NOVEL ECDYSONE RECEPTOR-BASED INDUCIBLE GENE EXPRESSION SYSTEM

Abstract: This invention relates to the field of biotechnology or genetic engineering. Specifically, this invention relates to the field of gene expression. More specifically, this invention relates to a novel inducible gene expression system and methods of modulating gene expression in a host cell for applications such as gene therapy, large scale production of proteins and antibodies, cell-based high throughput screening assays, functional genomics and regulation of traits in transgenic plants and animals.

Description: NOVEL ECDYSONE RECEPTOR-BASED INDUCIBLE GENE EXPRESSION SYSTEM This application claims priority to co-pending US provisional appUcation Serial number 60/191,355, filed March 22, 2000 and to co-pending US provisional appUcation Serial number 60/269,799, filed February 20, 2001. FIELD OF THE INVENTION This invention relates to the field of biotechnology or genetic engineering. Specifically, this invention relates to the field of gene expression. More specifically, this invention relates to a novel ecdysone receptor-based inducible gene expression system and methods of modulating the expression of a gene within a host ceU using this inducible gene expression system. BACKGROUND OF THE INVENTION In the field of genetic engineering, precise control of gene expression is a valuable tool for studying, manipulating, and continuing development and other physiological processes. Gene expression is a complex biological process involving a number of specific protein-protein interactions. In order for gene expression to be triggered, such that it produces the RNA necessary as the first step in protein synthesis, a transcriptional activator must be brought into proximity of a promoter that controls gene transcription. TypicaUy, the transcriptional activator itself is associated with a protein that has at least one DNA binding domain that binds to DNA binding sites present in the promoter regions of genes. Thus, for gene expression to occur, a protein comprising a DNA binding domain and a transactivation domain located at an appropriate distance from the DNA binding domain must be brought into the correct position in the promoter region of the gene. The traditional transgenic approach utilizes a cell-type specific promoter to drive the expression of a designed transgene. A DNA construct containing the transgene is first incorporated into a host genome. When triggered by a transcriptional activator, expression of the transgene occurs in a given cell type. Another means to regulate expression of foreign genes in ceUs is through inducible promoters. Examples of the use of such inducible promoters include the PRl-a promoter, prokaryotic repressor-operator systems, iinniunosuppressive-immunophilin systems, and higher eukaryotic transcription activation systems such as steroid hormone receptor systems and are described below. The PRl-a promoter from tobacco is induced during the systemic acquired resistance response following pathogen attack. The use of PRl-a may be Umited because it often responds to endogenous materials and external factors such as pathogens, UV-B radiation, and poUutants. Gene regulation systems based on promoters induced by heat shock, interferon and heavy metals have been described (Wurn et al., 1986, Proc. Natl. Acad. Sci. USA 83:5414- 5418; Arnheiter et al., 1990 Cell 62:51-61; Filmus et al., 1992 Nucleic Acids Research 20:27550-27560). However, these systems have limitations due to their effect on expression of non-target genes. These systems are also leaky. Prokaryotic repressor-operator systems utilize bacterial repressor proteins and the unique operator DNA sequences to which they bind. Both the tetracycline ("Tet") and lactose ("Lac") repressor-operator systems from the bacterium Escherichia coli have been used in plants and animals to control gene expression. In the Tet system, tetracycline binds to the TetR repressor protein, resulting in a conformational change which releases the repressor protein from the operator which as a result allows transcription to occur. In the Lac system, a lac operon is activated in response to the presence of lactose, or synthetic analogs such as isopropyl-b-D-thiogalactoside. Unfortunately, the use of such systems is restricted by unstable chemistry of the Ugands, i.e. tetracycUne and lactose, their toxicity, their natural presence, or the relatively high levels required for induction or repression. For similar reasons, utility of such systems in animals is limited. hnmunosuppressive molecules such as FK506, rapamycin and cyclosporine A can bind to immunophilins FKBP12, cyclophiUn, etc. Using this information, a general strategy has been devised to bring together any two proteins simply by placing FK506 on each of the two proteins or by placing FK506 on one and cyclosporine A on another one. A synthetic homodimer of FK506 (FK1012) or a compound resulted from fusion of FK506-cyclosporine (FKCsA) can then be used to induce dimerization of these molecules (Spencer et al., 1993, Science 262: 1019-24; Belshaw et al., 1996 Proc Natl Acad Sci USA 93:4604-7). Gal4 DNA binding domain fused to FKBP12 and VP16 activator domain fused to cyclophiUn, and FKCsA compound were used to show heterodimerization and activation of a reporter gene under the control of a promoter containing Gal4 binding sites. Unfortunately, this system includes immunosuppressants that can have unwanted side effects and therefore, limits its use for various mammalian gene switch appUcations. Higher eukaryotic transcription activation systems such as steroid hormone receptor systems have also been employed. Steroid hormone receptors are members of the nuclear receptor superfamily and are found in vertebrate and invertebrate ceUs. Unfortunately, use of steroidal compounds that activate the receptors for the regulation of gene expression, particularly in plants and mammals, is limited due to their involvement in many other natural biological pathways in such organisms. In order to overcome such difficulties, an alternative system has been developed using insect ecdysone receptors (EcR). Growth, molting, and development in insects are regulated by the ecdysone steroid hormone (molting hormone) and the juvenile hormones (Dhadialla, et al., 1998. Annu. Rev. Entomol. 43: 545-569). The molecular target for ecdysone in insects consists of at least ecdysone receptor (EcR) and ultraspiracle protein (USP). EcR is a member of the nuclear steroid receptor super family that is characterized by signature DNA and ligand binding domains, and an activation domain (KoeUe et al. 1991, Cell, 67:59-77). EcR receptors are responsive to a number of steroidal compounds such as ponasterone A and muristerone A. Recently, non-steroidal compounds with ecdysteroid agonist activity have been described, including the commerciaUy available insecticides tebufenozide and methoxyfenozide that are marketed world wide by Rohm and Haas Company (see International Patent Application No. PCT/EP96/00686 and US Patent 5,530,028). Both analogs have exceptional safety profiles to other organisms. International Patent Application No. PCT/US97/05330 (WO 97/38117) discloses methods for modulating the expression of an exogenous gene in which a DNA construct comprising the exogenous gene and an ecdysone response element is activated by a second DNA construct comprising an ecdysone receptor that, in the presence of a ligand therefor, and optionaUy in the presence of a receptor capable of acting as a silent partner, binds to the ecdysone response element to induce gene expression. The ecdysone receptor of choice was isolated from Drosophila melanogaster. TypicaUy, such systems require the presence of the silent partner, preferably retinoid X receptor (RXR), in order to provide optimum activation. In mammalian cells, insect ecdysone receptor (EcR) heterodimerizes with retinoid X receptor (RXR) and regulates expression of target genes in a ligand dependent manner. International Patent AppUcation No. PCT/US98/14215 (WO 99/02683) discloses that the ecdysone receptor isolated from the silk moth Bombyx mori is functional in mammaUan systems without the need for an exogenous dimer partner. U.S. Patent No. 5,880,333 discloses a Drosophila melanogaster EcR and ultraspiracle (USP) heterodimer system used in plants in which the transactivation domain and the DNA binding domain are positioned on two different hybrid proteins. Unfortunately, this system is not effective for inducing reporter gene expression in animal cells (for comparison, see Example 1.2, below). In each of these cases, the transactivation domain and the DNA binding domain (either as native EcR as in International Patent Application No. PCT/US98/14215 or as modified EcR as in International Patent Application No. PCT/US97/05330) were incorporated into a single molecule and the other heterodimeric partners, either USP or RXR, were used in their native state. Drawbacks of the above described EcR-based gene regulation systems include a considerable background activity in the absence of Ugands and that these systems are not applicable for use in both plants and animals (see U.S. Patent No. 5,880,333). For most appUcations that rely on modulating gene expression, these EcR-based systems are undesirable. Therefore, a need exists in the art for improved systems to precisely modulate the expression of exogenous genes in both plants and animals. Such improved systems would be useful for appUcations such as gene therapy, large scale production of proteins and antibodies, ceU-based high throughput screening assays, functional genomics and regulation of traits in transgenic animals. Improved systems that are simple, compact, and dependent on Ugands that are relatively inexpensive, readily available, and of low toxicity to the host would prove useful for regulating biological systems. Various pubUcations are cited herein, the disclosures of which are incorporated by reference in their entireties. However, the citation of any reference herein should not be construed as an admission that such reference is available as "Prior Art" to the instant application. SUMMARY OF THE INVENTION The present invention relates to a novel ecdysone receptor-based inducible gene expression system, novel receptor polynucleotides and polypeptides for use in the novel inducible gene expression system, and methods of modulating the expression q£ a gene within a host ceU using this inducible gene expression system. In particular, Applicants' invention relates to an improved gene expression modulation system comprising a polynucleotide encoding a receptor polypeptide comprising a truncation mutation. SpecificaUy, the present invention relates to a gene expression modulation system comprising: a) a first gene expression cassette that is capable of being expressed in a host cell comprising a polynucleotide that encodes a first polypeptide comprising: i) a DNA-binding domain that recognizes a response element associated with a gene whose expression is to be modulated; and u) a Ugand binding domain comprising a Ugand binding domain from a nuclear receptor; and b) a second gene expression cassette that is capable of being expressed in the host ceU comprising a polynucleotide sequence that encodes a second polypeptide comprising: i) a transactivation domain; and U) a ligand bmding domain comprising a Ugand binding domain from a nuclear receptor other than an ultraspiracle receptor; wherein the DNA binding domain and the transactivation domain are from a polypeptide other than an ecdysone receptor, a retinoid X receptor, or an ultraspiracle receptor; wherein the Ugand binding domains from the first polypeptide and the second polypeptide are different and dimerize. In a specific embodiment, the Ugand binding domain of the first polypeptide comprises an ecdysone receptor (EcR) Ugand binding domain In another specific embodiment, the ligand binding domain of the second polypeptide comprises a retinoid X receptor (RXR) ligand binding domain. In a preferred embodiment, the ligand binding domain of the first polypeptide comprises an ecdysone receptor Ugand binding domain and the Ugand binding domain of the second polypeptide comprises a retinoid X receptor Ugand binding domain The present invention also relates to a gene expression modulation system according to the invention further comprising c) a third gene expression cassette comprising: i) a response element to which the DNA-binding domain of the first polypeptide binds ; ii) a promoter that is activated by the transactivation domain of the second polypeptide; and in) the gene whose expression is to be modulated. The present invention also relates to an isolated polynucleotide encoding a truncated EcR or a truncated RXR polypeptide, wherein the truncation mutation affects ligand binding activity or ligand sensitivity. In particular, the present invention relates to an isolated polynucleotide encoding a truncated EcR or a truncated RXR polypeptide comprising a truncation mutation that reduces ligand binding activity or Ugand sensitivity of said EcR or RXR polypeptide. In a specific embodiment, the present invention relates to an isolated polynucleotide encoding a truncated EcR or a truncated RXR polypeptide comprising a truncation mutation that reduces steroid binding activity or steroid sensitivity of said EcR or RXR polypeptide. In another specific embodiment, the present invention relates to an isolated polynucleotide encoding a truncated EcR or a truncated RXR polypeptide comprising a truncation mutation that reduces non- steroid binding activity or non-steroid sensitivity of said EcR or RXR polypeptide. The present invention also relates to an isolated polynucleotide encoding a truncated EcR or a truncated RXR polypeptide comprising a truncation mutation that enhances Ugand binding activity or Ugand sensitivity of said EcR or RXR polypeptide. In a specific embodiment, the present invention relates to an isolated polynucleotide encoding a trancated EcR or a truncated RXR polypeptide comprising a truncation mutation that enhances steroid binding activity or steroid sensitivity of said EcR or RXR polypeptide. In another specific embodiment, the present invention relates to an isolated polynucleotide encoding a truncated EcR or a truncated RXR polypeptide comprising a truncation mutation that enhances non- steroid binding activity or non-steroid sensitivity of said EcR or RXR polypeptide. The present invention also relates to an isolated polynucleotide encoding a trancated RXR polypeptide comprising a truncation mutation that increases Ugand sensitivity of a heterodimer comprising the trancated retinoid X receptor polypeptide and a dimerization partner. In a specific embodiment, the dimerization partner is an ecdysone receptor polypeptide. The present invention also relates to an isolated polypeptide encoded by a polynucleotide according to AppUcants' invention. In particular, the present invention relates to an isolated truncated EcR or truncated RXR polypeptide comprising a truncation mutation, wherein the EcR or RXR polypeptide is encoded by a polynucleotide according to the invention. Thus, the present invention also relates to an isolated truncated EcR or trancated RXR polypeptide comprising a truncation mutation that affects Ugand binding activity or Ugand sensitivity of said EcR or RXR polypeptide. Applicants' invention also relates to methods of modulating gene expression in a host cell using a gene expression modulation system according to the invention. SpecificaUy, Applicants' invention provides a method of modulating the expression of a gene in a host cell comprising the gene to be modulated comprising the steps of: a) introducing into the host cell a gene expression modulation system according to the invention; and b) introducing into the host cell a Ugand that independently combines with the ligand binding domains of the first polypeptide and the second polypeptide of the gene expression modulation system; wherein the gene to be expressed is a component of a chimeric gene comprising: i) a response element comprising a domain to which the DNA binding domain from the first polypeptide binds; ii) a promoter that is activated by the transactivation domain of the second polypeptide; and in) the gene whose expression is to be modulated, whereby a complex is formed comprising the Ugand, the first polypeptide, and the second polypeptide, and whereby the complex modulates expression of the gene in the host ceU. Applicants' invention also provides an isolated host ceU comprising an inducible gene expression system according to the invention. The present invention also relates to an isolated host ceU comprising a polynucleotide or polypeptide according to the invention. Accordingly, Applicants' invention also relates to a non-human organism comprising a host ceU according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 : An ecdysone receptor-based gene expression system comprising a first gene expression cassette encoding a Gal4DBD-CfEcRDEF chimeric polypeptide and a second gene expression cassette encoding a VP16AD-MmRXRDEF chimeric polypeptide; prepared as described in Example 1 (switch 1.1). Figure 2: An ecdysone receptor-based gene expression system comprising a first gene expression cassette encoding a Gal4DBD-CfEcRDEF chimeric polypeptide and a second gene expression cassette encoding a VPlόAD-CfUSPDEF chimeric polypeptide; prepared as described in Example 1 (switch 1.2). Figure 3: An ecdysone receptor-based gene expression system comprising a first gene expression cassette encoding a Gal4DBD-MmRXRDEF chimeric polypeptide and a second gene expression cassette encoding a VP16AD-CfEcRCDEF chimeric polypeptide; prepared as described in Example 1 (switch 1.3). Figure 4: An ecdysone receptor-based gene expression system comprising a first gene expression cassette encoding a Gal4DBD-MmRXRDEF chimeric polypeptide and a second gene expression cassette encoding a VP16AD-DmEcRCDEF chimeric polypeptide; prepared as described in Example 1 (switch 1.4). Figure 5: An ecdysone receptor-based gene expression system comprising a first gene expression cassette encoding a Gal4DBD-CfUSPDEF chimeric polypeptide and a second gene expression cassette encoding a VP16AD-CfEcRCDEF chimeric polypeptide; prepared as described in Example 1 (switch 1.5). Figure 6: An ecdysone receptor-based gene expression system comprising a first gene expression cassette encoding a Gal4DBD-Cf__cRDEF-VP16AD chimeric polypeptide; prepared as described in Example 1 (switch 1.6). Figure 7: An ecdysone receptor-based gene expression system comprising a first gene expression cassette encoding a VP16AD-CfEcRCDEF chimeric polypeptide; prepared as described in Example 1 (switch 1.7). Figure 8: An ecdysone receptor-based gene expression system comprising a first gene expression cassette encoding a VP16AD-DmEcRCDEF chimeric polypeptide and a second gene expression cassette encoding a MmRXR polypeptide; prepared as described in Example 1 (switch 1.8). Figure 9: An ecdysone receptor-based gene expression system comprising a first gene expression cassette encoding a VP16AD-CfEcRCDEF chimeric polypeptide and a second gene expression cassette encoding a MmRXR polypeptide; prepared as described in Example 1 (switch 1.9). Figure 10: An ecdysone receptor-based gene expression system comprising a gene expression cassette encoding a Gal4DBD-CfEcRCDEF chimeric polypeptide; prepared as described in Example 1 (switch 1.10). Figure 11: Expression data of GAL4CfEcRA BCDEF, GA CfEcRCDEF, GAL4CfEcRl/2CDEF, GAL4CfEcRDEF, GAL4CfficREF, GAL4CfEcRDE truncation mutants tiransfected into NIH3T3 ceUs along with VP16MmRXRDE, pFRLUc and pTKRL plasmid DNAs. Figure 12: Expression data of GAL4CfEcRA BCDEF, GA CfEcRCDEF, GAL4CfEcRl/2CDEF, GAL4CfEcRDEF, GAL4CfEcREF, GAL4CfEcRDE truncation mutants tiransfected into 3T3 ceUs along with VPlόMmRXRE, pFRLUc and pTKRL plasmid DNAs. Figure 13: Expression data of VP16MmRXRA/BCDEF, VPlόMmRXRCDEF, VP16MmRXRDEF, VP16MmRXREF, VP16MmRXRBam-EF, VP16MmRXRAF2del constructs tiransfected into NIH3T3 cells along with GAL4CfEcRCDEF, pFRLUc and pTKRL plasmid DNAs. Figure 14: Expression data of VP16MmRXRA/BCDEF, VPlόMmRXRCDEF, VP16MmRXRDEF, VP16MmRXREF, VP16MmRXRBam-EF, VP16MmRXRAF2del constructs tiransfected into NIH3T3 ceUs along with GAL4CfEcRDEF, pFRLUc and pTKRL plasmid DNAs. Figure 15: Expression data of various trancated CfEcR and MmRXR receptor pairs fransfected into NIH3T3 cells along with GAL4CfEcRDEF, pFRLUc and pTKRL plasmid DNAs. DETAILED DESCRIPTION OF THE INVENTION AppUcants have now developed an improved ecdysone receptor-basedinducible gene expression system comprising a truncation mutant of an ecdysone receptor or a retinoid X receptor (RXR) polypeptide that affects Ugand binding activity or ligand sensitivity. This mutational effect may increase or reduce ligand binding activity or ligand sensitivity and may be steroid or non-steroid specific. Thus, AppUcants' invention provides an improved ecdysone receptor-based inducible gene expression system useful for modulating expression of a gene of interest in a host ceU. In a particularly desirable embodiment, AppUcants' invention provides an inducible gene expression system that has a reduced level of background gene expression and responds to submicromolar concentrations of non-steroidal Ugand. Thus, AppUcants' novel inducible gene expression system and its use in methods of modulating gene expression in a host cell overcome the Umitations of currently available inducible expression systems and provide the skilled artisan with an effective means to control gene expression. The present invention provides a novel inducible gene expression system that can be used to modulate gene expression in both prokaryotic and eukaryotic host cells. AppUcants' invention is useful for applications such as gene therapy, large scale production of proteins and antibodies, ceU-based high throughput