon; and (d) an unpaired splice-donor site. 2. The DNA construct of Claim 1 wherein the exon com- prises a CAP site. 3. The DNA construct of Claim 2 wherein the exon further comprises the nucleotide sequence ATG. 4. The DNA construct of Claim 3 wherein the exon further compriseε encoding DNA which iε in-frame with the targeted gene. 5. The DNA conεtruct of Claim 4 wherein the encoding DNA of the exon iε the same as the encoding DNA of the first exon of the targeted gene. 6. The DNA construct of Claim 4 wherein the encoding DNA of the exon is different from the encoding DNA of the first exon of the targeted gene. 7. The DNA construct of Claim 4 wherein the targeting sequence is homologous to a sequence within the targeted gene. 8. The DNA construct of Claim 4 wherein the targeting sequence is homologous to a sequence upstream of the coding region of the targeted gene. 9. The DNA construct of Claim 4 wherein the targeting sequence is homologous to a sequence upstream of the endogenous regulatory sequence of the targeted gene. 10. The DNA construct of Claim 4 wherein the construct further comprises a second targeting sequence homolo¬ gous to a sequence within the targeted gene. 11. The DNA construct of Claim 4 wherein the construct further comprises a second targeting sequence homolo¬ gous to a sequence upstream of the coding region of the targeted gene. 12. The DNA construct of Claim 4 wherein the construct further comprises a second targeting sequence homolo¬ gous to a sequence upstream of the endogenous regula¬ tory sequence of the targeted gene. 13. The DNA construct of Claim 4 wherein the targeted gene encodes a therapeutic protein. 14. The DNA construct of Claim 4 wherein the targeted gene encodes a hormone, a cytokine, an antigen, an antibody, an enzyme, a clotting factor, a tranεport protein, a receptor, a regulatory protein, a struc¬ tural protein or a transcription factor. 15. The DNA construct of Claim 4 wherein the targeted gene encodes a protein selected from the group con¬ sisting of erythropoietin, calcitonin, growth hor- mone, insulin, insulinotropin, insulin-like growth factors, parathyroid hormone, β-interferon, γ-inter- feron, nerve growth factors, FSH / 8, TGF-/S, tumor necrosis factor, glucagon, bone growth factor-2, bone growth factor-7, TSH-/S, interleukin 1, interleukin 2, interleukin 3, interleukin 6, interleukin 11, inter¬ leukin 12, CSF-granulocyte, CSF-macrophage, CSF- granulocyte/ macrophage, immunoglobulins, catalytic antibodieε, protein kinase C, glucocerebroεidaεe, 5 superoxide dismutaεe, tissue plasminogen activator, urokinase, antithrombin III, DNAse, α-galactosidase, tyrosine hydroxylase, blood clotting factor V, blood clotting factor VII, blood clotting factor VIII, blood clotting factor IX, blood clotting factor X, 1.0 blood clotting factor XIII, apolipoprotein E or apolipoprotein A-I, globinε, low density lipoprotein receptor, IL-2 receptor, IL-2 antagonists, alpha-1 antitrypsin, immune responεe modifiers, and soluble CD4. 15 16. The DNA construct of Claim 15 wherein the targeted gene encodeε growth hormone, FSH / 8, G-CSF or GM-CSF. 17. The DNA conεtruct of Claim 15 wherein the targeted gene encodeε erythropoietin. 18. The DNA conεtruct of Claim 17 wherein the encoding 0 DNA of the exon is the same as the encoding DNA of the first exon of erythropoietin. 19. The DNA construct of Claim 17 wherein the encoding DNA of the exon is different from the encoding DNA of the first exon of erythropoietin. 5 20. The DNA conεtruct of Claim 19 wherein the encoding DNA of the exon iε the εame aε the encoding DNA of the first exon of human growth hormone. 21. The DNA construct of Claim 1 wherein the regulatory sequence is a promoter, an enhancer, a scaffold- attachment region or a transcription factor binding site. 22. The DNA construct of Claim 21 wherein the regulatory εequence iε a promoter. 23. The DNA construct of Claim 22 further comprising an additional regulatory sequence. 24. The DNA construct of Claim 22 wherein the construct further comprises an enhancer. 25. The DNA construct of Claim 24 further comprising one or more selectable markers. 