screening assays, functional genomics and regulation of traits in transgenic organisms. DEFINITIONS In this disclosure, a number of terms and abbreviations are used. The foUowing definitions are provided and should be helpful in understanding the scope and practice of the present invention. In a specific embodiment, the term "about" or "approximately" means within 20%, preferably within 10%, more preferably within 5%, and even more preferably within 1 % of a given value or range. The term "substantiaUy free" means that a composition comprising "A" (where "A" is a single protein, DNA molecule, vector, recombinanthost cell, etc.) is substantiaUy free of "B" (where "B" comprises one or more contaminating proteins, DNA molecules, vectors, etc.) when at least about 75% by weight of the proteins, DNA, vectors (depending on the category of species to which A and B belong) in the composition is "A". Preferably, "A" comprises at least about 90% by weight of the A+B species in the composition, most preferably at least about 99% by weight. It is also preferred that a composition, which is substantiaUy free of contamination, contain only a single molecular weight species having the activity or characteristic of the species of interest. ^ The term "isolated" for the purposes of the present invention designates a biological material (nucleic acid or protein) that has been removed from its original environment (the environment in which it is naturally present). For example, a polynucleotide present in the natural state in a plant or an animal is not isolated. The same polynucleotide separated from the adjacent nucleic acids in which it is naturaUy present. The term "purified" does not require the material to be present in a form exhibiting absolute purity, exclusive of the presence of other compounds. It is rather a relative definition. A polynucleotide is in the "purified" state after purification of the starting material or of the natural material by at least one order of magnitude, preferably 2 or 3 and preferably 4 or 5 orders of magnitude. A "nucleic acid" is a polymeric compound comprised of covalently Uhked subunits called nucleotides. Nucleic acid includes polyribonucleic acid (RNA) and polydeoxyribonucleic acid (DNA), both of which may be single-stranded or double-stranded. DNA includes but is not limited to cDNA, genomic DNA, plasmids DNA, synthetic DNA, and semi-synthetic DNA. DNA may be Unear, circular, or supercoiled. A "nucleic acid molecule" refers to the phosphate ester polymeric form of ribonucleosides (adenosine, guanosine, uridine or cytidine; "RNA molecules") or deoxyribonucleosides (deoxyadenosine, deoxyguanosine, deoxythymidine, or deoxycytidine; "DNA molecules"), or any phosphoester anologs thereof, such as phosphorothioates and thioesters, in either single stranded form, or a double-stranded helix. Double stranded DNA- DNA, DNA-RNA and RNA-RNA helices are possible. The term nucleic acid molecule, and in particular DNA or RNA molecule, refers only to the primary and secondary structure of the molecule, and does not limit it to any particular tertiary forms. Thus, this term includes double-stranded DNA found, inter alia, in linear or circular DNA molecules (e.g., restriction fragments), plasmids, and chromosomes. In discussing the structure of particular double- stranded DNA molecules, sequences may be described herein according to the normal convention of giving only the sequence in the 5' to 3' direction along the non-transcribed strand of DNA (i. e. , the strand having a sequence homologous to the mRNA). A "recombinant DNA molecule" is a DNA molecule that has undergone a molecular biological manipulation. The term "fragment" will be understood to mean a nucleotide sequence of reduced length relative to the reference nucleic acid and comprising, over the common portion, a nucleotide sequence identical to the reference nucleic acid. Such a nucleic aci l. fragment according to the invention may be, where appropriate, included in a larger polynucleotide of which it is a constituent. Such fragments comprise, or alternatively consist of, oligonucleotides ranging in length from at least 8, 10, 12, 15, 18, 20 to 25, 30, 40, 50, 70, 80, 100, 200, 500, 1000 or 1500 consecutive nucleotides of a nucleic acid according to the invention. As used herein, an "isolated nucleic acid fragment" is a polymer of RNA or DNA that is single- or double-stranded, optionally containing synthetic, non-natural or altered nucleotide bases. An isolated nucleic acid fragment in the form of a polymer of DNA may be comprised of one or more segments of cDNA, genomic DNA or synthetic DNA. A "gene" refers to an assembly of nucleotides that encode a polypeptide, and includes cDNA and genomic DNA nucleic acids. "Gene" also refers to a nucleic acid fragment that expresses a specific protein or polypeptide, including regulatory sequences preceding (5' non- coding sequences) and foUowing (3 * non-coding sequences) the coding sequence. "Native gene" refers to a gene as found in nature with its own regulatory sequences. "Chimeric gene" refers to any gene that is not a native gene, comprising regulatory and/or coding sequences that are not found together in nature. Accordingly, a chimeric gene may comprise regulatory sequences and coding sequences that are derived from different sources, or regulatory sequences and coding sequences derived from the same source, but arranged in a manner different than that found in nature. A chimeric gene may comprise coding sequences derived from different sources and/or regulatory sequences derived from different sources. "Endogenous gene" refers to a native gene in its natural location in the genome of an organism. A "foreign" gene or "heterologous" gene refers to a gene not normaUy found in the host organism, but that is introduced into the host organism by gene transfer. Foreign genes can comprise native genes inserted into a non-native organism, or chimeric genes. A "transgene" is a gene that has been introduced into the genome by a transformation procedure. "Heterologous" DNA refers to DNA not naturaUy located in the ceU, or in a chromosomal site of the ceU. Preferably, the heterologous DNA includes a gene foreign to the ceU. The term "genome" includes chromosomal as well as mitochondrial, chloroplast and viral DNA or RNA. A nucleic acid molecule is "hybridizable" to another nucleic acid molecule, such as a cDNA, genomic DNA, or RNA, when a single stranded form of the nucleic acid molecule can anneal to the other nucleic acid molecule under the appropriate conditions of temperature and solution ionic strength (see Sambrook et al, 1989 infra). Hybridization and washing conditions are weU known and exempUfied in Sambrook, J., Fritsch, E. F. and Maniatis, T. Molecular Cloning: A Laboratory Manual, Second Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor (1989), particularly Chapter 11 and Table 11.1 therein (entirely incorporated herein by reference). The conditions of temperature and ionic strength determine the "stringency" of the hybridization. Stringency conditions can be adjusted to screen for moderately similar fragments, such as homologous sequences from distantly related organisms, to highly similar fragments, such as genes that dupUcate functional enzymes from closely related organisms. For preliminary screening for homologous nucleic acids, low stringency hybridization conditions, corresponding to a T m of 55°, can be used, e.g., 5x SSC, 0.1% SDS, 0.25% milk, and no formamide; or 30% formamide, 5x SSC, 0.5% SDS). Moderate stringency hybridization conditions correspond to a higher T m , e.g., 40% formamide, with 5x or 6x SCC. High stringency hybridization conditions correspond to the highest T m , e. g. , 50% formamide, 5x or 6x SCC. Hybridization requires that the two nucleic acids contain complementary sequences, although depending on the stringency of the hybridization, mismatches between bases are possible. The term "complementary" is used to describe the relationship between nucleotide bases that are capable of hybridizing to one another. For example, with respect to DNA, adenosine is complementary to thymine and cytosine is complementary to guanine. Accordingly, the instant invention also includes isolated nucleic acid fragments that are complementary to the complete sequences as disclosed or used herein as well as those substantiaUy similar nucleic acid sequences. In a specific embodiment, the term "standard hybridization conditions" refers to a T m of 55°C, and utilizes conditions as set forth above. In a preferred embodiment, the T m is 60°C; in a more preferred embodiment, the T m is 65°C. Post-hybridization washes also determine stringency conditions. One set of preferred conditions uses a series of washes starting with 6X SSC, 0.5% SDS at room temperature for 15 minutes (min), then repeated with 2X SSC, 0.5% SDS at 45°C for 30 minutes, and then repeated twice with 0.2X SSC, 0.5% SDS at 50°C for 30 minutes. A more preferred set of stringent conditions uses higher temperatures in which the washes are identical to those above except for the temperature of the final two 30 min washes in 0.2X SSC, 0.5% SDS was increased to 60°C. Another preferred set of highly stringent conditions uses two final washes in 0. IX SSC, 0.1 % SDS at 65°C. Hybridization requires that the two nucleic acids comprise complementary sequences, although depending on the stringency of the hybridation, mismatches between bases are possible. The appropriate stringency for hybridizing nucleic acids depends on the length of the nucleic acids and the degree of complementation, variables weU known in the art. The greater the degree of similarity or homology between two nucleotide sequences, the greater the value of T m for hybrids of nucleic acids having those sequences. The relative stability (corresponding to higher T m ) of nucleic acid hybridizations decreases in the foUowing order: RNA:RNA, DNA:RNA, DNA:DNA. For hybrids of greater than 100 nucleotides in length, equations for calculating T m have been derived (see Sambrook et al., supra, 9.50-0.51). For hybridization with shorter nucleic acids, i.e., oUgonucleotides, the position of mismatches becomes more important, and the length of the oUgonucleotide determines its specificity (see Sambrook et al., supra, 11.7-11.8). In one embodiment the length for a hybridizable nucleic acid is at least about 10 nucleotides. Preferable a minimum length for a hybridizable nucleic acid is at least about 15 nucleotides; more preferably at least about 20 nucleotides; and most preferably the length is at least 30 nucleotides. Furthermore, the skilled artisan wiU recognize that the temperature and wash solution salt concentration may be adjusted as necessary according to factors such as length of the probe. The term "probe" refers to a single-stranded nucleic acid molecule that can base pair with a complementary single stranded target nucleic acid to form a double-stranded molecule. As used herein, the term "oUgonucleotide" refers to a nucleic acid, generaUy of at least 18 nucleotides, that is hybridizable to a genomic DNA molecule, a cDNA molecule, a plasmid DNA or an mRNA molecule. OUgonucleotides can be labeled, e. g. , with 32 P-nucleotides or nucleotides to which a label, such as biotin, has been covalently conjugated. A labeled oUgonucleotide can be used as a probe to detect the presence of a nucleic acid. OUgonucleotides (one or both of which may be labeled) can be used as PCR primers, either for cloning fuU length or a fragment of a nucleic acid, or to detect the presence of a nucleic acid. An oUgonucleotide can also be used to form a triple helix with a DNA molecule. GeneraUy, oUgonucleotides are prepared syntheticaUy, preferably on a nucleic acid synthesizer. Accordingly, oUgonucleotides can be prepared with non-naturaUy occurring phosphoester analog bonds, such as thioester bonds, etc. A "primer" is an oUgonucleotide that hybridizes to a target nucleic acid sequence to create a double stranded nucleic acid region that can serve as an initiation point for DNA synthesis under suitable conditions. Such primers may be used in a polymera§_? chain reaction. "Polymerase chain reaction" is abbreviated PCR and means an in vitro method for enzymatically ampUfying specific nucleic acid sequences. PCR involves a repetitive series of temperature cycles with each cycle comprising three stages: denaturation of the template nucleic acid to separate the strands of the target molecule, annealing a single stranded PCR oUgonucleotide primer to the template nucleic acid, and extension of the annealed primer(s) by DNA polymerase. PCR provides a means to detect the presence of the target molecule and, under quantitative or semi-quantitative conditions, to determine the relative amount of that target molecule within the starting pool of nucleic acids. "Reverse transcription-polymerase chain reaction" is abbreviated RT-PCR and means an in vitro method for enzymatically producing a target cDNA molecule or molecules from an RNA molecule or molecules, foUowed by enzymatic ampUfication of a specific nucleic acid sequence or sequences within the target cDNA molecule or molecules as described above. RT- PCR also provides a means to detect the presence of the target molecule and, under quantitative or semi-quantitative conditions, to determine the relative amount of that target molecule within the starting pool of nucleic acids. A DNA "coding sequence" is a double-stranded DNA sequence that is transcribed and translated into a polypeptide in a cell in vitro or in vivo when placed under the control of appropriate regulatory sequences. "Suitable regulatory sequences" refer to nucleotide sequences located upstream (5' non-coding sequences), within, or downstream (3' non-coding sequences) of a coding sequence, and which influence the transcription, RNA processing or stability, or translation of the associated coding sequence. Regulatory sequences may include promoters, translation leader sequences, introns, polyadenylation recognition sequences, RNA processing site, effector binding site and stem-loop structure. The boundaries of the coding sequence are determined by a start codon at the 5' (amino) terminus and a translation stop codon at the 3' (carboxyl) teirminus. A coding sequence can include, but is not Umited to, prokaryotic sequences, cDNA frommRNA, genomic DNA sequences, and even synthetic DNA sequences. If the coding sequence is intended for expression in a eukaryotic cell, a polyadenylation signal and transcription termination sequence will usuaUy be located 3 ' to the coding sequence. "Open reading frame" is abbreviated ORF and means a length of nucleic acid sequence, either DNA, cDNA or RNA, that comprises a translation start signal or initiation codon, such as an ATG or AUG, and a termination codon and can be potentially translated into a polypeptide sequence. ^ The term "head-to-head" is used herein to describe the orientation of two polynucleotide sequences in relation to each other. Two polynucleotides are positioned in a head-to-head orientation when the 5' end of the coding strand of one polynucleotide is adjacent to the 5' end of the coding strand of the other polynucleotide, whereby the direction of transcription of each polynucleotide proceeds away from the 5' end of the other polynucleotide. The term "head-to-head" may be abbreviated (5')-to-(5') and may also be indicated by the symbols (<- →) or (3'<-5'5'→3'). The term "tail-to-tail" is used herein to describe the orientation of two polynucleotide sequences in relation to each other. Two polynucleotides are positioned in a tail-to-tail orientation when the 3' end of the coding strand of one polynucleotide is adjacent to the 3' end of the coding strand of the other polynucleotide, whereby the direction of transcription of each polynucleotide proceeds toward the other polynucleotide. The term "tail-to-tail" may be abbreviated (3')-to-(3') and may also be indicated by the symbols (→ <— ) or (5'— >3'3'<— 5'). The term "head-to-tail" is used herein to describe the orientation of two polynucleotide sequences in relation to each other. Two polynucleotides are positioned in a head-to-tail orientation when the 5' end of the coding strand of one polynucleotide is adjacent to the 3' end of the coding strand of the other polynucleotide, whereby the direction of transcription of each polynucleotide proceeds in the same direction as that of the other polynucleotide. The term "head-to-tail" may be abbreviated (5')-to-(3') and may also be indicated by the symbols (— > →) or (5'→3'5'→3'). The term "downstream" refers to a nucleotide sequence that is located 3' to reference nucleotide sequence. In particular, downstream nucleotide sequences generally relate to sequences that follow the starting point of transcription. For example, the translation initiation codon of a gene is located downstream of the start site of transcription. The term "upstream" refers to a nucleotide sequence that is located 5' to reference nucleotide sequence. In particular, upstream nucleotide sequences generally relate to sequences that are located on the 5' side of a coding sequence or starting point of transcription. For example, most promoters are located upstream of the start site of transcription. The terms "restriction endonuclease" and "restriction enzyme" refer to an enzyme that binds and cuts within a specific nucleotide sequence within double stranded DNA. "Homologous recombination" refers to the insertion of a foreign DNA sequence into another DNA molecule, e.g., insertion of a vector in a chromosome. Preferably, the vector targets a specific chromosomal site for homologous recombination. For specific homologous recombination, the vector wiU contain sufficiently long regions of homology to sequences of the chromosome to aUow complementary binding and incorporation of the vector into the chromosome. Longer regions of homology, and greater degrees of sequence similarity, may increase the efficiency of homologous recombination. Several methods known in the art may be used to propagate a polynucleotide according to the invention. Once a suitable host system and growth conditions are established, recombinant expression vectors can be propagated and prepared in quantity. As described herein, the expression vectors which can be used include, but are not limited to, the following vectors or their derivatives: human or animal viruses such as vaccinia virus or adeno virus; insect viruses such as baculovirus; yeast vectors; bacteriophage vectors (e.g., lambda), and plasmid and cosmid DNA vectors, to name but a few. A "vector" is any means for the cloning of and/or transfer of a nucleic acid into a host ceU. A vector may be a repUcon to which another DNA segment may be attached so as to bring about the replication of the attached segment. A "repUcon" is any genetic element (e.g., plasmid, phage, cosmid, chromosome, virus) that functions as an autonomous unit of DNA repUcation in vivo, i.e., capable of replication under its own control. The term "vector" includes both viral and nonviral means for introducing the nucleic acid into a cell in vitro, ex vivo or in vivo. A large number of vectors known in the art may be used to manipulate nucleic acids, incorporate response elements and promoters into genes, etc. Possible vectors include, for example, plasmids or modified viruses including, for example bacteriophages such as lambda derivatives, or plasmids such as PBR322 or pUC plasmid derivatives, or the Bluescript vector. For example, the insertion of the DNA fragments corresponding to response elements and promoters into a suitable vector can be accomplished by ligating the appropriate DNA fragments into a chosen vector that has complementary cohesive termini. Alternatively, the ends of the DNA molecules may be enzymatically modified or any site may be produced by ligating nucleotide sequences (linkers) into the DNA termini. Such vectors may be engineered to contain selectable marker genes that provide for the selection of cells that have incorporated the marker into the ceUular genome. Such markers aUo w identification and/or selection of host ceUs that incorporate and express the proteins encoded by the marker. Viral vectors, and particularly retroviral vectors, have been used in a wide variety of gene deUvery applications in cells, as well as Uving animal subjects. Viral vectors that can be used include but are not United to retrovirus, adeno-associated virus, pox, baculovirus, vaccinia, herpes simplex, Epstein-Barr, adeno virus, geminiviras, and caulimoyjrus vectors. Non-viral vectors include plasmids, liposomes, electricaUy charged Upids (cytofectins), DNA- protein complexes, and biopolymers. In addition to a nucleic acid, a vector may also comprise one or more regulatory regions, and/or selectable markers useful in selecting, measuring, and monitoring nucleic acid transfer results (transfer to which tissues, duration of expression, etc.). The term "plasmid" refers to an extra chromosomal element often carrying a gene that is not part of the central metaboUsm of the ceU, and usuaUy in the form of circular double- stranded DNA molecules. Such elements may be autonomously replicating sequences, genome integrating sequences, phage or nucleotide sequences, linear, circular, or supercoiled, of a single- or double-stranded DNA or RNA, derived from any source, in which a number of nucleotide sequences have been joined or recombined into a unique construction which is capable of introducing a promoter fragment and DNA sequence for a selected gene product along with appropriate 3' untranslated sequence into a ceU. A "cloning vector" is a "repUcon", which is a unit length of a nucleic acid, preferably DNA, that replicates sequentially and which comprises an origin of repUcation, such as a plasmid, phage or cosmid, to which another nucleic acid segment may be attached so as to bring about the repUcation of the attached segment. Cloning vectors may be capable of repUcation