26. The DNA construct of Claim 25 further comprising an amplifiable marker gene. 27. The DNA conεtruct of Claim 21 wherein the regulatory εequence is a regulatory sequence of the mouse metallothionein-I gene, a regulatory sequence of an SV-40 gene, a regulatory sequence of a cytomegalo- virus gene, a regulatory sequence of a collagen gene, a regulatory sequence of an actin gene, a regulatory sequence of an immunoglobulin gene, a regulatory sequence of the HMG-CoA reductase gene or a regulato¬ ry sequence of the EF-lα gene. 28. A method of making a homologously recombinant cell wherein the expression of a targeted gene is altered, comprising the steps of: (a) transfecting a cell with a DNA construct, the DNA construct compriεing: (i) a targeting sequence; (ii) a regulatory sequence; (iii) an exon; and (iv) an unpaired splice-donor site, thereby producing a transfected cell; and (b) maintaining the transfected cell under condi¬ tions appropriate for homologous recombination. 29. The method of Claim 28 wherein the exon comprises a CAP site. 30. The method of Claim 29 wherein the exon comprises the nucleotide sequence ATG. 31. The method of Claim 30 wherein the exon further comprises encoding DNA in-frame with the targeted gene. 32. The method of Claim 31 wherein the encoding DNA of the exon is the same as the encoding DNA of the first exon of erythropoietin. 33. The method of Claim 31 wherein the encoding DNA of the exon is different from the encoding DNA of the first exon of erythropoietin. 34. The method of Claim 31 wherein the targeting sequence is homologous to a sequence within the targeted gene. 35. The method of Claim 31 wherein the targeting εequence iε homologouε to a εequence upεtream of the coding region of the targeted gene. 36. The method of Claim 31 wherein the targeting sequence iε homologouε to a sequence upstream of the endoge¬ nous regulatory sequence of the targeted gene. 37. The method of Claim 31 wherein the construct further comprises a εecond targeting sequence homologouε to a sequence within the targeted gene. 38. The method of Claim 31 wherein the construct further comprises a second targeting sequence homologous to a sequence upstream of the coding region of the target- ed gene. 39. The method of Claim 31 wherein the construct further comprises a second targeting sequence homologous to a sequence upstream of the endogenous regulatory se¬ quence of the targeted gene. 40. The method of Claim 31 wherein the cell is a human cell. 41. The method of Claim 28 wherein the targeted gene encodes erythropoietin. 42. The method of Claim 31 wherein the encoding DNA is the same as the encoding DNA of the first exon of erythropoietin. 43. The method of Claim 31 wherein the encoding DNA is different from the encoding DNA of the firεt exon of erythropoietin. 44. The method of Claim 31 wherein the encoding DNA of the exon iε the same as the encoding DNA of the first exon of human growth hormone. 45. The method of Claim 28 further comprising the step of: (c) maintaining a homologously recombinant cell from step (b) under conditions appropriate for pro- duction of a protein. 46. The method of Claim 45 in which the gene whose ex¬ preεεion is altered is the erythropoietin gene. 47. Erythropoietin produced by the method of Claim 45. 48. A fusion protein containing amino acids encoded by exons from the DNA construct and amino acids encoded by an endogenouε gene produced by the method of Claim 45. 49. A fuεion protein of Claim 48 wherein the endogenous gene is erythropoietin. 50. A fusion protein of Claim 49 comprising amino acids 1-3 of human growth hormone and amino acids 6-165 of human erythropoietin. 51. A homologously recombinant cell produced by the method of Claim 28. 52. A homologously recombinant cell produced by the method of Claim 29. 53. A homologously recombinant cell produced by the method of Claim 30. 5 . A homologously recombinant cell produced by the method of Claim 31. 55. A homologouεly recombinant cell produced by the method of Claim 32. 56. A homologouεly recombinant cell produced by the method of Claim 33. 57. A homologously recombinant cell produced by the method of Claim 40. 58. A homologously recombinant cell produced by the method of Claim 41. 59. A homologously recombinant cell produced by the method of Claim 42. 60. A homologously recombinant cell produced by the method of Claim 44. 61. A homologously recombinant cell comprising an exoge¬ nous regulatory sequence, an exogenous exon and a splice-donor site, operatively linked to the second exon of an endogenous gene. 62. The homologously recombinant cell of Claim 61 wherein the exogenous exon comprises a CAP site. 63. The homologously recombinant cell of Claim 62 wherein the exogenous exon further compriseε the nucleotide εequence ATG. 64. The homologouεly recombinant cell of Claim 63 wherein the exogenouε exon further comprises encoding DNA in- frame with the targeted endogenouε gene. 65. The homologously recombinant cell of Claim 64 wherein the encoding DNA is the same as the encoding DNA of the first exon of the targeted gene. 66. The homologously recombinant cell of Claim 64 wherein the encoding DNA iε different from the encoding DNA of the first exon of the targeted gene. 67. The homologously recombinant cell of Claim 64 wherein the exogenous regulatory sequence, exogenous exon and εplice-donor εite are upstream of the coding region of the targeted gene. 68. The homologouεly recombinant cell of Claim 67 wherein the exogenouε regulatory sequence, exogenous exon and splice-donor site are upstream of the endogenous regulatory sequence of the targeted gene. 69. The homologously recombinant cell of Claim 61 wherein the endogenous regulatory sequence is deleted. 70. The homologously recombinant cell of Claim 69 wherein the first endogenous exon is deleted. 71. The homologously recombinant cell of Claim 64 wherein the targeted gene encodes a hormone, a cytokine, an antigen, an antibody, an enzyme, a clotting factor, a transport protein, a receptor, a regulatory protein, a structural protein or a transcription factor. 72. The homologously recombinant cell of Claim 64 wherein the targeted gene encodes a protein selected from the group consisting of erythropoietin, calcitonin, growth hormone, insulin, insulinotropin, insulin¬ like growth factorε, parathyroid hormone, S-inter- feron, γ-interferon, nerve growth factors, FSH / 8, TGF- β, tumor necrosis factor, glucagon, bone growth factor-2, bone growth factor-7, TSH-8, interleukin 1, interleukin 2, interleukin 3, interleukin 6, interleukin 11, interleukin 12, CSF-granulocyte, CSF- macrophage, CSF-granulocyte/ macrophage, immunoglobulins, catalytic antibodies, protein kinase C, glucocerebrosidase, superoxide dismutase, tissue plasminogen activator, urokinase, antithrombin III, DNAse, α-galactosidase, tyrosine hydroxylase, blood clotting factor V, blood clotting factor VII, blood clotting factor VIII, blood clotting factor IX, blood clotting factor X, blood clotting factor XIII, apoli¬ poprotein E or apolipoprotein A-I, globins, low denεity lipoprotein receptor, IL-2 receptor, IL-2 antagonists, alpha-1 antitrypsin, immune response modifiers, and εoluble CD4. 73. The homologously recombinant cell of Claim 61 wherein the cell iε a eukaryote. 74. The homologouεly recombinant cell of Claim 73 wherein the cell is of fungal, plant or animal origin. 75. The homologously recombinant cell of Claim 74 wherein the cell is of vertebrate origin. 76. The homologously recombinant cell of Claim 75 wherein the cell is a primary or secondary mammalian cell. 77. The homologously recombinant cell of Claim 75 wherein the cell is a primary or secondary human cell. 78. The homologously recombinant cell of Claim 75 wherein the cell is an immortalized mammalian cell. 79. The homologously recombinant cell of Claim 75 wherein the cell is an immortalized human cell. 80. The homologously recombinant cell of Claim 75 wherein the cell is selected from the group consiεting of: HT1080 cellε, HeLa cellε and derivatives of HeLa cellε, MCF-7 breast cancer cells, K-562 leukemia cells, KB carcinoma cells, 2760AD ovarian carcinoma cellε, Raji cellε, Jurkat cellε, Namalwa cells, HL-60 cells, Daudi cells, RPMI 6226 cells, U-937 cells, Boweε Melanoma cellε, WI-36VA13 εubline 2R4 cells, and MOLT-4 cells. 81. The homologously recombinant cell of Claim 80 wherein the targeted gene encodes erythropoietin. 