in one cell type and expression in another ("shuttle vector"). Vectors may be introduced into the desired host ceUs by methods known in the art, e.g., transfection, electroporation, microinjection, transduction, cell fusion, DEAE dextran, calcium phosphate precipitation, lipofection (lysosome fusion), use of a gene gun, or a DNA vector transporter (see, e.g., Wu et al., 1992, J. Biol. Chem. 267:963-967; Wu and Wu, 1988, J. Biol. Chem 263:14621-14624; and Hartmut et al., Canadian Patent AppUcation No. 2,012,311, filed March 15, 1990). A polynucleotide according to the invention can also be introduced in vivo by lipofection. For the past decade, there has been increasing use of liposomes for encapsulation and transfection of nucleic acids in vitro. Synthetic cationic Upids designed to limit the difficulties and dangers encountered with Uposome mediated transfection can be used to prepare liposomes for in vivo transfection of a gene encoding a marker (Feigner et al., 1987. PNAS 84:7413; Mackey, et al., 1988. Proc. Natl. Acad. Sci. U.S.A 85:8027-8031; and Ulmer et al., 1993. Science 259:1745- 1748). The use of cationic lipids may promote encapsulation of negatively charged nucleic acids, and also promote fusion with negatively charged ceU membranes (Feigner and Ringold, 1989. Science 337:387-388). Particularly useful Upid compounds and compositions for transfer of nucleic acids are described in International Patent PubUcations W095/18863 and WO96/17823, and in U.S. Patent No. 5,459,127. The use of lipofection to introduce exogenous genes into the specific organs in vivo has certain practical advantages. Molecular targeting of liposomes to specific ceUs represents one area of benefit. It is clear that directing transfection to particular ceU types would be particularly preferred in a tissue with cellular heterogeneity, such as pancreas, Uver, kidney, and the brain. Lipids may be chemically coupled to other molecules for the purpose of targeting (Mackey, et al., 1988, supra). Targeted peptides, e.g., hormones or neurotransmitters, and proteins such as antibodies, or non-peptide molecules could be coupled to Uposomes chemicaUy. Other molecules are also useful for facilitating transfection of a nucleic acid in vivo, such as a cationic oUgopeptide (e.g., WO95/21931), peptides derived from DNA binding proteins (e.g., WO96/25508), or a cationic polymer (e.g., WO95/21931). It is also possible to introduce a vector in vivo as a naked DNA plasmid (see U.S. Patents 5,693,622, 5,589,466 and 5,580,859). Receptor-mediated DNA delivery approaches can also be used (Curiel et al., 1992. Hum Gene Ther. 3:147-154; and Wu and Wu, 1987. J. Biol. Chem 262:4429-4432). The term "transfection" means the uptake of exogenous or heterologous RNA or DNA by a ceU. A ceU has been "transfected" by exogenous or heterologous RNA or DNA when such RNA or DNA has been introduced inside the ceU. A cell has been "transformed" by exogenous or heterologous RNA or DNA when the transfected RNA or DNA effects a phenotypic change. The transforming RNA or DNA can be integrated (covalentiy linked) into chromosomal DNA making up the genome of the ceU. "Transformation" refers to the transfer of a nucleic acid fragment into the genome of a host organism, resulting in geneticaUy stable inheritance. Host organisms containing the transformed nucleic acid fragments are referred to as "transgenic" or "recombinant" or "transformed" organisms. The term "genetic region" will refer to a region of a nucleic acid molecule or a nucleotide sequence that comprises a gene encoding a polypeptide. In addition, the recombinant vector comprising a polynucleotide according to the invention may include one or more origins for replication in the ceUular hosts in which their ampUfication or their expression is sought, markers or selectable markers. The term "selectable marker" means an identifying factor, usuaUy an antibiotic or chemical resistance gene, that is able to be selected for based upon the marker gene's effect, i.e., resistance to an antibiotic, resistance to a herbicide, colorimetric markers, enzymes, fluorescent markers, and the like, wherein the effect is used to track the inheritance of a nucleic acid of interest and/or to identify a cell or organism that has inherited the nucleic acid of interest. Examples of selectable marker genes known and used in the art include: genes providing resistance to ampicUUn, streptomycin, gentamycin, kanamycin, hygromycin, bialaphos herbicide, sulfonamide, and the Uke; and genes that are used as phenotypic markers, i.e., anthocyanin regulatory genes, isopentanyl transferase gene, and the like. The term "reporter gene" means a nucleic acid encoding an identifying factor that is able to be identified based upon the reporter gene's effect, wherein the effect is used to track the inheritance of a nucleic acid of interest, to identify a ceU or organism that has inherited the nucleic acid of interest, and/or to measure gene expression induction or transcription. Examples of reporter genes known and used in the art include: luciferase (Luc), green fluorescent protein (GFP), chloramphenicol acetyltransferase (CAT), β-galactosidase (LacZ), β-glucuronidase (Gus), and the like. Selectable marker genes may also be considered reporter genes. "Promoter" refers to a DNA sequence capable of controlling the expression of a coding sequence or functional RNA. In general, a coding sequence is located 3' to a promoter sequence. Promoters may be derived in their entirety from a native gene, or be composed of different elements derived from different promoters found in nature, or even comprise synthetic DNA segments. It is understood by those skilled in the art that different promoters may direct the expression of a gene in different tissues or cell types, or at different stages of development, or in response to different environmental or physiological conditions. Promoters that cause a gene to be expressed in most ceU types at most times are commonly referred to as "constitutive promoters". Promoters that cause a gene to be expressed in a specific ceU type are commonly referred to as "ceU-specific promoters" or "tissue-specific promoters". Promoters that cause a gene to be expressed at a specific stage of development or ceU differentiation are commonly referred to as "developmentaUy-specific promoters" or "cell differentiation-specific promoters". Promoters that are induced and cause a gene to be expressed foUowing exposure or treatment of the ceU with an agent, biological molecule, chemical, Ugand, Ught, or the like that induces the promoter are commonly referred to as "inducible promoters" or "regulatable promoters". It is further recognized that since in most cases the exact boundaries of regulatory sequences have not been completely defined, DNA fragments of different lengths may have identical promoter activity. A "promoter sequence" is a DNA regulatory region capable of binding- RNA polymerase in a cell and initiating transcription of a downstream (3' direction) coding sequence. For purposes of defining the present invention, the promoter sequence is bounded at its 3' teπninus by the transcription initiation site and extends upstream (5' direction) to include the minimum number of bases or elements necessary to initiate transcription at levels detectable above background. Within the promoter sequence will be found a transcription initiation site (conveniently defined for example, by mapping with nuclease S 1), as weU as protein binding domains (consensus sequences) responsible for the binding of RNA polymerase. A coding sequence is "under the control" of transcriptional and translational control sequences in a cell when RNA polymerase transcribes the coding sequence into mRNA, which is then trans-RNA spliced (if the coding sequence contains introns) and translated into the protein encoded by the coding sequence. "Transcriptional and translational control sequences" are DNA regulatory sequences, such as promoters, enhancers, teirrninators, and the like, that provide for the expression of a coding sequence in a host cell. In eukaryotic cells, polyadenylation signals are control sequences. The term "response element" means one or more cis-acting DNA elements which confer responsiveness on a promoter mediated through interaction with the DNA-binding domains of the first chimeric gene. This DNA element may be either paUndromic (perfect or imperfect) in its sequence or composed of sequence motifs or half sites separated by a variable number of nucleotides. The half sites can be similar or identical and arranged as either direct or inverted repeats or as a single half site or multimers of adjacent half sites in tandem. The response element may comprise a minimal promoter isolated from different organisms depending upon the nature of the cell or organism into which the response element wiU be incorporated. The DNA binding domain of the first hybrid protein binds, in the presence or absence of a Ugand, to the DNA sequence of a response element to initiate or suppress transcription of downstream gene(s) under the regulation of this response element. Examples of DNA sequences for response elements of the natural ecdysone receptor include: RRGG/TTCANTGAC/ACYY (see Cherbas L., et. al., (1991), Genes Dev. 5, 120-131); AGGTCAN ( „ ) AGGTCA, where N (n) can be one or more spacer nucleotides (see DAvino PP., et. al., (1995), Mol. Cell. Endocήnol, 113, 1-9); and GGGTTGAATGAATTT (see Antoniewski C, et. al., (1994). Mol. Cell Biol. 14, 4465-4474). The term "operably linked" refers to the association of nucleic acid sequences on a single nucleic acid fragment so that the function of one is affected by the other. For example, a promoter is operably Unked with a coding sequence when it is capable of affecting the expression of that coding sequence (i.e., that the coding sequence is under the transcriptional control of the promoter). Coding sequences can be operably linked to regulatory sequences in sense or antisense orientation. The term "expression", as used herein, refers to the transcription and stable accumulation of sense (mRNA) or antisense RNA derived from a nucleic acid or polynucleotide. Expression may also refer to translation of mRNA into a protein or polypeptide. The terms "cassette", "expression cassette" and "gene expression cassette" refer to a segment of DNA that can be inserted into a nucleic acid or polynucleotide at specific restriction sites or by homologous recombination. The segment of DNA comprises a polynucleotide that encodes a polypeptide of interest, and the cassette and restriction sites are designed to ensure insertion of the cassette in the proper reading frame for transcription and translation. "Transformation cassette" refers to a specific vector comprising a polynucleotide that encodes a polypeptide of interest and having elements in addition to the polynucleotide that f aciUtate transformation of a particular host ceU. Cassettes, expression cassettes, gene expression cassettes and transformation cassettes of the invention may also comprise elements that aUow for enhanced expression of a polynucleotide encoding a polypeptide of interest in a host cell. These elements may include, but are not limited to: a promoter, a minimal promoter, an enhancer, a response element, a terminator sequence, a polyadenylation sequence, and the like. For purposes of this invention, the term "gene switch" refers to the combination of a response element associated with a promoter, and an EcR based system which, in the presence of one or more Ugands, modulates the expression of a gene into which the response element and promoter are incorporated. The terms "modulate" and "modulates" mean to induce, reduce or inhibit nucleic acid or gene expression, resulting in the respective induction, reduction or inhibition of protein or polypeptide production. The plasmids or vectors according to the invention may further comprise at least one promoter suitable for driving expression of a gene in a host ceU. The term "expression vector" means a vector, plasmid or vehicle designed to enable the expression of an inserted nucleic acid sequence foUowing transformation into the host. The cloned gene, i.e., the inserted nucleic acid sequence, is usually placed under the control of control elements such as a promoter, a minimal promoter, an enhancer, or the like. Initiation control regions or promoters, whjch are useful to drive expression of a nucleic acid in the desired host cell are numerous and famiUar to those skilled in the art. VirtuaUy any promoter capable of driving these genes is suitable for the present invention including but not limited to: viral promoters, plant promoters, bacterial promoters, animal promoters, mammalian promoters, synthetic promoters, constitutive promoters, tissue specific promoter, developmental specific promoters, inducible promoters, light regulated promoters; CYC1, HIS3, GAL1, GALA, GAL10, ADH1, PGK, PH05, GAPDH, ADC1, TRP1, URA3, LEU2, ENO, TPI, alkaline phosphatase promoters (useful for expression in Saccharomyces); AOX1 promoter (useful for expression in Pichia); b-lactamase, lac, or a, tet, trp, IPj j IPR, T7, tac, and trc promoters (useful for expression in Escherichia coli); and light regulated-, seed specific-, pollen specific-, ovary specific-, pathogenesis or disease related-, cauliflower mosaic viras 35S, CMV 35S minimal, cassava vein mosaic viras (CsVMV), chlorophyll a/b binding protein, ribulose 1, 5-bisphosphate carboxylase, shoot- specific, root specific, chitinase, stress inducible, rice tungro bacUUform virus, plant super- promoter, potato leucine aminopeptidase, nitrate reductase, mannopine synthase, nopaline synthase, ubiquitin, zein protein, and anthocyanin promoters (useful for expression in plant ceUs); animal and mammaUan promoters known in the art include, but are not limited to, the SV40 early (SV40e) promoter region, the promoter contained in the 3' long terminal repeat (LTR) of Rous sarcoma virus (RSV), the promoters of the El A or major late promoter (MLP) genes of adenoviruses, the cytomegalovirus early promoter, the herpes simplex virus (HSV) thymidine kinase (TK) promoter, an elongation factor 1 alpha (EF1) promoter, a phosphoglycerate kinase (PGK) promoter, a ubiquitin (Ubc) promoter, an albumin promoter, the regulatory sequences of the mouse metallothionein-L promoter, and transcriptional control regions, the ubiquitous promoters (HPRT, vimentin, -actin, tubulin and the Uke), the promoters of the intermediate filaments (desmin, neurofilaments, keratin, GFAP, and the like), the promoters of therapeutic genes (of the MDR, CFTR or factor VIII type, and the Uke), and promoters that exhibit tissue specificity and have been utiUzed in transgenic animals, such as the elastase I gene control region which is active in pancreatic acinar cells; insuUn gene control region active in pancreatic beta cells, immunoglobulin gene control region active in lymphoid ceUs, mouse πiammary tumor virus control region active in testicular, breast, lymphoid and mast ceUs; albumin gene, Apo Al and Apo All control regions active in liver, alpha-fetoprotein gene control region active in Uver, alpha 1-antitrypsin gene control region active in the Uver, beta-globin gene control region active in myeloid ceUs, myelin basic protein gene control region active in oUgodendrocyte ceUs in the brain, myosin Ught chain-2 gene control region active in skeletal muscle, and gonadotropic releasing hormone gene control region active in the hypothalamus, pyruvate kinase promoter, vilUn promoter, promoter of the fatty acid binding intestinal protein, promoter of the smooth muscle cell α-actin, and the Uke. In a preferred embodiment of the invention, the promoter is selected from the group consisting of a cauliflower mosaic viras 35S promoter, a cassava vein mosaic viras promoter, and a cauliflower mosaic virus 35S minimal promoter, an elongation factor 1 alpha (EF1) promoter, a phosphoglycerate kinase (PGK) promoter, a ubiquitin (Ubc) promoter, and an albumin promoter. In addition, these expression sequences may be modified by addition of enhancer or regulatory sequences and the Uke. Enhancers that may be used in embodiments of the invention include but are not limited to: tobacco mosaic virus enhancer, cauliflower mosaic virus 35S enhancer, tobacco etch virus enhancer, ribulose 1, 5-bisphosphate carboxylase enhancer, rice tungro bacilUform viras enhancer, and other plant and viral gene enhancers, and the like. Termination control regions, i.e., teiminator or polyadenylation sequences, may also be derived from various genes native to the preferred hosts. OptionaUy, a termination site may be unnecessary, however, it is most preferred if included. In a preferred embodiment of the invention, the termination control region may be comprise or be derived from a synthetic sequence, synthetic polyadenylation signal, an SV40 late polyadenylation signal, an SV40 polyadenylation signal, a bovine growth hormone (BGH) polyadenylation signal, nopaline synthase (nos), cauUflower mosaic virus (CaMV), octopine synthase (ocs), Agrocateum, viral, and plant terminator sequences, or the like. The terms "3' non-coding sequences" or "3' untranslated region (UTR)" refer to DNA sequences located downstream (3') of a coding sequence and may comprise polyadenylation tpoly(A)] recognition sequences and other sequences encoding regulatory signals capable of affecting mRNA processing or gene expression. The polyadenylation signal is usuaUy characterized by affecting the addition of polyadenyUc acid tracts to the 3' end of the mRNA precursor. "Regulatory region" means a nucleic acid sequence which regulates the expression of a second nucleic acid sequence. A regulatory region may include sequences which are naturaUy responsible for expressing a particular nucleic acid (a homologous region) or may include sequences of a different origin that are responsible for expressing different proteins or even synthetic proteins (a heterologous region). In particular, the sequences can be sequences of prokaryotic, eukaryotic, or viral genes or derived sequences that stimulate or repress transcription of a gene in a specific or non-specific manner and in an inducible or non-inducible manner. Regulatory regions include origins of repUcation, RNA spUce sites, promoters, enhancers, transcriptional termination sequences, and signal sequences which direct the polypeptide into the secretory pathways of the target cell. A regulatory region from a "heterologous source" is a regulatory region that is not naturaUy associated with the expressed nucleic acid. Included among the heterologous regulatory regions are regulatory regions from a different species, regulatory regions from a different gene, hybrid regulatory sequences, and regulatory sequences which do not occur in nature, but which are designed by one having ordinary skill in the art. "RNA transcript" refers to the product resulting from RNA polymerase-catalyzed transcription of a DNA sequence. When the RNA transcript is a perfect complementary copy of the DNA sequence, it is referred to as the primary transcript or it may be a RNA sequence derived from post-transcriptional processing of the primary transcript and is referred to as the mature RNA. "Messenger RNA (mRNA)" refers to the RNA that is without introns and that can be translated into protein by the cell. "cDNA" refers to a double-stranded DNA that is complementary to and derived from mRNA. "Sense" RNA refers to RNA transcript that includes the mRNA and so can be translated into protein by the cell. "Antisense RNA" refers to a RNA transcript that is complementary to all or part of a target primary transcript or mRNA and that blocks the expression of a target gene. The complementarity of an antisense RNA may be with any part of the specific gene transcript, i.e., at the 5' non-coding sequence, 3' non-coding sequence, or the coding sequence. "Functional RNA" refers to antisense RNA, ribozyme RNA, or other RNA that is not translated yet has an effect on ceUular processes. A "polypeptide" is a polymeric compound comprised of covalently linked amino acid residues. Amino acids have the following general structure: H R-C-COOH ' I NH 2 Amino acids are classified into seven groups on the basis of the side chain R: (1) aUphatic side chains, (2) side chains containing a hydroxylic (OH) group, (3) side chains containing sulfur atoms, (4) side chains containing an acidic or amide group, (5) side chains containing a basic group, (6) side chains containing an aromatic ring, and (7) proUne, an imino acid in which the side chain is fused to the amino group. A polypeptide of the invention preferably comprises at least about 14 amino acids. A "protein" is a polypeptide that performs a structural or functional role in a living ceU. An "isolated polypeptide" or "isolated protein" is a polypeptide or protein that is substantiaUy free of those compounds that are normally associated therewith in its natural state (e.g., other proteins or polypeptides, nucleic acids, carbohydrates, lipids). "Isolated" is not meant to exclude artificial or synthetic mixtures with other compounds, or the presence of impurities which do not interfere with biological activity, and which may be present, for example, due to incomplete purification, addition of stabiUzers, or compounding into a pharmaceuticaUy acceptable preparation. "Fragment" of a polypeptide according to the invention wiU be understood to mean a polypeptide whose amino acid sequence is shorter than that of the reference polypeptide and which comprises, over the entire portion with these reference polypeptides, an identical amino acid sequence. Such fragments may, where appropriate, be included in a larger polypeptide of which they are a part. Such fragments of a polypeptide according to the invention may have a length of 10, 15, 20, 30 to 40, 50, 100, 200 or 300 amino acids. A "variant" of a polypeptide or protein is any analogue, fragment, derivative, or