82. The homologously recombinant cell of Claim 81 capable of expressing erythropoietin. 83. The homologously recombinant cell of Claim 82 wherein the encoding DNA is the same aε the encoding DNA of the firεt exon of erythropoietin. 84. The homologouεly recombinant cell of Claim 81 wherein the encoding DNA iε different from the encoding DNA of the firεt exon of erythropoietin. 85. The homologously recombinant cell of Claim 84 wherein the encoding DNA is the same as the encoding DNA of the firεt exon of human growth hormone. 86. The homologouεly recombinant cell of Claim 61 capable of expreεεing a fuεion protein comprising amino acids encoded by the exogenous exon and amino acids encoded by the endogenous gene. 87. A fusion protein of Claim 86 wherein the endogenous gene is erythropoietin. 86. A fusion protein of Claim 87 comprising amino acids 1-3 of human growth hormone and amino acids 6-165 of human erythropoietin. 89. The homologously recombinant cell of Claim 66 wherein the regulatory sequence is a promoter, an enhancer, a scaffold-attachment region or a transcription factor binding εite. 90. The homologously recombinant cell of Claim 89 wherein the exogenous regulatory sequence is a promoter. 91. The homologously recombinant cell of Claim 89 wherein the exogenous regulatory sequence is a regulatory sequence of the mouse metallothionein-I gene, a regulatory sequence of an SV-40 gene, a regulatory sequence of a cytomegalovirus gene, a regulatory sequence of a collagen gene, a regulatory sequence of an actin gene, a regulatory sequence of an immuno¬ globulin gene, a regulatory sequence of the HMG-CoA reductase gene or a regulatory sequence of the EF-lα gene. 92. A method of altering the expression of a gene in a cell, comprising the steps of: (a) transfecting a cell with a DNA construct, the DNA construct comprising: (i) a targeting sequence; (ii) a regulatory sequence; (iii) an exon; and (iv) an unpaired splice-donor site, thereby producing a transfected cell; (b) maintaining the tranεfected cell under condi¬ tions appropriate for homologous recombination, thereby producing a homologously recombinant cell,- and (c) maintaining the homologously recombinant cell under conditions appropriate for expresεion of the gene. 93. The method of Claim 92 wherein the exon comprises the nucleotide sequence ATG. 9 . The method of Claim 92 wherein the exon further compriεeε a CAP site. 95. The method of Claim 94 wherein the exon further comprises encoding DNA which is in-frame with the targeted gene. 96. The method of Claim 95 wherein the encoding DNA is the same aε the encoding DNA of the first exon of the targeted gene. 97. The method of Claim 96 wherein the targeted gene iε the erythropoietin gene. 9δ. The method of Claim 96 wherein the encoding DNA is different from the encoding DNA of the first exon of the targeted gene. 99. The method of Claim 98 wherein the targeted gene is the erythropoietin gene. 100. The method of Claim 98 wherein the targeting sequence is homologous to a sequence within the targeted gene. 101. The method of Claim 98 wherein the targeting sequence is homologous to a sequence upstream of the coding region of the targeted gene. 102. The method of Claim 98 wherein the targeting sequence is homologous to a sequence upstream of the endoge¬ nous regulatory sequence for the targeted gene. 103. The method of Claim 98 wherein the conεtruct further comprises a second targeting sequence homologous to a sequence within the targeted gene. 104. The method of Claim 98 wherein the construct further comprises a second targeting sequence homologouε to a εequence upstream of the coding region of the target¬ ed gene. 105. The method of Claim 98 wherein the construct further comprises a second targeting sequence homologous to a sequence upstream of the endogenous regulatory se¬ quence for the targeted gene. 106. The method of Claim 92 further comprising the step of: (c) maintaining a homologously recombinant cell under conditions appropriate for production of a protein. 107. The method of Claim 106 in which the gene whose ex¬ presεion iε altered is the erythropoietin gene. 108. Erythropoietin produced by the method of Claim 107. 