mutant which is derived from a polypeptide or protein and which retains at least one biological property of the polypeptide or protein. Different variants of the polypeptide or protein may exist in nature. These variants may be aUeUc variations characterized by differences in the nucleotide sequences of the structural gene coding for the protein, or may involve differential spUcing or post-translational modification. The skilled artisan can produce variants having single or multiple amino acid substitutions, deletions, additions, or replacements. These variants may include, inter alia: (a) variants in which one or more amino acid residues are substituted with conservative or non-conservative amino acids, (b) variants in which one or more amino acids are added to the polypeptide or protein, (c) variants in which one or more of the amino acids includes a substituent group, and (d) variants in which the polypeptide or protein is fused with another polypeptide such as serum albumin. The techniques for obtaining these variants, including genetic (suppressions, deletions, mutations, etc.), chemical, and enzymatic techniques, are known to persons having ordinary skill in the art. A variant polypeptide preferably comprises at least about 14 amino acids. A "heterologous protein" refers to a protein not naturaUy produced in the ceU. A "mature protein" refers to a post-translationally processed polypeptide; i.e., one from which any pre- or propeptides present in the primary translation product have been removed. "Precursor" protein refers to the primary product of translation of mRNA; i.e., with pre- and propeptides stiU present. Pre- and propeptides may be but are not Umited to intracellular localization signals. The term "signal peptide" refers to an amino terminal polypeptide preceding the secreted mature protein. The signal peptide is cleaved from and is therefore not present in the mature protein. Signal peptides have the function of directing and translocating secreted proteins across cell membranes. Signal peptide is also referred to as signal protein. A "signal sequence" is included at the beginning of the coding sequence of a protein to be expressed on the surface of a ceU. This sequence encodes a signal peptide, N-teiminal to the mature polypeptide, that directs the host ceU to translocate the polypeptide. The term "translocation signal sequence" is used herein to refer to this sort of signal sequence. Translocation signal sequences can be found associated with a variety of proteins native to eukaryotes and prokaryotes, and are often functional in both types of organisms. The term "homology" refers to the percent of identity between two polynucleotide or two polypeptide moieties. The correspondence between the sequence from one moiety to anolher can be determined by techniques known to the art. For example, homology can be determined by a direct comparison of the sequence information between two polypeptide molecules by aUgning the sequence information and using readily available computer programs. Alternatively, homology can be determined by hybridization of polynucleotides under conditions that form stable duplexes between homologous regions, foUowed by digestion with single-stranded-specific nuclease(s) and size determination of the digested fragments. As used herein, the term "homologous" in aU its grammatical forms and speUing variations refers to the relationship between proteins that possess a "common evolutionary origin," including proteins from superfamilies (e.g., the immunoglobulin superfamily) and homologous proteins from different species (e.g., myosin light chain, etc.) (Reeck et al., 1987, Cell 50:667.). Such proteins (and their encoding genes) have sequence homology, as reflected by their high degree of sequence similarity. Accordingly, the term "sequence similarity" in aU its grammatical forms refers to the degree of identity or correspondence between nucleic acid or amino acid sequences of proteins that may or may not share a common evolutionary origin (see Reeck et al., 1987, Cell 50:667). As used herein, the term "homologous" in aU its grammatical forms and speUύjg variations refers to the relationship between proteins that possess a "common evolutionary origin," including proteins from superfamilies and homologous proteins from different species (Reeck et al., supra). Such proteins (and their encoding genes) have sequence homology, as reflected by their high degree of sequence similarity. However, in common usage and in the instant application, the term "homologous," when modified with an adverb such as "highly," may refer to sequence similarity and not a common evolutionary origin. In a specific embodiment, two DNA sequences are "substantiaUy homologous" or "substantially similar" when at least about 50% (preferably at least about 75%, and most preferably at least about 90 or 95%) of the nucleotides match over the defined length of the DNA sequences. Sequences that are substantiaUy homologous can be identified by comparing the sequences using standard software available in sequence data banks, or in a Southern hybridization experiment under, for example, stringent conditions as defined for that particular system. Defining appropriate hybridization conditions is within the skUl of the art. See, e.g., Sambrook et al., 1989, supra. As used herein, "substantiaUy similar" refers to nucleic acid fragments wherein changes in one or more nucleotide bases results in substitution of one or more amino acids, but do not affect the functional properties of the protein encoded by the DNA sequence. "SubstantiaUy similar" also refers to nucleic acid fragments wherein changes in one or more nucleotide bases does not affect the abiUty of the nucleic acid fragment to mediate alteration of gene expression by antisense or co-suppression technology. "SubstantiaUy similar" also refers to modifications of the nucleic acid fragments of the instant invention such as deletion or insertion of one or more nucleotide bases that do not substantially affect the functional properties of the resulting transcript. It is therefore understood that the invention encompasses more than the specific exemplary sequences. Each of the proposed modifications is weU within the routine skUl in the art, as is determination of retention of biological activity of the encoded products. Moreover, the skiUed artisan recognizes that substantially similar sequences encompassed by this invention are also defined by their abiUty to hybridize, under stringent conditions (0.1X SSC, 0.1% SDS, 65°C and washed with 2X SSC, 0.1% SDS followed by 0.1X SSC, 0.1% SDS), with the sequences exempUfied herein. SubstantiaUy similar nucleic acid fragments of the instant invention are those nucleic acid fragments whose DNA sequences are at least 70% identical to the DNA sequence of the nucleic acid fragments reported herein. Preferred substantiaUy nucleic acid fragments of the instant invention are thos^nucleic acid fragments whose DNA sequences are at least 80% identical to the DNA sequence of the nucleic acid fragments reported herein. More preferred nucleic acid fragments are at least 90% identical to the DNA sequence of the nucleic acid fragments reported herein. Even more preferred are nucleic acid fragments that are at least 95% identical to the DNA sequence of the nucleic acid fragments reported herein. Two amino acid sequences are "substantiaUy homologous" or "substantiaUy simUar" when greater than about 40% of the amino acids are identical, or greater than 60% are similar (functionally identical). Preferably, the similar or homologous sequences are identified by alignment using, for example, the GCG (Genetics Computer Group, Program Manual for the GCG Package, Version 7, Madison, Wisconsin) pileup program. The term "corresponding to" is used herein to refer to similar or homologous sequences, whether the exact position is identical or different from the molecule to which the similarity or homology is measured. A nucleic acid or amino acid sequence alignment may include spaces. Thus, the term "corresponding to" refers to the sequence similarity, and not the numbering of the amino acid residues or nucleotide bases. A "substantial portion" of an amino acid or nucleotide sequence comprises enough of the amino acid sequence of a polypeptide or the nucleotide sequence of a gene to putatively identify that polypeptide or gene, either by manual evaluation of the sequence by one skilled in the art, or by computer-automated sequence comparison and identification using algorithms such as BLAST (Basic Local AUgnment Search Tool; Altschul, S. F., et al, (1993) /. Mol. Biol. 215:403-410; see also www.ncbi.nlm.nih.gov/BLAST/). In general, a sequence often or more contiguous amino acids or thirty or more nucleotides is necessary in order to putatively identify a polypeptide or nucleic acid sequence as homologous to a known protein or gene. Moreover, with respect to nucleotide sequences, gene specific oUgonucleotide probes comprising 20-30 contiguous nucleotides may be used in sequence-dependent methods of gene identification (e.g., Southern hybridization) and isolation (e.g., in situ hybridization of bacterial colonies or bacteriophage plaques). In addition, short oUgonucleotides of 12-15 bases may be used as ampUfication primers in PCR in order to obtain a particular nucleic acid fragment comprising the primers. Accordingly, a "substantial portion" of a nucleotide sequence comprises enough of the sequence to specifically identify and/or isolate a nucleic acid fragment comprising the sequence. The term "percent identity", as known in the art, is a relationship between two or more polypeptide sequences or two or more polynucleotide sequences, as deterrnine^,by comparing the sequences. In the art, "identity" also means the degree of sequence relatedness between polypeptide or polynucleotide sequences, as the case may be, as determined by the match between strings of such sequences. "Identity" and "simUarity" can be readUy calculated by known methods, including but not limited to those described in: Computational Molecular Biology (Lesk, A. M., ed.) Oxford University Press, New York (1988); Biocomputing: Informatics and Genome Projects (Smith, D. W., ed.) Academic Press, New York (1993); Computer Analysis of Sequence Data, Part I (Griffin, A. M., and Griffin, H. G., eds.) Humana Press, New Jersey (1994); Sequence Analysis in Molecular Biology (von Heinje, G., ed.) Academic Press (1987); and Sequence Analysis Primer (Gribskov, M. and Devereux, J., eds.) Stockton Press, New York (1991). Preferred methods to determine identity are designed to give the best match between the sequences tested. Methods to determine identity and similarity are codified in pubUcly available computer programs. Sequence aUgnments and percent identity calculations may be performed using the Megalign program of the LASERGENE bioinfoimatics computing suite (DNASTAR Inc., Madison, WI). Multiple alignment of the sequences may be performed using the Clustal method of aUgnment (Higgins and Sharp (1989) CABIOS. 5:151-153) with the default parameters (GAP PENALTY=10, GAP LENGTH PENALTY=10). Default parameters for pairwise aUgnments using the Clustal method may be selected: KTUPLE 1 , GAP PENALTY=3 , WINDOW=5 and DIAGONALS SAVED=5. The term "sequence analysis software" refers to any computer algorithm or software program that is useful for the analysis of nucleotide or amino acid sequences. "Sequence analysis software" may be commercia y available or independently developed. Typical sequence analysis software wiU include but is not limited to the GCG suite of programs (Wisconsin Package Version 9.0, Genetics Computer Group (GCG), Madison, WI), BLASTP, BLASTN, BLASTX (Altschul et al., /. Mol. Biol. 215:403-410 (1990), and DNASTAR (DNASTAR, Inc. 1228 S. Park St. Madison, WI 53715 USA). Within the context of this appUcation it wiU be understood that where sequence analysis software is used for analysis, that the results of the analysis wiU be based on the "default values" of the program referenced, unless otherwise specified. As used herein "default values" wiU mean any set of values or parameters which originaUy load with the software when first initiaUzed. "Synthetic genes" can be assembled from oUgonucleotide buUding blocks that are chemically synthesized using procedures known to those skUled in the art. These building blocks are Ugated and annealed to form gene segments that are then enzymatiqaUy assembled to construct the entire gene. "Chemically synthesized", as related to a sequence of DNA, means that the component nucleotides were assembled in vitro. Manual chemical synthesis of DNA may be accompUshed using weU estabUshed procedures, or automated chemical synthesis can be performed using one of a number of commercially available machines. Accordingly, the genes can be tailored for optimal gene expression based on optimization ofnucleoti.de sequence to reflect the codon bias of the host cell. The skilled artisan appreciates the likelihood of successful gene expression if codon usage is biased towards those codons favored by the host. Determination of preferred codons can be based on a survey of genes derived from the host ceU where sequence information is available. GENE EXPRESSION MODULATION SYSTEM OF THE INVENTION AppUcants have now shown that separating the transactivation and DNA binding domains by placing them on two different proteins results in greatly reduced background activity in the absence of a Ugand and significantly increased activity over background in the presence of a ligand. AppUcants' improved gene expression system comprises two chimeric gene expression; the first encoding a DNA binding domain fused to a nuclear receptor polypeptide and the second encoding a transactivation domain fused to a nuclear receptor polypeptide. The interaction of the first protein with the second protein effectively tethers the DNA binding domain to the transactivation domain. Since the DNA binding and transactivation domains reside on two different molecules, the background activity in the absence of Ugand is greatly reduced. In general, the inducible gene expression modulation system of the invention comprises a) a first chimeric gene that is capable of being expressed in a host ceU comprising a polynucleotide sequence that encodes a first hybrid polypeptide comprising i) a DNA-binding domain that recognizes a response element associated with a gene whose expression is to be modulated; and ii) a Ugand binding domain comprising the ligand binding domain from a nuclear receptor; and b) a second chimeric gene that is capable of being expressed in the host ceU comprising a polynucleotide sequence that encodes a second hybrid polypeptide comprising: i) a transactivation domain; and n) a Ugand binding domain comprising the ligand binding domain from a nuclear receptor other than ultraspiracle (USP); wherein the transactivation domain are from other than EcR, RXR, or USP; and wherein the ligand binding domains from the first hybrid polypeptide and the second hybrid polypeptide are different and dimerize. ,_, This two-hybrid system exploits the abiUty of a pair of interacting proteins to bring the transcription activation domain into a more favorable position relative to the DNA binding domain such that when the DNA binding domain binds to the DNA binding site on the gene, the transactivation domain more effectively activates the promoter (see, for example, U.S. Patent No. 5,283,173). This two-hybrid system is a significantly improved inducible gene expression modulation system compared to the two systems disclosed in International Patent Applications PCT/US97/05330 and PCT/US98/14215. 5 The ecdysone receptor-based gene expression modulation system of the invention may be either heterodimeric and homodimeric. A functional EcR complex generaUy refers to a heterodimeric protein complex consisting of two members of the steroid receptor family, an ecdysone receptor protein obtained from various insects, and an ultraspiracle (USP) protein or the vertebrate homolog of USP, retinoid X receptor protein (see Yao, et al. (1993) Nature 366, 10 476-479; Yao, et al., (1992) Cell 71, 63-72). However, the complex may also be a homodimer as detaUed below. The functional ecdysteroid receptor complex may also include additional protein(s) such as immunophiUns. Additional members of the steroid receptor family of proteins, known as transcriptional factors (such as DHR38 or betaFTZ-1), may also be Ugand dependent or independent partners for EcR, USP, and/or RXR. AdditionaUy, other cofactors 15 may be required such as proteins generaUy known as coactivators (also termed adapters or mediators). These proteins do not bind sequence-specificaUy to DNA and are not involved in basal transcription. They may exert their effect on transcription activation through various mechanisms, including stimulation of DNA-binding of activators, by affecting chromatin structure, or by mediating activator-initiation complex interactions. Examples of such 20 coactivators include RIP140, TIF1, RAP46/Bag-1, ARA70, SRC-l/NCoA-1, ΗF2/GRIP/NCoA-2, ACTR/AIBl/RAC3/pCIP as weU as the promiscuous coactivator C response element B binding protein, CBP/p300 (for review see Glass et al, Curr. Opin. Cell Biol. 9:222-232, 1997). Also, protein cofactors generally known as corepressors (also known as repressors, silencers, or silencing mediators) may be required to effectively inhibit 25 transcriptional activation in the absence of ligand. These corepressors may interact with the unliganded ecdysone receptor to silence the activity at the response element. Current evidence suggests that binding of Ugand changes the conformation of the receptor, which results in release of the corepressor and recruitment of the above described coactivators, thereby . aboUshing their silencing activity. Examples of corepressors include N-CoR and SMRT (for 30 review, see Horwitz et al. Mol Endocrinol. 10: 1167-1177, 1996). These cofactors may either be endogenous within the cell or organism, or may be added exogenously as transgenes to be expressed in either a regulated or unregulated fashion. Homodimer complexes of the ecdysone receptor protein, USP, or RXR may also be functional under some circumstances. The ecdysone receptor complex typicaUy includes proteins which are members of the nuclear receptor superfamUy wherein all members are characterized by the presence of an amino-terminal transactivation domain, a DNA binding domain ("DBD"), and a ligand binding domain ("LBD") separated from the DBD by a hinge region. As used herein, the term "DNA binding domain" comprises a niinimal peptide sequence of a DNA binding protein, up to the entire length of a DNA binding protein, so long as the DNA binding domain functions to associate with a particular response element. Members of the nuclear receptor superfamily are also characterized by the presence of four or five domains: A B, C, D, E, and in some members F (see Evans, Science 240:889-895 (1988)). The "A/B" domain corresponds to the transactivation domain, "C" corresponds to the DNA binding domain, "D" corresponds to the hinge region, and "E" corresponds to the Ugand binding domain. Some members of the family may also have another transactivation domain on the carboxy-terminal side of the LBD corresponding to "F". The DBD is characterized by the presence of two cysteine zinc fingers between which are two amino acid motifs, the P-box and the D-box, which confer specificity for ecdysone response elements. These domains may be either native, modified, or chimeras of different domains of heterologous receptor proteins. This EcR receptor, like a subset of the steroid receptor family, also possesses less weU defined regions responsible for heterodimerization properties. Because the domains of EcR, USP, and RXR are modular in nature, the LBD, DBD, and transactivation domains may be interchanged. Gene switch systems are known that incorporate components from the ecdysone receptor complex. However, in these known systems, whenever EcR is used it is associated with native or modified DNA binding domains and transactivation domains on the same molecule. USP or RXR are typically used as silent partners. We have now shown that when DNA binding domains and transactivation domains are on the same molecule the background activity in the absence of ligand is high and that such activity is dramaticaUy reduced when DNA binding domains and transactivation domains are on different molecules, that is, on each of two partners of a heterodimeric or homodimeric complex. This two-hybrid system also provides improved sensitivity to non-steroidal Ugands for example, diacylhydrazines, when compared to steroidal Ugands for example, ponasterone A ("PonA") or muristerone A ("MurA"). That is, when compared to steroids, the non-steroidal Ugands provide higher activity at a lower concentration. In addition, since transactivation based on EcR gene switches is often ceU-line dependent, it is easier to tailor switching system to obtain maximum transactivation capabUity for each appUcation. Furthermore, this two-hybrid system avoids some side effects due to overexpression of RXR that often occur when unmodified RXR is used as a switching partner. In this two-hybrid system, native DNA binding and transactivation domains of EcR or RXR are eliminated. As a result, these chimeric molecules have less chance of interacting with other steroid hormone receptors present in the cell resulting in reduced side effects. SpecificaUy, AppUcants' invention relates to a gene expression modulation system comprising: a) a first gene expression cassette that is capable of being expressed in a host ceU, wherein the first gene expression cassette comprises a polynucleotide that encodes a first polypeptide comprising i) a DNA-binding domain that recognizes a response element associated with a gene whose expression is to be modulated; and U) a ligand binding domain comprising a