109. A fusion protein produced by the method of Claim 106. 110. A fusion protein of Claim 109 wherein the endogenous gene is erythropoietin. 111. A fusion protein of Claim 110 comprising amino acids 1-3 of human growth hormone and amino acids 6-165 of human erythropoietin. 112. A method of making a protein by altering the expres¬ εion of a gene in a cell, compriεing the steps of: (a) transfecting a cell with a DNA construct, the DNA construct comprising: (i) a targeting sequence; (ii) a regulatory sequence; (iii) an exon; and (iv) an unpaired splice-donor site, thereby producing a transfected cell; (b) maintaining the transfected cell under condi¬ tions appropriate for homologous recombination, thereby producing a homologously recombinant cell; and (c) maintaining the homologously recombinant cell under conditions appropriate for production of the protein. 113. The method of Claim 112 wherein the exon comprises a CAP site. 114. The method of Claim 113 wherein the exon comprises the nucleotide sequence ATG. 115. The method of Claim 114 wherein the exon further comprises encoding DNA which is in-frame with the targeted endogenouε gene. 116. The method of Claim 115 wherein the encoding DNA is the same as the encoding DNA of the first exon of the targeted gene. 117. The method of Claim 116 wherein the encoding DNA is different from the encoding DNA of the first exon of the targeted gene. 118. The method of Claim 117 wherein the targeting se¬ quence is homologous to a sequence within the target¬ ed gene. 119. The method of Claim 117 wherein the targeting se¬ quence is homologous to a sequence upstream of the coding region of the targeted gene. 120. The method of Claim 117 wherein the targeting se¬ quence is homologous to a sequence upstream of the endogenous regulatory sequence for the targeted gene. 121. The method of Claim 117 wherein the construct f rther comprises a second targeting sequence homologous to a sequence within the targeted gene. 122. The method of Claim 117 wherein the construct further compriseε a second targeting sequence homologous to a sequence upstream of the coding region of the target¬ ed gene. 123. The method of Claim 117 wherein the construct further comprises a second targeting sequence homologous to a sequence upstream of the endogenous regulatory se¬ quence for the targeted gene. 124. An erythropoietin produced by the method of Claim 112. 125. The erythropoietin of Claim 124 wherein the cell is of human origin. 126. A protein produced by the method of Claim 112. 127. The protein of Claim 126 which is a fusion protein. 128. The fusion protein of Claim 127 wherein the endoge¬ nous gene is the erythropoietin gene. 129. The fusion protein of Claim 128 comprising amino acids 1-3 of human growth hormone and amino acids 6- 165 of human erythropoietin. 130. The DNA plasmid pREPOlβ. 131. A DNA construct capable of altering the expression of a targeted gene when inserted into the chromosomal DNA of a cell, comprising: (a) a targeting sequence; (b) a regulatory sequence; (c) an exon; (d) a splice-donor site; (e) an intron; and (f) a splice-acceptor site. 132. The DNA construct of Claim 131 wherein the targeting sequence is homologous to a sequence within the targeted gene. 133. The DNA construct of Claim 131 wherein the targeting sequence is homologous to a sequence upstream of the coding region of the targeted gene. 134. The DNA conεtruct of Claim 131 wherein the targeting εequence is homologouε to a εequence upstream of the endogenous regulatory sequence of the targeted gene. 135. The DNA construct of Claim 131 wherein the construct further comprises a second targeting sequence homolo¬ gous to a sequence within the targeted gene. 136. The DNA construct of Claim 131 wherein the construct further comprises a second targeting sequence homolo¬ gouε to a sequence upstream of the coding region of the targeted gene. 137. The DNA construct of Claim 131 wherein the conεtruct further comprises a second targeting sequence homolo¬ gous to a sequence upstream of the endogenous regula¬ tory sequence of the targeted gene. 138. A homologously recombinant cell compriεing a regula¬ tory sequence, an exon, a splice-donor site, an intron and a splice-acceptor site introduced by homologous recombination upstream of the coding region of a targeted gene. 139. The homologously recombinant cell of Claim 138 where¬ in the targeted gene iε the α-interferon gene. 140. The homologously recombinant cell of Claim 138 where¬ in the targeted gene is the erythropoietin gene. 141. A homologously recombinant cell comprising the dhfr gene, the neo gene, the CMV promoter, hGH exon 1 and an unpaired splice-donor site targeted to a position upstream of the endogenous erythropoietin regulatory region. 