Ugand binding domain from a nuclear receptor; and b) a second gene expression cassette that is capable of being expressed in the host ceU, wherein the second gene expression cassette comprises a polynucleotide sequence that encodes a second polypeptide comprising i) a transactivation domain; and U) a ligand binding domain comprising a Ugand binding domain from a nuclear receptor other than ultraspiracle (USP); wherein the DNA binding domain and the transactivation domain are from other than EcR, RXR, or USP; wherein the ligand binding domains from the first polypeptide and the second polypeptide are different and dimerize. The present invention also relates to a gene expression modulation system according to the present invention further comprising c) a third gene expression cassette comprising: i) the response element to which the DNA-binding domain of the first polypeptide binds; n) a promoter that is activated by the transactivation domain of the second polypeptide; and iu) the gene whose expression is to be modulated. In a specific embodiment, the gene whose expression is to be modulated is a homologous gene with respect to the host ceU. In another specific embodiment, the gene whose expression is to be modulated is a heterologous gene with respect to the host cell. In a specific embodiment, the Ugand binding domain of the first polypeptide comprises an ecdysone receptor ligand binding domain. In another specific embodiment, the ligand binding domain of the first polypeptide comprises a retinoid X receptor ligand binding domain. ,_, In a specific embodiment, the ligand binding domain of the second polypeptide comprises an ecdysone receptor Ugand binding domain. In another specific embodiment, the ligand binding domain of the second polypeptide comprises a retinoid X receptor ligand binding domain. In a preferred embodiment, the ligand binding domain of the first polypeptide comprises an ecdysone receptor Ugand binding domain, and the Ugand binding domain of the second polypeptide comprises a retinoid X receptor ligand binding domain. In another preferred embodiment, the ligand binding domain of the first polypeptide is from a retinoid X receptor polypeptide, and the ligand binding domain of the second polypeptide is from an ecdysone receptor polypeptide. Preferably, the Ugand binding domain is an EcR or RXR related steroid/thyroid hormone nuclear receptor family member ligand binding domain, or analogs, combinations, or modifications thereof. More preferably, the LBD is from EcR or RXR. Even more preferably, the LBD is from a truncated EcR or RXR. A truncation mutation may be made by any method used in the art, including but not limited to restriction endonuclease digestion/deletion, PCR-mediated/oUgonucleotide-directed deletion, chemical mutagenesis, UV strand breakage, and the like. Preferably, the EcR is an insect EcR selected from the group consisting of a Lepidopteran EcR, a Dipteran EcR, an Arthropod EcR, a Homopteran EcR and a Hemipteran EcR. More preferably, the EcR for use is a spruce budworm Choristoneurafumiferana EcR ("CfEcR"), a Tenebrio molitor EcR ("TmEcR"), a Manduca sexta EcR ("MsEcR"), a Heliothies virescens EcR ("HvEcR"), a silk moth Bombyx mori EcR ("BmEcR"), a fruit fly Drosophila melanogaster EcR ("DmEcR"), a mosquito Aedes aegypti EcR ("AaEcR"), a blowfly LuciUa capitata EcR ("LcEcR"), a Mediterranean fruit fly Ceratitis capitata EcR ("CcEcR"), a locust Locusta migratoria EcR ("LmEcR"), an aphid Myzus persicae EcR ("MpEcR"), a fiddler crab Uca pugilator EcR ("UpEcR"), or an ixodid tick Amblyomma ameήcanum EcR ("AmaEcR"). Even more preferably, the LBD is from spruce budworm (Choristoneurafumiferana) EcR ("CfEcR") or fruit fly Drosophila melanogaster EcR ("DmEcR"). Preferably, the LBD is from a trancated insect EcR. The insect EcR polypeptide truncation comprises a deletion of at least 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 2^5, 240, 245, 250, 255, 260, or 265 amino acids. More preferably, the insect EcR polypeptide truncation comprises a deletion of at least a partial polypeptide domain. Even more preferably, the insect EcR polypeptide truncation comprises a deletion of at least an entire polypeptide domain. In a specific embodiment, the insect EcR polypeptide truncation comprises a deletion of at least an A B-domain deletion, a C-domain deletion, a D-domain deletion, an E-domain deletion, an F- domain deletion, an A B/C-domains deletion, an A/B/l/2-C-domains deletion, an A/B/C/D- domains deletion, an A/B/C/D/F-domains deletion, an A/B/F-domains, and an A/B/C/F- domains deletion. A combination of several complete and/or partial domain deletions may also be performed. In a preferred embodiment, the ecdysone receptor Ugand binding domain is encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, and SEQ ID NO: 10. In another preferred embodiment, the ecdysone receptor Ugand binding domain comprises a polypeptide sequence selected from the group consisting of SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, and SEQ ID NO: 20. Preferably, the RXR polypeptide is a mouse Mus musculus RXR ("MmRXR") or a human Homo sapiens RXR ("HsRXR"). The RXR polypeptide may be an RXR α , RXRp, or RXRγisoform. Preferably, the LBD is from a trancated RXR. The RXR polypeptide truncation comprises a deletion of at least 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, or 265 amino acids. More preferably, the RXR polypeptide truncation comprises a deletion of at least a partial polypeptide domain. Even more preferably, the RXR polypeptide truncation comprises a deletion of at least an entire polypeptide domain. In a specific embodiment, the RXR polypeptide truncation comprises a deletion of at least an A/B-domain deletion, a C-domain deletion, a D-domain deletion, an E-domain deletion, an F-domain deletion, an A/B/C-domains deletion, an A/B/l/2-C-domains deletion, an A B/C/D-domains deletion, an A/B/C D/F-domains deletion, an A/B/F-domains, and an A/B/C/F-domains deletion. A combination of several complete and/or partial domain deletions may also be performed. ^ In a preferred embodiment, the retinoid X receptor ligand binding domain is encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, and SEQ ID NO: 30. In another preferred embodiment, the retinoid X receptor Ugand binding domain comprises a polypeptide sequence selected from the group consisting of SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, and SEQ ID NO: 40. For purposes of this invention EcR and RXR also include synthetic and chimeric EcR and RXR and their homologs. The DNA binding domain can be any DNA binding domain with a known response element, including synthetic and chimeric DNA binding domains, or analogs, combinations, or modifications thereof. Preferably, the DBD is a GAL4 DBD, a LexA DBD, a transcription factor DBD, a steroid/thyroid hormone nuclear receptor superfamily member DBD, a bacterial LacZ DBD, or a yeast put DBD. More preferably, the DBD is a GAL4 DBD [SEQ ID NO: 41 (polynucleotide) or SEQ ID NO: 42 (polypeptide)] or a LexA DBD [(SEQ ID NO: 43 (polynucleotide) or SEQ ID NO: 44 (polypeptide)]. The transactivation domain (abbreviated "AD" or "TA") may be any steroid/thyroid hormone nuclear receptor AD, synthetic or chimeric AD, polyglutamine AD, basic or acidic amino acid AD, a VP16 AD, a GAL4 AD, an NF-κB AD, a BP64 AD, or an analog, combination, or modification thereof. Preferably, the AD is a synthetic or chimeric AD, or is obtained from a VP16, GAL4, or NF-kB. Most preferably, the AD is a VP16 AD [SEQ ID NO: 45 (polynucleotide) or SEQ ID NO: 46 (polypeptide)]. The response element ("RE") may be any response element with a known DNA binding domain, or an analog, combination, or modification thereof. Preferably, the RE is an RE from GAL4 ("GAL4RE"), LexA, a steroid/thyroid hormone nuclear receptor RE, or a synthetic RE that recognizes a synthetic DNA binding domain. More preferably, the RE is a GAL4RE comprising a polynucleotide sequence of SEQ ID NO: 47 or a LexA 8X operon comprising a polynucleotide sequence of SEQ ID NO: 48. Preferably, the first hybrid protein is substantiaUy free of a transactivation domain and the second hybrid protein is substantially free of a DNA binding domain. For purposes of this invention, "substantiaUy free" means that the protein in question does not contain a sufficient sequence of the domain in question to provide activation or binding activity. The Ugands for use in the present invention as described below, when combined with the ligand binding domain of an EcR, USP, RXR, or another polypeptide which in turn are bound to the response element Unked to a gene, provide the means for external temporal regulation of expression of the gene. The binding mechanism or the order in which the various components of this invention bind to each other, that is, ligand to receptor, first polypeptide to response element, second polypeptide to promoter, etc., is not critical. Bmding of the ligand to the ligand binding domains of an EcR, USP, RXR, or another protein, enables expression or suppression of the gene. This mechanism does not exclude the potential for Ugand binding to EcR, USP, or RXR, and the resulting formation of active homodimer complexes (e.g. EcR+EcR or USP+USP). Preferably, one or more of the receptor domains can be varied producing a chimeric gene switch. TypicaUy, one or more of the three domains, DBD, LBD, and transactivation domain, may be chosen from a source different than the source of the other domains so that the chimeric genes and the resulting hybrid proteins are optimized in the chosen host cell or organism for transactivating activity, complementary binding of the ligand, and recognition of a specific response element. In addition, the response element itself can be modified or substituted with response elements for other DNA binding protein domains such as the GAL-4 protein from yeast (see Sadowski, et al. (1988) Nature, 335:563-564) or LexA protein fromE. coli (see Brent and Ptashne (1985), CeU, 43:729-736), or synthetic response elements specific for targeted interactions with proteins designed, modified, and selected for such specific interactions (see, for example, Kim, et al. (1997), Proc. Natl. Acad. Sci., USA, 94:3616-3620) to accommodate chimeric receptors. Another advantage of chimeric systems is that they aUow choice of a promoter used to drive the gene expression according to a desired end result. Such double control can be particularly important in areas of gene therapy, especially when cytotoxic proteins are produced, because both the liming of expression as well as the ceUs wherein expression occurs can be contiroUed. When genes, operatively linked to a suitable promoter, are introduced into the ceUs of the subject, expression of the exogenous genes is contiroUed by the presence of the system of this invention. Promoters may be constitutively or inducibly regulated or may be tissue-specific (that is, expressed only in a particular type of ceUs) or specific to certain developmental stages of the organism. GENE EXPRESSION CASSETTES OF THE INVENTION The novel ecdysone receptor-based inducible gene expression system of the invention comprises a novel gene expression cassette that is capable of being expressed n a host ceU, wherein the gene expression cassette comprises a polynucleotide encoding a hybrid polypeptide. Thus, Applicants' invention also provides novel gene expression cassettes for use in the gene expression system of the invention. SpecificaUy, the present invention provides a gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide. The hybrid polypeptide comprises either 1) a DNA-binding domain that recognizes a response element and a Ugand binding domain of a nuclear receptor or 2) a transactivation domain and a ligand binding domain of a nuclear 5 receptor, wherein the transactivation domain is from a nuclear receptor other than an EcR, an RXR, or a USP. In a specific embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a DNA-binding domain that recognizes a response element and an ecdysone receptor Ugand binding domain, wherein the DNA binding domain is from a nuclear receptor 10 other than an ecdysone receptor. In another specific embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a DNA-binding domain that recognizes a response element and a retinoid X receptor Ugand binding domain, wherein the DNA binding domain is from a nuclear receptor other than a retinoid X receptor. 15 The DNA binding domain can be any DNA binding domain with a known response element, including synthetic and chimeric DNA binding domains, or analogs, combinations, or modifications thereof. Preferably, the DBD is a GAL4 DBD, a LexA DBD, a transcription factor DBD, a steroid/thyroid hormone nuclear receptor superfamily member DBD, a bacterial LacZ DBD, or a yeast put DBD. More preferably, the DBD is a GAL4 DBD [SEQ ID NO: 20 41 (polynucleotide) or SEQ ID NO: 42 (polypeptide)] or a LexA DBD [(SEQ ID NO: 43 (polynucleotide) or SEQ ID NO: 44 (polypeptide)]. In another specific embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a transactivation domain and an ecdysone receptor Ugand binding domain, wherein the transactivation domain is from a nuclear receptor other than an ecdysone 25 receptor. In another specific embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a transactivation domain and a retinoid X receptor Ugand binding domain, wherein the transactivation domain is from a nuclear receptor other than a retinoid X receptor. 30 The transactivation domain (abbreviated "AD" or "TA") may be any steroid/thyroid hormone nuclear receptor AD, synthetic or chimeric AD, polyglutamine AD, basic or acidic amino acid AD, a VP16 AD, a GAL4 AD, an NF-κB AD, a BP64 AD, or an analog, combination, or modification thereof. Preferably, the AD is a synthetic or chimeric AD, or is obtained from a VP16, GAL4, or NF-kB. Most preferably, the AD is a VP16 AD [SEQ ID NO: 45 (polynucleotide) or SEQ ID NO: 46 (polypeptide)]. Preferably, the Ugand binding domain is an EcR or RXR related steroid/thyroid hormone nuclear receptor family member ligand bmding domain, or analogs, combinations, or modifications thereof. More preferably, the LBD is from EcR or RXR. Even more preferably, the LBD is from a truncated EcR or RXR. Preferably, the EcR is an insect EcR selected from the group consisting of a Lepidopteran EcR, a Dipteran EcR, an Arthropod EcR, a Homopteran EcR and a Hemipteran EcR. More preferably, the EcR for use is a spruce budworm Choristoneurafumiferana EcR ("CfEcR"), a Tenebrio molitor EcR ("TmEcR"), a Manduca sexta EcR ("MsEcR"), a Heliothies virescens EcR ("HvEcR"), a sUk moth Bombyx mori EcR ("BmEcR"), a fruit fly Drosophila melanogaster EcR ("DmEcR"), a mosquito Aedes aegypti EcR ("AaEcR"), a blowfly LuciUa capitata EcR ("LcEcR"), a Mediterranean fruit fly Ceratitis capitata EcR ("CcEcR"), a locust Locusta migratoria EcR ("LmEcR"), an aphid Myzus persicae EcR ("MpEcR"), a fiddler crab Uca pugilator EcR ("UpEcR"), or an ixodid tick A mblyomma americanum EcR ("AmaEcR"). Even more preferably, the LBD is from spruce budworm (Choristoneurafumiferana) EcR ("CfEcR") or fruit fly Drosophila melanogaster EcR ("DmEcR"). Preferably, the LBD is from a truncated insect EcR. The insect EcR polypeptide truncation comprises a deletion of at least 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, or 265 amino acids. More preferably, the insect EcR polypeptide truncation comprises a deletion of at least a partial polypeptide domain. Even more preferably, the insect EcR polypeptide truncation comprises a deletion of at least an entire polypeptide domain. In a specific embodiment, the insect EcR polypeptide truncation comprises a deletion of at least an A/B-domain deletion, a C-domain deletion, a D-domain deletion, an E-domain deletion, an F- domain deletion, an A B/C-domains deletion, anA/B/l/2-C-domains deletion, an A/B/C/D- domains deletion, an A/B/C/D/F-domains deletion, an A/B/F-domains, and an A B/C/F- domains deletion. A combination of several complete and/or partial domain deletions may also be performed. In a prefened embodiment, the ecdysone receptor Ugand binding domain is encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, and SEQ ID NO: 10. In another preferred embodiment, the ecdysone receptor Ugand binding domain comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 11 , SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, and SEQ ID NO: 20. Preferably, the RXR polypeptide is a mouse Mus musculus RXR ("MmRXR") or a human Homo sapiens RXR ("HsRXR"). The RXR polypeptide may be an RXR α , RXR β , or RXR y isoform. Preferably, the LBD is from a trancated RXR. The RXR polypeptide truncation comprises a deletion of at least 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, or 265 amino acids. More preferably, the RXR polypeptide truncation comprises a deletion of at least a partial polypeptide domain. Even more preferably, the RXR polypeptide truncation comprises a deletion of at least an entire polypeptide domain. In a specific embodiment, the RXR polypeptide truncation comprises a deletion of at least an A/B-domain deletion, a C-domain deletion, a D-domain deletion, an E-domain deletion, an F-domain deletion, an A B/C-domains deletion, an A/B/l/2-C-domains deletion, an A/B/C/D-domains deletion, an A/B/C D/F-domains deletion, an A/B/F-domains, and an A/B/C/F-domains deletion. A combination of several complete and/or partial domain deletions may also be performed. In a preferred embodiment, the retinoid X receptor ligand binding domain is encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, and SEQ ID NO: 30. In another preferred embodiment, the retinoid X receptor Ugand binding domain comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 31 , SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, and SEQ ID NO: 40. In a prefened embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a DNA-binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of a GAL4 DBD (SEQ ID NO: 41) or a LexA DBD (SEQ ID NO: 43) and an ecdysone receptor Ugand binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, and SEQ ID NO: 10. In another prefened embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a DNA-binding domain comprising a polypeptide sequence selected from the group consisting of a GAL4 DBD (SEQ ID NO: 42) or a LexA DBD (SEQ ID NO: 44) and an ecdysone receptor ligand binding domain comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 11 , SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, and SEQ ID NO: 20. In another prefened embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a DNA-binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of a GAL4 DBD (SEQ ID NO: 41) or a LexA DBD (SEQ ID NO: 43) and a retinoid X receptor ligand binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, and SEQ ID NO: 30. In another prefened embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a DNA-binding domain comprising a polypeptide sequence selected from the group consisting of a GAL4 DBD (SEQ ID NO: 42) or a LexA DBD (SEQ ID NO: 44) and a retinoid X receptor Ugand binding domain comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, and SEQ ID NO: 40. In another prefened embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a transactivation domain encoded by a polynucleotide comprising a nucleic acid sequence of SEQ ID NO: 45 and an ecdysone receptor ligand binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4 SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, and SEQ ID NO: 10. In another prefened embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a transactivation domain comprising a polypeptide sequence of SEQ ID NO: 46 and an ecdysone receptor ligand binding domain comprising a polypeptide sequence selected from the group consisting of SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, and SEQ ID NO: 20. In another prefened embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a transactivation domain encoded by a polynucleotide comprising a nucleic acid sequence of SEQ ID NO: 45 and a retinoid X receptor ligand binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, and SEQ ID NO: 30. In another prefened embodiment, the gene expression cassette encodes a hybrid polypeptide comprising a transactivation domain comprising a polypeptide sequence of SEQ ID NO: 46 and a retinoid X receptor Ugand binding domain comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, and SEQ ID NO: 40. For purposes of this invention EcR and RXR also include synthetic and chimeric EcR and RXR and their homologs. POLYNUCLEOTIDES OF THE INVENTION The novel ecdysone receptor-based inducible gene expression system of the invention comprises a gene expression cassette comprising a polynucleotide that encodes a trancated EcR or RXR polypeptide comprising a truncation mutation and is useful in methods of modulating the expression of a gene within a host cell. Thus, the present invention also relates to a polynucleotide that encodes an EcR or RXR polypeptide comprising a truncation mutation. Specifically, the present invention relates to an isolated polynucleotide encoding an EcR or RXR polypeptide comprising a truncation mutation that affects ligand binding activity or Ugand sensitivity. Preferably, the truncation mutation results in a polynucleotide that encodes a trancated EcR polypeptide or a truncated RXR polypeptide comprising a deletion of at least 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, or 265 amino acids. More preferably, the EcR or RXR polypeptide truncation comprises a deletion of at least a partial polypeptide domain. Even more preferably, the EcR or RXR polypeptide truncation comprises a deletion of at least an entire polypeptide domain. In a specific embodiment, the EcR or RXR polypeptide truncation comprises a deletion of at least an A/B-domain deletion, a C-domain deletion, a D-domain deletion, an E-domain deletion, an F-domain deletion, an A/B/C-domains deletion, an A/B/l/2-C-domair_s deletion, an A B/C D-domains deletion, an A/B/C/D/F- domains deletion, an A/B/F-domains, and an A/B/C/F-domains deletion. A combination of several complete and/or partial domain deletions may also be performed. In a specific embodiment, the EcR polynucleotide according to the invention comprises a polynucleotide sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, and SEQ ID NO: 10. In a specific embodiment, the polynucleotide according to the invention encodes a ecdysone receptor polypeptide comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 11 (CfEcR-CDEF), SEQ ID NO: 12 (CfEcR- 1/2CDEF, which comprises the last 33 carboxy-terminal amino acids of C domain), SEQ ID NO: 13 (CfEcR-DEF), SEQ ID NO: 14 (CfEcR-EF), SEQ ID NO: 15 (CfEcR-DE ), SEQ ID NO: 16 (DmEcR-CDEF), SEQ ID NO: 17 (DmEcR- 1/2CDEF), SEQ ID NO: 18 (DmEcR-DEF), SEQ ID NO: 19 (DmEcR-EF), and SEQ ID NO: 20 (DmEcR- DE). In another specific embodiment, the RXR polynucleotide according to the invention comprises a polynucleotide sequence selected from the group consisting of SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, and SEQ ID NO: 30. In another specific embodiment, the polynucleotide according to the invention encodes a truncated RXR polypeptide comprising an amino acid sequence consisting of SEQ ID NO: 31 (MmRXR- CDEF), SEQ ID NO: 32 (MmRXR-DEF), SEQ ID NO: 33 (M RXR-EF), SEQ ID NO: 34 (MmRXR-truncatedEF), SEQ ID NO: 35 (MmRXR-E), SEQ ID NO: 36 (HsRXR-CDEF), SEQ ID NO: 37 (HsRXR-DEF), SEQ ID NO: 38 (HsRXR-EF), SEQ ID NO: 39 (HsRXR- trancated EF), and SEQ ID NO: 40 (HsRXR-E). In particular, the present invention relates to an isolated polynucleotide encoding an EcR or RXR polypeptide comprising a truncation mutation, wherein the mutation reduces ligand binding activity or Ugand sensitivity of the EcR or RXR polypeptide. In a specific embodiment, the present invention relates to an isolated polynucleotide encoding an EcR or RXR polypeptide comprising a truncation mutation that reduces steroid binding activity or steroid sensitivity of the EcR or RXR polypeptide. In a prefened embodiment, the present invention relates to an isolated polynucleotide encoding an EcR polypeptide comprising a truncation mutation that reduces steroid binding activity or steroid sensitivity of the EcR polypeptide, wherein the polynucleotide comprises a nucleic acid sequence of SEQ ID NO: 3 (CfEcR-DEF), SEQ ID NO: 4 (CfEcR-EF), SEQ ID NO: 8 (DmEcR-DEF), or SEQ ID NO: 9 (DmEcR-EF). In another specific embodiment, the present invention relates to an isolated polynucleotide encoding an EcR or RXR polypeptide comprising a truncation mutation that reduces non-steroid binding activity or non-steroid sensitivity of the EcR or RXR polypeptide. In a preferred embodiment, the present invention relates to an isolated polynucleotide encoding an EcR polypeptide comprising a truncation mutation that reduces non-steroid binding activity or non-steroid sensitivity of the EcR polypeptide, wherein the polynucleotide comprises a nucleic acid sequence of SEQ ID NO: 4 (CfEcR-EF) or SEQ ID NO: 9 (DmEcR-EF). The present invention also relates to an isolated polynucleotide encoding an EcR or RXR polypeptide comprising a truncation mutation, wherein the mutation enhances ligand binding activity or Ugand sensitivity of the EcR or RXR polypeptide. In a specific embodiment, the present invention relates to an isolated polynucleotide encoding an EcR or RXR polypeptide comprising a truncation mutation that enhances steroid bmding activity or steroid sensitivity of the EcR or RXR polypeptide. In another specific embodiment, the present invention relates to an isolated polynucleotide encoding an EcR or RXR polypeptide comprising a truncation mutation that enhances non-steroid binding activity or non-steroid sensitivity of the EcR or RXR polypeptide. In a prefened embodiment, the present invention relates to an isolated polynucleotide encoding an EcR polypeptide comprising a truncation mutation that enhances non-steroid binding activity or non-steroid sensitivity of the EcR polypeptide, wherein the polynucleotide comprises a nucleic acid sequence of SEQ ID NO: 3 (CfEcR-DEF) or SEQ ID NO: 8 (DmEcR-DEF). The present invention also relates to an isolated polynucleotide encoding a retinoid X receptor polypeptide comprising a truncation mutation that increases Ugand sensitivity of a heterodimer comprising the mutated retinoid X receptor polypeptide and a dimerization partner. Preferably, the isolated polynucleotide encoding a retinoid X receptor polypeptide comprising a truncation mutation that increases Ugand sensitivity of a heterodimer comprises a polynucleotide sequence selected from the group consisting of SEQ ID NO: 23 (MmRXR-EF), SEQ ID NO: 24 (MmRXR-truncatedEF , SEQ ID NO: 28 (HsRXR-EF), or SEQ ID NO: 29 (HsRXR-truncated EF). In a specific embodiment, the dimerization partner is an ecdysone receptor polypeptide. Preferably, the dimerization partner is a truncated EcR polypeptide. More preferably, the dimerization partner is an EcR polypeptide in which domains A/B/C have 5 been deleted. Even more preferably, the dimerization partner is an EcR polypeptide comprising an amino acid sequence of SEQ ID NO: 13 (CfEcR-DEF) or SEQ ID NO: 18 (DmEcR-DEF). POLYPEPTIDES OF THE INVENTION The novel ecdysone receptor-based inducible gene expression system of the invention 10 comprises a polynucleotide that encodes a truncated EcR or RXR polypeptide and is useful in methods of modulating the expression of a gene within a host cell. Thus, the present invention also relates to an isolated truncated EcR or RXR polypeptide encoded by a polynucleotide or a gene expression cassette according to the invention. SpecificaUy, the present invention relates to an isolated truncated EcR or RXR polypeptide comprising a truncation mutation that affects 15 ligand binding activity or Ugand sensitivity encoded by a polynucleotide according to the invention. The present invention also relates to an isolated truncated EcR or RXR polypeptide comprising a truncation mutation. Specifically, the present invention relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that affects Ugand binding activity or 20 ligand sensitivity. Preferably, the truncation mutation results in a trancated EcR polypeptide or a truncated RXR polypeptide comprising a deletion of at least 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 25 230, 235, 240, 245, 250, 255, 260, or 265 amino acids. More preferably, the EcR or RXR polypeptide truncation comprises a deletion of at least a partial polypeptide domain. Even more preferably, the EcR or RXR polypeptide truncation comprises a deletion of at least an entire polypeptide domain. In a specific embodiment, the EcR or RXR polypeptide truncation comprises a deletion of at least an A/B-domain deletion, a C-domain deletion, a D-domain 30 deletion, an E-domain deletion, an F-domain deletion, an A B/C-domains deletion, an A/B/l/2- C-domains deletion, an A/B/C/D-domains deletion, an A/B/C/D/F-domains deletion, an A/B/F- domains, and an A/B/C/F-domains deletion. A combination of several complete and/or partial domain deletions may also be performed. In a prefened embodiment, the isolated trancated ecdysone receptor polypeptide is encoded by a polynucleotide comprising a polynucleotide sequence selected from the group consisting of SEQ ID NO: 1 (CfEcR-CDEF), SEQ ID NO: 2 (CfEcR- 1/2CDEF), SEQ ID NO: 3 (CfEcR-DEF), SEQ ID NO: 4 (CfEcR-EF), SEQ ID NO: 5 (CfEcR-DE), SEQ ID NO: 6 (DmEcR-CDEF), SEQ ID NO: 7 (DmEcR- 1/2CDEF), SEQ ID NO: 8 (DmEcR-DEF), SEQ ID NO: 9 (DmEcR-EF), and SEQ ID NO: 10 (DmEcR-DE). In another prefened embodiment, the isolated truncated ecdysone receptor polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 11 (CfEcR-CDEF), SEQ ID NO: 12 (CfEcR-l/2CDEF), SEQ ID NO: 13 (CfEcR-DEF), SEQ ID NO: 14 (CfEcR-EF), SEQ ID NO: 15 (CfEcR-DE), SEQ ID NO: 16 (DmEcR-CDEF), SEQ ID NO: 17 (DmEcR-l/2CDEF), SEQ ID NO: 18 (DmEcR-DEF), SEQ ID NO: 19 (DmEcR-EF), and SEQ ID NO: 20 (DmEcR-DE). In a prefened embodiment, the isolated trancated RXR polypeptide is encoded by a polynucleotide comprising a polynucleotide sequence selected from the group consisting of SEQ ID NO: 21 (MmRXR-CDEF), SEQ ID NO: 22 (MmRXR-DEF), SEQ ID NO: 23 (MmRXR-EF), SEQ ID NO: 24 (MmRXR-trancatedEF), SEQ ID NO: 25 (MmRXR-E), SEQ ID NO: 26 (HsRXR-CDEF), SEQ ID NO: 27 (HsRXR-DEF), SEQ ID NO: 28 (HsRXR-EF), SEQ ID NO: 29 (HsRXR-truncatedEF) and SEQ ID NO: 30 (HsRXR-E). In another prefened embodiment, the isolated trancated RXR polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 31 (MmRXR-CDEF), SEQ ID NO: 32 (MmRXR-DEF), SEQ ID NO: 33 (MmRXR-EF), SEQ ID NO: 34 (MmRXR- trancatedEF), SEQ ID NO: 35 (MmRXR-E), SEQ ID NO: 36 (HsRXR-CDEF), SEQ ID NO: 37 (HsRXR-DEF), SEQ ID NO: 38 (HsRXR-EF), SEQ ID NO: 39 (HsRXR-truncatedEF), and SEQ ID NO: 40 (HsRXR-E). The present invention relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that reduces Ugand binding activity or Ugand sensitivity of the EcR or RXR polypeptide, wherein the polypeptide is encoded by a polynucleotide comprising a polynucleotide sequence selected from the group consisting of SEQ ID NO: 1 (CfEcR-CDEF), SEQ ID NO: 2 (CfEcR- 1/2CDEF), SEQ ID NO: 3 (CfEcR-DEF), SEQ ID NO: 4 (CfEcR- EF), SEQ ID NO: 5 (CfEcR-DE), SEQ ID NO: 6 (DmEcR-CDEF), SEQ ID $0: 7 (DmEcR- 1/2CDEF), SEQ ID NO: 8 (DmEcR-DEF), SEQ ID NO: 9 (DmEcR-EF), SEQ ID NO: 10 (DmEcR-DE), SEQ ID NO: 21 (MmRXR-CDEF), SEQ ID NO: 22 (MmRXR-DEF), SEQ ID NO: 23 (MmRXR-EF), SEQ ID NO: 24 (MmRXR-trancatedEF), SEQ ID NO: 25 (MmRXR- E), SEQ ID NO: 26 (HsRXR-CDEF), SEQ ID NO: 27 (HsRXR-DEF), SEQ ID NO: 28 (HsRXR-EF), SEQ ID NO: 29 (HsRXR-truncatedEF), and SEQ ID NO: 30 (HsRXR-E). Thus, the present invention relates to an isolated trancated EcR or RXR polypeptide comprising a truncation mutation that reduces Ugand binding activity or ligand sensitivity of the EcR or RXR polypeptide, wherein the polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 11 (CfEcR-CDEF), SEQ ID NO: 12 (CfEcR-l/2CDEF), SEQ ID NO: 13 (CfEcR-DEF), SEQ ID NO: 14 (CfEcR-EF), SEQ ID NO: 15 (CfEcR-DE), SEQ ID NO: 16 (DmEcR-CDEF), SEQ ID NO: 17 (DmEcR- 1/2CDEF), SEQ ID NO: 18 (DmEcR-DEF), SEQ ID NO: 19 (DmEcR-EF), SEQ ID NO: 20 (DmEcR- DE), SEQ ID NO: 31 (MmRXR-CDEF), SEQ ID NO: 32 (MmRXR-DEF), SEQ ID NO: 33 (MmRXR-EF), SEQ ID NO: 34 (MmRXR-trancatedEF), SEQ ID NO: 35 (MmRXR-E), SEQ ID NO: 36 (HsRXR-CDEF), SEQ ID NO: 37 (HsRXR-DEF), SEQ ID NO: 38 (HsRXR-EF), SEQ ID NO: 39 (HsRXR-truncatedEF), and SEQ ID NO: 40 (HsRXR-E). In a specific embodiment, the present invention relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that reduces steroid binding activity or steroid sensitivity of the EcR or RXR polypeptide. In a prefened embodiment, the present invention relates to an isolated EcR polypeptide comprising a truncation mutation that reduces steroid binding activity or steroid sensitivity of the EcR polypeptide, wherein the EcR polypeptide is encoded by a polynucleotide comprising a nucleic acid sequence of SEQ ID NO: 3 (CfEcR- DEF), SEQ ID NO: 4 (CfEcR-EF), SEQ ID NO: 8 (DmEcR-DEF), or SEQ ID NO: 9 (DmEcR-EF). Accordingly, the present invention also relates to an isolated truncated EcR or RXR polypeptide comprising a truncation mutation that reduces steroid binding activity or steroid sensitivity of the EcR or RXR polypeptide. In a prefened embodiment, the present invention relates to an isolated EcR polypeptide comprising a truncation mutation that reduces steroid binding activity or steroid sensitivity of the EcR polypeptide, wherein the EcR polypeptide comprises an amino acid sequence of SEQ ID NO: 13 (CfEcR-DEF), SEQ ID NO: 14 (CfEcR-EF), SEQ ID NO: 18 (DmEcR-DEF), or SEQ ID NO: 19 (DmEcR-EF). In another specific embodiment, the present invention relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that reduces non-steroid binding activity or non-steroid sensitivity of the EcR or RXR polypeptide. In a prefened embodiment, the present invention relates to an isolated EcR polypeptide comprising a truncation mutation that reduces non-steroid binding activity or non-steroid sensitivity of the EcR polypeptide, wherein the EcR polypeptide is encoded by a polynucleotide comprising a nucleic acid sequence of SEQ ID NO: 4 (CfEcR-EF) or SEQ ID NO: 9 (DmEcR-EF). Accordingly, the present invention also relates to an isolated trancated EcR or RXR polypeptide comprising a trancation mutation that reduces non-steroid binding activity or steroid sensitivity of the EcR or RXR polypeptide. In a prefened embodiment, the present invention relates to an isolated EcR polypeptide comprising a trancation mutation that reduces non-steroid binding activity or non-steroid sensitivity of the EcR polypeptide, wherein the EcR polypeptide comprises an amino acid sequence of SEQ ID NO: 14 (CfEcR-EF) or SEQ ID NO: 19 (DmEcR-EF). In particular, the present invention relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that enhances Ugand binding activity or ligand sensitivity of the EcR or RXR polypeptide, wherein the polypeptide is encoded by a polynucleotide comprising a polynucleotide sequence selected from the group consisting of SEQ ID NO: 1 (CfEcR-CDEF), SEQ ID NO: 2 (CfEcR- 1/2CDEF), SEQ ID NO: 3 (CfEcR-DEF), SEQ ID NO: 4 (CfEcR-EF), SEQ ID NO: 5 (CfEcR-DE), SEQ ID NO: 6 (DmEcR-CDEF), SEQ ID NO: 7 (DmEcR- 1/2CDEF), SEQ ID NO: 8 (DmEcR-DEF), SEQ ID NO: 9 (DmEcR-EF), SEQ ID NO: 10 (DmEcR-DE), SEQ ID NO: 21 (MmRXR-CDEF), SEQ ID NO: 22 (MmRXR-DEF), SEQ ID NO: 23 (MmRXR-EF), SEQ ID NO: 24 (MmRXR-trancatedEF), SEQ ID NO: 25 (MmRXR-E), SEQ ID NO: 26 (HsRXR-CDEF), SEQ ID NO: 27 (HsRXR- DEF), SEQ ID NO: 28 (HsRXR-EF), SEQ ID NO: 29 (HsRXR-trancated EF), and SEQ ID NO: 30 (HsRXR-E). The present invention relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that enhances ligand binding activity or Ugand sensitivity of the EcR or RXR polypeptide, wherein the polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 11 (CfEcR-CDEF), SEQ ID NO: 12 (CfEcR- 1/2CDEF), SEQ ID NO: 13 (CfEcR-DEF), SEQ ID NO: 14 (CfEcR-EF), SEQ ID NO: 15 (CfEcR-DE), SEQ ID NO: 16 (DmEcR-CDEF), SEQ ID NO: 17 (DmEcR- 1 2CDEF), SEQ ID NO: 18 (DmEcR-DEF), SEQ ID NO: 19 (DmEcR-EF), SEQ ID NO: 20 (DmEcR-DE), SEQ ID NO: 31 (MmRXR-CDEF), SEQ ID NO: 32 (MmRXR-DEF), SEQ ID NO: 33 (MmRXR-EF), SEQ ID NO: 34 (MmRXR-trancatedEF), SEQ ID NO: 35 (MmRXR-E), SEQ ID NO: 36 (HsRXR-CDEF), SEQ ID NO: 37 (HsRXR-DEF), SEQ ID NO: 39 (HsRXR-EF), SEQ ID NO: 39 (HsRXR-truncatedEF), and SEQ ID NO: 40 (HsRXR-E). The present invention relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that enhances ligand binding activity or Ugand sensitivity of the EcR or RXR polypeptide. In a specific embodiment, the present invention relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that enhances steroid binding activity or steroid sensitivity of the EcR or RXR polypeptide. Accordingly, the present invention also relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that enhances steroid binding activity or steroid sensitivity of the EcR or RXR polypeptide. In another specific embodiment, the present invention relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that enhances non-steroid binding activity or non-steroid sensitivity of the EcR or RXR polypeptide. In a prefened embodiment, the present invention relates to an isolated EcR polypeptide comprising a truncation mutation that enhances non-steroid binding activity or non-steroid sensitivity of the EcR polypeptide, wherein the EcR polypeptide is encoded by a polynucleotide comprising a nucleic acid sequence of SEQ ID NO: 3 (CfEcR-DEF) or SEQ ID NO: 8 (DmEcR-DEF). Accordingly, the present invention also relates to an isolated EcR or RXR polypeptide comprising a truncation mutation that enhances non-steroid binding activity or steroid sensitivity of the EcR or RXR polypeptide. In a prefened embodiment, the present invention relates to an isolated EcR polypeptide comprising a truncation mutation that enhances non-steroid binding activity or non-steroid sensitivity of the EcR polypeptide, wherein the EcR polynucleotide comprises an amino acid sequence of SEQ ID NO: 13 (CfEcR-DEF) or SEQ ID NO: 18 (DmEcR-DEF). The present invention also relates to an isolated retinoid X receptor polypeptide comprising a truncation mutation that increases Ugand sensitivity of a heterodimer comprising the mutated retinoid X receptor polypeptide and a dimerization partner. Preferably, the isolated retinoid X receptor polypeptide comprising a trancation mutation that increases ligand sensitivity of a heterodimer is encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 23 (MmRXR-EF), SEQ ID NO: 24 (MmRXR-trancatedEF), SEQ ID NO: 28 (HsRXR-EF), or SEQ ID NO: 29 (HsRXR- trancatedEF). More preferably, the isolated polynucleotide encoding a retinoid X receptor polypeptide comprising a truncation mutation that increases ligand sensitivity of a heterodimer comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 33 (MmRXR-EF), SEQ ID NO: 34 (MmRXR-trancatedEF), SEQ ID NO: 38 (HsRXR-EF), or SEQ ID NO: 39 (HsRXR-truncatedEF). In a specific embodiment, the dimerization partner is an ecdysone receptor polypeptide. Preferably, the dimerization partner is a truncated EcR polypeptide. More preferably, the dimerization partner is an EcR polypeptide in which domains A/B/C have been deleted. Even more preferably, the dimerization partner is an EcR polypeptide comprising an amino acid sequence of SEQ ID NO: 13 (CfEcR-DEF) or SEQ ID NO: 18 (DmEcR-DEF). METHOD OF MODULATING GENE EXPRESSION OF THE INVENTION Applicants' invention also relates to methods of modulating gene expression in a host cell using a gene expression modulation system according to the invention. SpecificaUy, Applicants' invention provides a method of modulating the expression of a gene in a host cell comprising the steps of: a) introducing into the host cell a gene expression modulation system according to the invention; and b) introducing into the host cell a ligand that independently combines with the Ugand binding domains of the first polypeptide and the second polypeptide of the gene expression modulation system; wherein the gene to be expressed is a component of a gene expression cassette comprising: i) a response element comprising a domain to which the DNA binding domain of the first polypeptide binds; ii) a promoter that is activated by the transactivation domain of the second polypeptide; and Ui) a gene whose expression is to be modulated, whereby a complex is formed comprising the Ugand, the first polypeptide of the gene expression modulation system and the second polypeptide of the gene expression modulation system, and whereby the complex modulates expression of the gene in the host cell. Genes of interest for expression in a host ceU using Applicants' methods may be endogenous genes or heterologous genes. Nucleic acid or amino acid sequence information for a desired gene or protein can be located in one of many pubUc access databases, for example, GENB ANK, EMBL, Swiss-Prot, and PIR, or in many biology related journal pubUcations. Thus, those skilled in the art have access to nucleic acid sequence information for virtually all known genes. Such information can then be used to construct the desired constructs for the insertion of the gene of interest within the gene expression cassettes used in AppUcants' methods described herein. Examples of genes of interest for expression in a host ceU using AppUcants' methods include, but are not Umited to: antigens produced in plants as vaccines, enzymes Uke alpha- amylase, phytase, glucanes, and xylanse, genes for resistance against insects, nematodes, fungi, bacteria, viruses, and abiotic stresses, nutraceuticals, pharmaceuticals, vitamins, genes for modifying amino acid content, herbicide resistance, cold, drought, and heat tolerance, industrial products, oils, protein, carbohydrates, antioxidants, male sterile plants, flowers, fuels, other output traits, genes encoding therapeuticaUy desirable polypeptides or products, such as genes that can provide, modulate, alleviate, conect and/or restore polypeptides important in treating a condition, a disease, a disorder, a dysfunction, a genetic defect, and the Uke. Acceptable Ugands are any that modulate expression of the gene when binding of the DNA binding domain of the two hybrid system to the response element in the presence of the ligand results in activation or suppression of expression of the genes. Prefened Ugands include ponasterone, muristerone A, N,N'-diacylhydrazines such as those disclosed inU. S. Patents No. 6,013,836; 5,117,057; 5,530,028; and 5,378,726; dibenzoylalkyl cyanohydrazines such as those disclosed in European AppUcation No. 461,809; N-alkyl-N,N'-diaroylhydrazines such as those disclosed in U. S. Patent No. 5,225,443; N-acyl-N-alkylcarbonylhydrazines such as those disclosed in European AppUcation No. 234,994; N-aroyl-N-alkyl-N'-aroylhydrazines such as those described in U. S. Patent No. 4,985,461 ; each of which is incorporated herein by reference and other similar materials including 3 ,5-di-tert-butyl-4-hydroxy-N-isobutyl- benzamide, 8-O-acetylharpagide, and the like. Preferably, the Ugand for use in AppUcants' method of modulating expression of gene is a compound of the formula: wherein: E is a (C 4 -Ce)alkyl containing a tertiary carbon or a cyano(C3-Cs)alkyl containing a tertiary carbon; R 1 is H, Me, Et, i-Pr, F, for yl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 C1, CH 2 OH, CH 2 OMe, CH 2 CN, CN, C°CH, 1-propynyl, 2-proρynyl, vinyl, OH, OMe, OEt, cyclopropyl, CF 2 CF 3 , CH=CHCN, allyl, azido, SCN, or SCHF 2 ; R 2 is H, Me, Et, n-Pr, i-Pr, formyl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 C1, CH 2 OH, CH 2 OMe, CH 2 CN, CN, C°CH, 1-propynyl, 2-propynyl, vinyl, Ac, F, CI, OH, OMe, OEt, O-n- Pr, OAc, NMez, NEt 2 , SMe, SEt, SOCF 3 , OCF 2 CF 2 H, COEt, cyclopropyl, CF 2 CF 3 , CH=CHCN, allyl, azido, OCF 3) OCHF 2 , O-i-Pr, SCN, SCHF 2 , SOMe, NH-CN, or