142. The homologously recombinant cell of Claim 141 pro¬ duced by the integration of DNA from pREPOlδ. 143. A method of making a homologously recombinant cell wherein the expreεsion of a targeted gene is altered, comprising the stepε of: (a) transfecting a cell with a DNA construct, the construct comprising: (i) a targeting sequence; (ii) a regulatory sequence; (iii) an exon; (iv) a εplice-donor site; (v) an intron; and (vi) a splice-acceptor site; wherein the targeting sequence directs the inte- gration of elements (b) - (f) upstream such that they are operatively linked to the first exon of a targeted gene; and (b) maintaining the transfected cell under condi¬ tionε appropriate for homologouε recombination. 144. A homologouεly recombinant cell produced by the method of Claim 143. 145. A method of altering the expression of a gene in a cell, comprising the stepε of: (a) transfecting a cell with a DNA conεtruct, the conεtruct comprising: (i) a targeting sequence; (ii) a regulatory sequence; (iii) an exon; (iv) a splice-donor site; (v) an intron; and (vi) a splice-acceptor site; wherein the targeting sequence directs the inte¬ gration of elements (b) - (f) upstream such that they are operatively linked to the first exon of a targeted gene; (b) maintaining the transfected cell under condi¬ tions appropriate for homologous recombination; and (c) maintaining the homologously recombinant cell under conditions appropriate for expresεion of the gene. 146. A method of making a protein by altering the expres¬ sion of a gene in a cell, comprising the steps of: (a) transfecting a cell with a DNA construct, the construct comprising: (i) a targeting sequence; (ii) a regulatory sequence; (iii) an exon; (iv) a splice-donor site; (v) an intron; and (vi) a splice-acceptor site; wherein the targeting sequence directs the inte¬ gration of elements (b)-(f) upstream such that they are operatively linked to the first exon of a targeted gene; (b) maintaining the transfected cell under condi¬ tions appropriate for homologous recombination; and (c) maintaining the homologously recombinant cell under conditions appropriate for expresεion of the protein. 147. The method of Claim 146 wherein the targeted gene iε the α-interferon gene or the erythropoietin gene. 148. DNA sequences located between about 5 kilobases and 30 kilobases upstream of the ATG of the erythropoie¬ tin gene. 149. A method for targeting the erythropoietin gene in a mammalian cell comprising transfecting the cell with a construct comprising a DNA sequence homologous to a εequence upstream of the sequence ATG of the erythro¬ poietin gene. 150. The method of Claim 149 wherein the construct com- prising a DNA sequence homologous to a sequence located between about 5 kilobaεeε and 30 kilobaεeε upstream of the sequence ATG of the erythropoietin gene. 151. The method of Claim 150 wherein the mammalian cell is a human cell. 152. A method for targeting the erythropoietin gene in a mammalian cell comprising transfecting the cell with a construct comprising a DNA sequence homologous to a sequence within the erythropoietin gene. 153. The method of Claim 152 wherein the mammalian cell is a human cell.

Inventors: HEARTLEIN MICHAEL W, SELDEN RICHARD F, TRECO DOUGLAS A, HEARTLEIN, MICHAEL, W., SELDEN, RICHARD, F., TRECO, DOUGLAS, A.
Applicants: HEARTLEIN MICHAEL W, SELDEN RICHARD F, TRANSKARYOTIC THERAPIES INC, TRECO DOUGLAS A, HEARTLEIN, MICHAEL, W., SELDEN, RICHARD, F., TRANSKARYOTIC THERAPIES, INC., TRECO, DOUGLAS, A.
Application year: 1995
Main classification: C12N15
IPC codes: A61K31, A61K38, A61K39, A61K48, A61P43, C07H21, C07K14, C12N1, C12N5, C12N15, C12P21, C12R1