joined with R 3 and the phenyl carbons to which R 2 and R 3 are attached to form an ethylenedioxy, a dihydrofuryl ring with the oxygen adjacent to a phenyl carbon, or a dihydropyryl ring with the oxygen adjacent to a phenyl carbon; R 3 is H, Et, or joined with R 2 and the phenyl carbons to which R 2 and R 3 are attached to form an ethylenedioxy, a dihydrofuryl ring with the oxygen adjacent to a phenyl carbon, or a dihydropyryl ring with the oxygen adjacent to a phenyl carbon; R 4 , R s , and R 6 are independently H, Me, Et, F, CI, Br, formyl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 CI, CH 2 OH, CN, C°CH, 1-propynyl, 2-propynyl, vinyl, OMe, OEt, SMe, or SEt. Applicants' mvention provides for modulation of gene expression in prokaryotic and eukaryotic host cells. Thus, the present invention also relates to a method for modulating gene expression in a host ceU selected from the group consisting of a bacterial ceU, a fungal ceU, a yeast cell, a plant cell, an animal cell, and a mammaUan cell. Preferably, the host cell is a yeast cell, a plant ceU, a murine ceU, or a human cell. Expression in transgenic host ceUs may be useful for the expression of various polypeptides of interest including but not limited to therapeutic polypeptides, pathway intermediates; for the modulation of pathways already existing in the host for the synthesis of new products heretofore not possible using the host; cell based assays; and the like. Additionally the gene products may be useful for conferring higher growth yields of the host or for enabling alternative growth mode to bs utilized. HOST CELLS AND NON-HUMAN ORGANISMS OF THE INVENTION As described above, the gene expression modulation system of the present invention may be used to modulate gene expression in a host cell. Expression in transgenic host cells may be useful for the expression of various genes of interest. Thus, AppUcants' invention also provides an isolated host cell comprising a gene expression system according to the invention. The present invention also provides an isolated host ceU comprising a gene expression cassette according to the invention. AppUcants' invention also provides an isolated host cell comprising a polynucleotide or polypeptide according to the invention. The isolated host cell may be either a prokaryotic or a eukaryotic host ceU. Preferably, the host ceU is selected from the group consisting of a bacterial ceU, a fungal ceU, a yeast cell, a plant cell, an animal cell, and a mammaUan cell. Examples of preferred host ceUs include, but are not Umited to, fungal or yeast species such as Aspergillus, Tήchoderma, Saccharomyces, Pichia, Candida, Hansenula, or bacterial species such as those in the genera Synechocystis, Synechococcus, Salmonella, Bacillus, Acinetobacter, Rhodococcus, Streptomyces, Escherichia, Pseudomonas, Methylomonas, Methylobacter, Alcaligenes, Synechocystis, Anabaena, Thiobacillus, Methanobacterium and Klebsieϊla, plant, animal, and mammaUan host ceUs. More preferably, the host cell is a yeast ceU, a plant ceU, a murine ceU, or a human cell. In a specific embodiment, the host ceU is a yeast ceU selected from the group consisting of a Saccliaromyces, a Pichia, and a Candida host cell. In another specific embodiment, the host cell is a plant ceU selected from the group consisting of an apple, Arabidopsis, bajra, banana, barley, bean, beet, blackgram, chickpea, chiU, cucumber, eggplant, favabean, maize, melon, millet, mungbean, oat, okra, Panicum, papaya, peanut, pea, pepper, pigeonpea, pineapple, Phaseolus, potato, pumpkin, rice, sorghum, soybean, squash, sugarcane, sugarbeet, sunflower, sweet potato, tea, tomato, tobacco, watermelon, and wheat host cell. In another specific embodiment, the host cell is a murine ceU. In another specific embodiment, the host cell is a human cell. Host cell transformation is weU known in the art and may be achieved by a variety of methods including but not limited to electroporation, viral infection, plasmid vector transfection, non-viral vector mediated transfection, Agrobacterium-meάiateά transformation, particle bombardment, and the like. Expression of desired gene products involves culturing the transformed host cells under suitable conditions and inducing expression of the transformed gene. Culture conditions and gene expression protocols in prokaryotic and eukaryotic cells are well known in the art (see General Methods section of Examples). CeUs may be harvested and the gene products isolated according to protocols specific for the gene product. In addition, a host cell may be chosen which modulates the expression of the inserted polynucleotide, or modifies and processes the polypeptide product in the specific fashion desired. Different host cells have characteristic and specific mechanisms for the translational and post-translational processing and modification (e.g., glycosylation, cleavage [e.g., of signal sequence]) of proteins. Appropriate cell lines or host systems can be chosen to ensure the desired modification and processing of the foreign protein expressed. For example, expression in a bacterial system can be used to produce a non-glycosylated core protein product. However, a polypeptide expressed in bacteria may not be properly folded. Expression in yeast can produce a glycosylated product. Expression in eukaryotic ceUs can increase the likelihood of "native" glycosylation and folding of a heterologous protein. Moreover, expression in mammaUan cells can provide a tool for reconstituting, or constituting, the poljjpeptide's activity. Furthermore, different vector/host expression systems may affect processmg reactions, such as proteolytic cleavages, to a different extent. Applicants' invention also relates to a non-human organism comprising an isolated host cell according to the invention. Preferably, the non-human organism is selected from the group consisting of a bacterium, a fungus, a yeast, a plant, an animal, and a mammal. More preferably, the non-human organism is a yeast, a plant, a mouse, a rat, a rabbit, a cat, a dog, a bovine, a goat, a pig, a horse, a sheep, a monkey, or a chimpanzee. In a specific embodiment, the non-human organism is a yeast selected from the group consisting of Saccharomyces, Pichia, and Candida. In another specific embodiment, the non-human organism is a plant selected from the group consisting of an apple, Arabidopsis, bajra, banana, barley, beans, beet, blackgram, chickpea, chiU, cucumber, eggplant, favabean, maize, melon, miUet, mungbean, oat, okra, Panicum, papaya, peanut, pea, pepper, pigeonpea, pineapple, Phaseolus, potato, pumpkin, rice, sorghum, soybean, squash, sugarcane, sugarbeet, sunflower, sweet potato, tea, tomato, tobacco, watermelon, and wheat. In another specific embodiment, the non-human organism is a Mus musculus mouse. MEASURING GENE EXPRESSION/TRANSCRIPTION One useful measurement of AppUcants' methods of the invention is that of the transcriptional state of the ceU including the identities and abundances of RNA, preferably mRNA species. Such measurements are conveniently conducted by measuring cDNA abundances by any of several existing gene expression technologies. Nucleic acid anay technology is a useful technique for determining differential mRNA expression. Such technology includes, for example, oUgonucleotide chips and DNA microanays. These techniques rely on DNA fragments or oUgonucleotides which conespond to different genes or cDNAs which are immobUized on a soUd support and hybridized to probes prepared from total mRNA pools extracted from cells, tissues, or whole organisms and converted to cDNA. OUgonucleotide chips are anays of oUgonucleotides synthesized on a substrate using photoUthographic techniques. Chips have been produced which can analyze for up to 1700 genes. DNA microanays are arrays of DNA samples, typicaUy PCR products, that are robotically printed onto a microscope sUde. Each gene is analyzed by a full or partial- length target DNA sequence. Microanays with up to 10,000 genes are now rqutinely prepared commerciaUy. The primary difference between these two techniques is that oUgonucleotide chips typically utiUze 25-mer oUgonucleotides which allow fractionation of short DNA molecules whereas the larger DNA targets of microanays, approximately 1000 base pairs, may provide more sensitivity in fractionating complex DNA mixtures. Another useful measurement of AppUcants' methods of the invention is that of deteirnining the translation state of the ceU by measuring the abundances of the constituent protein species present in the ceU using processes well known in the art. Where identification of genes associated with various physiological functions is desired, an assay may be employed in which changes in such functions as cell growth, apoptosis, senescence, differentiation, adhesion, binding to a specific molecules, binding to another cell, cellular organization, organogenesis, intracellular transport, transport facUitation, energy conversion, metaboUsm, myogenesis, neurogenesis, and/or hematopoiesis is measured. In addition, selectable marker or reporter gene expression may be used to measure gene expression modulation using AppUcants' invention. Other methods to detect the products of gene expression are weU known in the art and include Southern blots (DNA detection), dot or slot blots (DNA, RNA), Northern blots (RNA), and RT-PCR (RNA) analyses. Although less prefened, labeled proteins can be used to detect a particular nucleic acid sequence to which it hybidizes. In some cases it is necessary to ampUfy the amount of a nucleic acid sequence. This may be carried out using one or more of a number of suitable methods including, for example, polymerase chain reaction ("PCR"), Ugase chain reaction ("LCR"), strand displacement ampUfication ("SDA"), transcription-based ampUfication, and the Uke. PCR is carried out in accordance with known techniques in which, for example, a nucleic acid sample is treated in the presence of a heat stable DNA polymerase, under hybridizing conditions, with one oUgonucleotide primer for each strand of the specific sequence to be detected. An extension product of each primer that is synthesized is complementary to each of the two nucleic acid strands, with the primers sufficiently complementary to each strand of the specific sequence to hybridize therewith. The extension product synthesized from each primer can also serve as a template for further synthesis of extension products using the same primers. FoUowing a sufficient number of rounds of synthesis of extension products, the sample may be analyzed as described above to assess whether the sequence or sequences to be detected are present. The present invention may be better understood by reference to the foljpwing non- limiting Examples, which are provided as exemplary of the invention. EXAMPLES GENERAL METHODS Standard recombinant DNA and molecular cloning techniques used herein are well known in the art and are described by Sambrook, J., Fritsch, E. F. and Maniatis, T. Molecular Cloning: A Laboratory Manual; Cold Spring Harbor Laboratory Press: Cold Spring Harbor, (1989) (Maniatis) and by T. J. Silhavy, M. L. Bennan, and L. W. Enquist, Experiments with Gene Fusions, Cold Spiring Harbor Laboratory, Cold Spring Harbor, N.Y. (1984) and by Ausubel, F. M. et al., Current Protocols in Molecular Biology, Greene PubUshing Assoc. and WUey-Interscience (1987). Methods for plant tissue culture, transformation, plant molecular biology, and plant, general molecular biology may be found in Plant Tissue Culture Concepts and Laboratory Exercises edited by RN Trigiano and DJ Gray, 2 nd edition, 2000, CRC press, New York; Agrobacterium Protocols edited by KMA Gartland and MR Davey, 1995, Humana Press, Totowa, New Jersey; Methods in Plant Molecular Biology, P. MaUga et al., 1995, Cold Spring Harbor Lab Press, New York; and Molecular Cloning, J. Sambrook et al., 1989, Cold Spring Harbor Lab Press, New York. Materials and methods suitable for the maintenance and growth of bacterial cultures are well known in the art. Techniques suitable for use in the following examples may be found as set out in Manual of Methods for General Bacteriology (PhilUpp Gerhardt, R. G. E. Munay, Ralph N. Costilow, Eugene W. Nester, WiUis A. Wood, Noel R. Krieg and G. Briggs PhiUips, eds), American Society for Microbiology, Washington, DC. (1994)) or by Thomas D. Brock in Biotechnology: A Textbook of Industrial Microbiology, Second Edition, Sinauer Associates, Inc., Sunderland, MA (1989). All reagents, restriction enzymes and materials used for the growth and maintenance of host ceUs were obtained from Aldrich Chemicals (Milwaukee, WI), DIFCO Laboratories (Detroit, MI), GIBCO/BRL (Gaithersburg, MD), or Sigma Chemical Company (St. Louis, MO) unless otherwise specified. Manipulations of genetic sequences may be accompUshed using the suite of programs available from the Genetics Computer Group Inc. (Wisconsin Package Version 9.0, Genetics Computer Group (GCG), Madison, WI). Where the GCG program "Pileup" is used the gap creation default value of 12, and the gap extension default value of 4 may be used. Where the CGC "Gap" or "Bestfit" programs is used the default gap creation penalty of 50 and the default gap extension penalty of 3 may be used. In any case where GCG program parameters are not prompted for, in these or any other GCG program, default values may be used. The meaning of abbreviations is as follows: "h" means hour(s), "min" means minute(s), "sec" means second(s), "d" means day(s), "μl" means microUter(s), "ml" means milUUter(s), "L" means liter(s), "μM" means micromolar, "mM" means miUimolar, "μg" means microgram(s), "mg" means milUgram(s), "A" means adenine or adenosine, "T" means thymine or thymidine, "G" means guanine or guanosine, "C" means cytidine or cytosme, "x g" means times gravity, "nt" means nucleotide(s), "aa" means amino acid(s), "bp" means base pair(s), "kb" means kilobase(s), "k" means kilo, "μ" means micro, and "°C" means degrees Celsius. EXAMPLE 1 AppUcants' improved EcR-based inducible gene modulation system was developed for use in various appUcations including gene therapy, expression of proteins of interest in host ceUs, production of transgenic organisms, and ceU-based assays. This Example describes the construction and evaluation of several gene expression cassettes for use in the EcR-based inducible gene expression system of the invention. In various ceUular backgrounds, including mammaUan cells, insect ecdysone receptor (EcR) heterodimerizes with retinoid X receptor (RXR) and, upon binding of ligand, transactivates genes under the control of ecdysone response elements. AppUcants constructed several EcR-based gene expression cassettes based on the spruce budworm Choristoneura fumiferana EcR ("CfEcR"; fuU length polynucleotide and amino acid sequences are set forth in SEQ ID NO: 49 and SEQ ID NO: 50, respectively), C. fumiferana ultraspiracle ("CfUSP"; fuU length polynucleotide and amino acid sequences are set forth in SEQ ID NO: 51 and SEQ ID NO: 52, respectively), and mouse Mus musculus RXR (MmRXRα; full length polynucleotide and amino acid sequences are set forth in SEQ ID NO: 53 and SEQ ID NO: 54, respectively). The prepared receptor constracts comprise a Ugand binding domain of EcR and of RXR or of USP; a DNA binding domain of GAL4 or of EcR; and an activation domain of VP16. The reporter constructs include a reporter gene, luciferase or LacZ, operably linked to a synthetic promoter construct that comprises either GAL4 or EcR/USP binding sites (response elements). Various combinations of these receptor and reporter constructs were cotransfected into CHO, NIH3T3, CV1 and 293 ceUs. Gene induction potential (magnitude of induction) and ligand specificity and sensitivity were examined using four different Ugands: two steroidal Ugands (ponasterone A and muristerone A) and two non-steroidal Ugands (N-(2-ethyl-3- memoxybenzoyl)-N'-(3,5-(_imethylbenzoyl)-N'-tert-butylhydrazine and N-(3,4-(l ,2- ethylene-Uoxy)-2-methylbenzoyl)-N'-(3,5-d_methylbenzoyl)-N'-tert-butylhydrazine) in a dose- dependent induction of reporter gene expression in the transfected ceUs. Reporter gene expression activities were assayed at 24hr or 48hr after ligand addition. Gene Expression Cassettes: Ecdysone receptor-based, chemicaUy inducible gene expression cassettes (switches) were constructed as foUowed, using standard cloning methods available in the art. The following is brief description of preparation and composition of each switch. 1.1 - GAL4EcR/VP16RXR: The D, E, and F domains from spruce budworm Choristoneura fumiferana EcR ("CfEcRDEF"; SEQ ID NO: 3) were fused to GAL4 DNA binding domain ("DNABD"; SEQ ID NO: 41) and placed under the control of an SV40e promoter (SEQ ID NO: 55). The DEF domains from mouse (Mus musculus) RXR ("MmRXRDEF"; SEQ ID NO: 22) were fused to the activation domain from VP16 ("VP16AD"; SEQ ID NO: 45) and placed under the control of an SV40e promoter (SEQ ID NO: 55). Five consensus GAL4 binding sites ("5XGAL4RE"; comprising 5, GAL4RE comprising SEQ ID NO: 47) were fused to a synthetic Elb minimal promoter (SEQ ID NO: 56) and placed upstream of the luciferase gene (SEQ ID NO: 57). 1.2 - GAL4EcR/VP16USP: This construct was prepared in the same way as in switch 1.1 above except MmRXRDEF was replaced with the D, E and F domains from spruce budworm USP ("CfUSPDEF"; SEQ ID NO: 58). The constracts used in this example are simUar to those disclosed in U. S. Patent No. 5,880,333 except that Choristoneurafumiferana USP rather than Drosophila melanogaster USP was utilized. 1.3 - GAL4RXR/VP16CfEcR: MmRXRDEF (SEQ ID NO: 22) was fused to a GAL4DNABD (SEQ ID NO: 41) and CfEcRCDEF (SEQ ID NO: 1) was fused to a VP16AD (SEQ ID NO: 45). 1.4 - GAL4RXR/VP16DmEcR: This construct was prepared in the same way as switch 1.3 except CfEcRCDEF was replaced with DmEcRCDEF (SEQ ID NO: 6). 1.5 - GAL4USP/VP16CfEcR: This construct was prepared in the same way as switch 1.3 except MmRXRDEF was replaced with CfUSPDEF (SEQ ID NO: 58). 1.6 - GAL4RXRCfEcRVP16: This construct was prepared so that both the GAL4 DNABD and the VP16AD were placed on the same molecule. GAL4DNABD (SEQ ID NO: 41) and VP16AD (SEQ ID NO: 45) were fused to CfEcRDEF (SEQ ID NO: 3) at N-and C-termini respectively. The fusion was placed under the control of an S V40e promoter (SEQ ID NO: 55). 1.7 - VP16CfEcR: This construct was prepared such that CfEcRCDEF (SEQ ID NO: 1) was fused to VP16AD (SEQ ID NO: 45) and placed under the control of an SV40e promoter (SEQ ID NO: 55). Six ecdysone response elements ("EcRE"; SEQ ID NO: 59) from the hsp27 gene were placed upstream of the promoter and a luciferase gene (SEQ ID NO: 57). This switch most probably uses endogenous RXR. 1.8 - DmVgRXR: This system was purchased from Invitrogen Corp., Carlsbad, CaUfornia. It comprises a Drosophila melanogaster EcR ("DmEcR") with a modified DNABD fused to VP16AD and placed under the control of a CMV promoter (SEQ ID NO: 60). FuU length MmRXR (SEQ ID NO: 53) was placed under the control of the RSV promoter (SEQ ID NO: 61). The reporter, pIND(SPl)LacZ, contains five copies of a modified ecdysone response element ("EcRE", E/GRE), three copies of an SP1 enhancer, and a minimal heat shock promoter, aU of which were placed upstream to the LacZ reporter gene. .9 - CfVgRXR: This example was prepared in the same way as switch 1.8 except DmEcR was replaced with a truncated CfEcR comprising a partial A B domain and the complete CDEF domains [SEQ ID NO: 62 (polynucleotide) and SEQ ID NO: 63 (polypeptide)]. 1.10 - CfVgRXRdel: This example was prepared in the same way as switch 1.9 except MmRXR (SEQ ID NO: 53) was deleted. Cell lines: Four ceU lines: CHO, Chinese hamster Cricetulus griseus ovarian ceU line; NIH3T3 (3T3) mouse Mus musculus ceU line; 293 human Homo sapiens kidney ceU line, and CV1 African green monkey kidney ceU line were used in these experiments. CeUs were maintained in their respective media and were subcultured when they reached 60% confluency. Standard methods for culture and maintenance of the ceUs were followed. Transfections: Several commercially available lipofactors as well as electroporation methods were evaluated and the best conditions for transfection of each cell line were developed. CHO, NIH3T3, 293 and CV1 cells were grown to 60% confluency. DNAs conesponding to the various switch constructs outlined in Examples 1.1 through 1.10 were transfected into CHO ceUs, NIH3T3 ceUs, 293 cells, or CV1 cells as foUows. CHO ceUs: Cells were harvested when they reach 60-80% confluency and plated in 6- or 12- or 24- weU plates at 250,000, 100,000, or 50,000 ceUs in 2.5, 1.0, or 0.5 ml of growth medium containing 10% Fetal bovine serum respectively. The next day, the cells were rinsed with growth medium and transfected for four hours. LipofectAMINE™ 2000 (Life Technologies Inc,) was found to be the best transfection reagent for these cells. For 12- well plates, 4 μl of LipofectAMINE™ 2000 was mixed with 100 μl of growth medium. 1.0 μg of reporter construct and 0.25 μg of receptor construct(s) were added to the transfection mix. A second 5 reporter construct was added [pTKRL (Promega), 0.1 μg/transfection mix] and comprised a Renilla luciferase gene (SEQ ID NO: 64) operably linked and placed under the control of a thymidine kinase (TK) constitutive promoter and was used for normalization. The contents of the transfection mix were mixed in a vortex mixer and let stand at room temperature for 30 min. At the end of incubation, the transfection mix was added to the cells maintained in 400 μl 10 growth medium. The cells were maintained at 37°C and 5% C0 2 for four hours. At the end of incubation, 500 μl of growth medium containing 20% FBS and either DMSO (control) or a DMSO solution of appropriate Ugands were added and the ceUs were maintained at 37 °C and 5% CO 2 for 24-48 hr. The cells were harvested and reporter activity was assayed. The same procedure was followed for 6 and 24 weU plates as weU except aU the reagents were doubled 15 for 6 well plates and reduced to half for 24- well plates. NIH3T3 CeUs: Superfect™ (Qiagen Inc.) was found to be the best transfection reagent for 3T3 ceUs. The same procedures described for CHO cells were foUowed for 3T3 ceUs as well with two modifications. The ceUs were plated when they reached 50% confluency. 125,000 or 50,000 or 25,000 ceUs were plated per well of 6- or 12- or 24-weU plates respectively. The 20 GA14EcR/VP16RXR and reporter vector DNAs were transfected into NIH3T3 cells, the transfected cells were grown in medium containing PonA, MurA, N-(2-ethyl-3- methoxybenzoyl)-N'-(3,5-dimethylbenzoyl)-N'-t-butylhydrazine, or N-(3,4-(l ,2- ethylenedioxy)-2-methylbenzoyl)-N'-(3,5-dimethylbenzoyl)-N'-tert-butylhydrazine for 48 hr. The Ugand treatments were performed as described in the CHO cell section above. 25 293 Cells: LipofectAMINE™ 2000 (Life Technologies) was found to be the best lipofactor for 293 ceUs. The same procedures described for CHO were foUowed for 293 ceUs except that the cells were plated inbiocoated plates to avoid clumping. The Ugand treatments were performed as described in the CHO cell section above. CVl CeUs: LipofectAMINE™ plus (Life Technologies) was found to be the best lipofactor 30 for CVl cells. The same procedures described for NIH3T3 ceUs were followeS for CVl ceUs Ligands: Ponasterone A and Muristerone A were purchased from Sigma Chemical Company. The two non-steroids N-(2-ethyl-3-methoxybenzoyl)-N'-(3,5-dimethylbenzoyl)-N'-t- butylhydrazine, or N-(3,4-(l ,2-ethylenedioxy)-2-methylbenzoyl)-N'-(3 ,5-dimethylbe__zoyl)-N'- tert-butylhydrazine are synthetic stable ecdysteroids synthesized at Rohm and Haas Company. All Ugands were dissolved in DMSO and the final concentration of DMSO was maintained at 0.1% in both controls and treatments. Reporter Assays: CeUs were harvested 24-48 hr after adding Ugands. 125, 250, or 500 μl of passive lysis buffer (part of Dual-luciferase™ reporter assay system from Promega Corporation) were added to each well of 24- or 12- or 24-well plates respectively. The plates were placed on a rotary shaker for 15 min. Twenty μl of lysate was assayed. Luciferase activity was measured using Dual-luciferase™ reporter assay system from Promega Corporation foUowing the manufacturer's instructions. β-Galactosidase was measured using Galacto-Star™ assay kit from TROPIX following the manufacturer' s instructions. All luciferase and β-galactosidase activities were normaUzed using Renilla luciferase as a standard. Fold activities were calculated by dividing normalized relative Ught units ("RLU") in ligand treated cells with normalized RLU in DMSO treated cells (untreated control). The results of these experiments are provided in the following tables. Table 1 Transactivation of reporter genes through various switches in CHO cells Table 2 Transactivation of reporter genes through various switches in 3T3 cells Table 3 Transactivation of reporter genes through various switches in 293 cells Table 4 Transactivation of reporter genes through various switches in CVl cells Table 5 Transactivation of reporter gene GAL4CfEcRDEF/VP16MmRXRDEF (switch 1.1) through steroids and non-steroids in 3T3 cells. Table 6 Transactivation of reporter gene GAL4MmRXRDEF/VP16CfEcRCDEF (switch 1.3) through steroids and non-steroids in 3T3 cells. Applicants' results demonstrate that the non-steroidal ecdysone agonists, N-(2-ethyl-3- methoxybe__zoyl)-N' -(3,5-dimethylbenzoyl)-N' -tert-butylhydrazine and N' -(3 ,4-( 1 ,2- ethylenedioxy)-2-methylbenzoyl)-N' -(3 ,5-dimethylbenzoyl)-N' -tert-butylhydrazine, were more potent activators of CfEcR as compared to Drosophila melanogaster EcR (DmEcR). (see Tables 1-4). Also, in the mammaUan ceU Unes tested, MmRXR performed better than CfUSP as a heterodimeric partner for CfEcR. (see Tables 1-4). AdditionaUy, Applicants' inducible gene expression modulation system performed better when exogenous MmRXR was used than when the system reUed only on endogenous RXR levels (see Tables 1-4). Applicants' results also show that in a CfEcR-based inducible gene expression system, the non-steroidal ecdysone agonists induced reporter gene expression at a lower concentration (i.e., increased ligand sensitivity) as compared to the steroid Ugands, ponasterone A and muristerone A (see Tables 5 and 6). Out of 10 EcR based gene switches tested, the GAL4EcR/VP16RXR switch (Switch 1.1) performed better than any other switch in aU four ceU Unes examined and was more sensitive to non-steroids than steroids. The results also demonstrate that placing the activation domain (AD) and DNA binding domain (DNABD) on each of the two partners reduced background when compared to placing both AD and DNABD together on one of the two partners. Therefore, a switch format where the AD and DNABD are separated between two partners, works well for EcR-based gene switch appUcations. In addition, the MmRXR/EcR-based switches performed better than CfUSP/EcR- based switches, which have a higher background activity than the MmRXR/EoR switches in the absence of Ugand. FinaUy, the GAL4EcR/VP16RXR switch (Switch 1.1) was more sensitive to non- steroid Ugands than to the steroid Ugands (see Tables 5 and 6). In particular, steroid Ugands initiated transactivation at concentrations of 50 μM, whereas the non-steroid Ugands initiated transactivation at less than 1 μM (submicromolar) concentration. EXAMPLE 2 This Example describes Applicants' further analysis of truncated EcR and RXR polypeptides in the improved EcR-based inducible gene expression system of the invention. To identify the best combination and length of two receptors that give a switch with a) maximum induction in the presence of ligand; b) minimum background in the absence of Ugand; c) highly sensitive to Ugand concentration; and d) minimum cross-talk among Ugands and receptors, AppUcants made and analyzed several truncation mutations of the CfEcR and MmRXR receptor polypeptides in NIH3T3 ceUs. Briefly, polynucleotides encoding EcR or RXR receptors were trancated at the junctions of A/B, C, D, E and F domains and fused to either a GAL4 DNA binding domain encoding polynucleotide (SEQ ID NO: 41) for CfEcR, or a VP16 activation domain encoding polynucleotide (SEQ ID NO: 45) for MmRXR as described in Example 1. The resulting receptor truncation/fusion polypeptides were assayed in NIH3T3 ceUs. Plasmid pFRLUC (Stratagene) encoding a luciferase polypeptide was used as a reporter gene construct and pTKRL (Promega) encoding a Renilla luciferase polypeptide under the control of the constitutive TK promoter was used to normaUze the transfections as described above. The analysis was performed in triplicates and mean luciferase counts were determined as described above. Gene Expression Cassettes Encoding Truncated Ecdysone Receptor Polypeptides Gene expression cassettes comprising polynucleotides encoding either full length or truncated CfEcR polypeptides fused to a GAL4 DNA binding domain (SEQ ID NO: 41): GAL4CfEcRA/BCDEF (full length CfEcRA/BCDEF; SEQ ID NO: 49), GAL4CfEcRCDEF (CfEcRCDEF; SEQ ID NO: 1), GAL4CfEcRl/2CDEF (CfEcRl/2CDEF; SEQ ID NO: 2), GAL4CfEcRDEF (CfEcRDEF; SEQ ID NO: 3), GAL4CfEcREF (CfEcREF; SEQ ID NO: 4), and GAL4CfEcRDE (CfEcRDE; SEQ ID NO: 5) were transfected into NIH3T3 ceUs along with VP16MmRXRDEF (constructed as in Example 1.1; Figure 11) or VPlόj imRXREF [constracted as in Example 1.1 except that MmRXRDEF was replaced with MmRXREF (SEQ ID NO: 23); Figure 12], and pFRLUc and pTKRL plasmid DNAs. The transfected ceUs were grown in the presence 0, 1, 5 or 25 uM of N-(2-ethyl-3-methoxybenzoyl)-N'-(3,5- dimethylbenzoyl)-N' -tert-butylhydrazine or PonA for 48 hr. The cells were harvested, lysed and luciferase reporter activity was measured in the cell lysates. Total fly luciferase relative light units are presented. The number on the top of each bar is the maximum fold induction for that treatment. AppUcants' results show that the EF domain of MmRXR is sufficient and performs better than DEF domains of this receptor (see Figures 11 and 12). Applicants have also shown that, in general, EcR/RXR receptor combinations are insensitive to PonA (see Figures 11 and 12). As shown in the Figures 11 and 12, the GAL4CfEcRCDEF hybrid polypeptide (SEQ ID NO: 7) performed better than any other CfEcR hybrid polypeptide. Gene Expression Cassettes Encoding Trancated Retinoid X Receptor Polypeptides Gene expression cassettes comprising polynucleotides encoding either full length or truncated MmRXR polypeptides fused to a VP16 transactivation domain (SEQ ID NO: 45): VP16MmRXRA/BCDEF (full length MmRXRA/BCDEF; SEQ ID NO: 53), VP16MmRXRCDEF (MmRXRCDEF; SEQ ID NO: 21), VP16MmRXRDEF (MmRXRDEF; SEQ ID NO: 22), VP16MmRXREF (MmRXREF; SEQ ID NO: 23), VP16MmRXRBam-EF ("MmRXRBam-EF" or "MmRXR-trancatedEF"; SEQ ID NO: 24), and VP16MmRXRAF2del ("MmRXRAF2del" or "MmRXR-E"; SEQ ID NO: 25) constracts were transfected into NIH3T3 cells along with GAL4CfEcRCDEF (constracted as in Example 1.1; Figure 13) or GAL4CfEcRDEF [constracted as in Example 1.1 except CfEcRCDEF was replaced with CfEcRDEF (SEQ ID NO: 3); Figure 14], pFRLUc and pTKRL plasmid DNAs as described above. The transfected ceUs were grown in the presence 0, 1, 5 and 25 uM of N- (2-ethyl-3-methoxybenzoyl)-N'-(3,5-dimethylbenzoyl)-N'-tert-butylhydrazine or PonA for 48 hr. The ceUs were harvested and lysed and reporter activity was measured in the cell lysate. Total fly luciferase relative light units are presented. The number on top of each bar is the maximum fold induction in that treatment. Of all the truncations of MmRXR tested, AppUcants' results show that the MmRXREF receptor was the best partner for CfEcR (Figures 13 and 14). CfEcRCDEF showed better induction than CfEcRDEF using MmRXREF. Deleting AF2 (abbreviated "EF- AF2del") or helices 1-3 of the E domain (abbreviated "EF-Bamdel") resulted in an RXR receptor that reduced gene induction and Ugand sensitivity when partnered wittø. either CfEcRCDEF (Figure 13) or CfEcRDEF (Figure 14) in NIH3T3 cells. In general, the CfEcR/RXR-based switch was much more sensitive to the non-steroid N-(2-ethyl-3- methoxybenzoyl)-N' -(3 ,5-dimethylbenzoyl)-N' -tert-butylhydrazine than to the steroid PonA. EXAMPLE 3 This Example describes AppUcants' further analysis of gene expression cassettes encoding truncated EcR or RXR receptor polypeptides that affect either ligand binding activity or ligand sensitivity, or both. Briefly, six different combinations of chimeric receptor pairs, constructed as described in Examples 1 and 2, were further analyzed in a single experiment in NIH3T3 ceUs. These six receptor pair combinations and their conesponding sample numbers are depicted in Table 7. Table 7 CfEcR + MmRXR Truncation Receptor Combinations in NIH3T3 Cells The above receptor construct pairs, along with the reporter plasmid pFRLuc were transfected into NIH3T3 cells as described above. The six CfEcR trancation receptor combinations were duplicated into two groups and treated with either steroid (odd numbers on x-axis of Figure 15) or non-steroid (even numbers on x-axis of Figure 15). In particular, the ceUs were grown in media containing 0, 1, 5 or 25 uM PonA (steroid) or N-(2-ethyl-3- methoxybenzoyl)-N'-(3,5-dimethylbenzoyl)-N'-tert-butylhydrazine (non-steroid) ligand. The reporter gene activity was measured and total RLU are shown. The number on top of each bar is the maximum fold induction for that treatment and is the mean of three repUcates. As shown in Figure 15, the CfEcRCDEF/MmRXREF receptor combinations were the best switch pairs both in terms of total RLU and fold induction (compare columns 1-6 to columns 7-12). This confirms Applicants' earUer findings as described in Example 2 (Figures 11-14). The same gene expression cassettes encoding the truncated EcR and RXR polypeptides were also assayed in a human lung carcinoma ceU Une A549 (ATCC) and similar results were observed (data not shown).

Claims: WE CLAIM: 1. A gene expression modulation system comprising : a) a first gene expression cassette that is capable of being expressed in a host cell comprising a polynucleotide sequence that encodes a first polypeptide comprising: i) a DNA-binding domain that recognizes a response element associated with a gene whose expression is to be modulated; i) a Ugand binding domain comprising a Ugand binding domain from a nuclear receptor; b) a second gene expression cassette that is capable of being expressed in the host ceU comprising a polynucleotide sequence that encodes a second polypeptide comprising: i) a transactivation domain; and n) a ligand binding domain comprising a Ugand binding domain from a nuclear receptor other than ultraspiracle (USP) ; wherein the transactivation domain is from a nuclear receptor other than an ecdysone receptor, a retinoid X receptor, or an ultraspiracle receptor; and wherein the Ugand binding domains from the first polypeptide and the second polypeptide are different and dimerize. 2. The gene expression modulation system according to claim 1 , further comprising a third gene expression cassette comprising: i) a response element to which the DNA-binding domain of the first polypeptide binds; ii) a promoter that is activated by the transactivation domain of the second polypeptide; and iii) the gene whose expression is to be modulated. 3. The gene expression modulation system according to claim 1 , wherein the ligand binding domain of the first polypeptide is an ecdysone receptor polypeptide. 4. The gene expression modulation system according to claim 1 , wherein the ligand binding domain of the second polypeptide is a retinoid X receptor polypeptide. 5. A gene expression modulation system comprising: a) a first gene expression cassette that is capable of being expressed in a host cell comprising a polynucleotide sequence that encodes a first polypeptide comprising: i) a DNA-binding domain that recognizes a response element associated with a gene whose expression is to be modulated; and U) a ligand binding domain comprising a Ugand binding domain from an ecdysone receptor; and b) a second gene expression cassette that is capable of being expressed in the host ceU comprising a polynucleotide sequence that encodes a second polypeptide comprising: i) a transactivation domain; and n) a ligand binding domain comprising a Ugand binding domain from a retinoid X receptor; wherein the Ugand binding domains from the first polypeptide and the second polypeptide are different and dimerize. 6. The gene expression modulation system according to claim 5, further comprising a third gene expression cassette comprising: i) a response element to which the DNA-binding domain of the first polypeptide binds; ii) a promoter that is activated by the transactivation domain of the second polypeptide; and in) the gene whose expression is to be modulated. 7. The gene expression modulation system according to claim 5, wherein the ligand binding domain of the first polypeptide is encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, and SEQ ID NO: 10. 8. The gene expression modulation system according to claim 5, wherein the ligand binding domain of the first polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, and SEQ ID NO: 20. 9. The gene expression modulation system according to claim 5, wherein the Ugand binding domain of the second polypeptide is encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 21, SJ3Q ID NO: 22, SEQ ID NO: 23, SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, and SEQ ID NO: 30. 10. The gene expression modulation system according to claim 5, wherein the ligand binding domain of the second polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, and SEQ ID NO: 40. 11. A gene expression modulation system comprising: a) a first gene expression cassette that is capable of being expressed in a host cell comprising a polynucleotide sequence that encodes a first polypeptide comprising: i) a DNA-binding domain that recognizes a response element associated with a gene whose expression is to be modulated; and U) a ligand binding domain comprising a Ugand binding domain from a retinoid X receptor; and b) a second gene expression cassette that is capable of being expressed in the host ceU comprising a polynucleotide sequence that encodes a second polypeptide comprising: i) a transactivation domain; and n) a ligand binding domain comprising a Ugand bmding domain from an ecdysone receptor; wherein the Ugand binding domains from the first polypeptide and the second polypeptide are different and dimerize. 12. The gene expression modulation system according to claim 11 , further comprising a third gene expression cassette comprising: i) a response element to which the DNA-binding domain of the first polypeptide binds; ii) a promoter that is activated by the transactivation domain of the second polypeptide; and ϋi) the gene whose expression is to be modulated. 13. The gene expression modulation system according to claim 11 , wherein the ligand binding domain of the first polypeptide is encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ EQ,NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, and SEQ ID NO: 30. 14. The gene expression modulation system according to claim 11 , wherein the ligand binding domain of the first polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, and SEQ ID NO: 40. 15. The gene expression modulation system according to claim 11 , wherein the ligand binding domain of the second polypeptide is encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, and SEQ ID NO: 10. 16. The gene expression modulation system according to claim 11 , wherein the ligand binding domain of the second polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, and SEQ ID NO: 20. 17. A gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide comprising a DNA-binding domain and an ecdysone receptor Ugand binding domain, wherein the DNA binding domain is from a nuclear receptor other than an ecdysone receptor. 18. The gene expression cassette according to claim 18, wherein the DNA-binding domain is a GAL4 DNA-binding domain or a LexA DNA-binding domain. 19. A gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide comprising a DNA-binding domain and a retinoid X receptor ligand binding domain, wherein the DNA binding domain is from a nuclear receptor other than a retinoid X receptor. 20. The gene expression cassette according to claim 19, wherein the DNA-binding domain is a GAL4 DNA-binding domain or a LexA DNA-binding domain. 21. A gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide comprising a transactivation domain and an ecdysone receptor Ugand binding domain, wherein the transactivation domain is from a nuclear receptor other than an ecdysone receptor. 22. The gene expression cassette according to claim 21, wherein the transactivation domain is a VP16 transactivation domain. 23. A gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide comprising a transactivation domain and a retinoid X receptor ligand binding domain, wherein the transactivation domain is from a nuclear receptor other than a retinoid X receptor. 24. The gene expression cassette according to claim 22, wherein the transactivation domain is a VP16 transactivation domain. 5 25. A gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide comprising a DNA-binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of a GAL4 DBD (SEQ ID NO: 41) or a LexA DBD (SEQ ID NO: 43) and an ecdysone receptor Ugand binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ 10 ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, and SEQ ID NO: 10. 26. A gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide comprising a DNA-binding domain comprising an amino acid sequence selected from the group consisting of a GAL4 DBD (SEQ ID NO: 42) or a LexA DBD (SEQ ID NO: 15 44) and an ecdysone receptor Ugand binding domain comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, and SEQ ID NO: 20. 27. A gene expression cassette comprising a polynucleotide encoding a hybrid 20 polypeptide comprising a DNA-binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of a GAL4 DBD (SEQ ID NO: 41) or a LexA DBD (SEQ ID NO: 43) and a retinoid X receptor ligand binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID 25 NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, and SEQ ID NO: 30. 28. A gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide comprising a DNA-binding domain comprising an amino acid sequence selected from the group consisting of a GAL4 DBD (SEQ ID NO: 42) or a LexA DBD (SEQ ID NO: 44) and a retinoid X receptor Ugand binding domain comprising an amino acid sequence 30 selected from the group consisting of SEQ ID NO: 31, SEQ ID NO: 32, SEQ.ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, and SEQ ID NO: 40. 29. A gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide comprising a transactivation domain encoded by a polynucleotide comprising a nucleic acid sequence of SEQ ID NO: 45 and an ecdysone receptor ligand binding domain encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, 5 SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, and SEQ ID NO: 10. 30. A gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide comprising a transactivation domain comprising an amino acid sequence of SEQ ID NO: 46 and an ecdysone receptor Ugand binding domain comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 10 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, and SEQ ID NO: 20. 31. A gene expression cassette comprising a polynucleotide encoding a hybrid polypeptide comprising a transactivation domain encoded by a polynucleotide comprising a nucleic acid sequence of SEQ ID NO: 45 and a retinoid X receptor ligand binding domain 15 encoded by a polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, and SEQ ID NO: 30. 32. A gene expression cassette comprising a polynucleotide encoding a hybrid 20 polypeptide comprising a transactivation domain comprising an amino acid sequence of SEQ ID NO: 46 and a retinoid X receptor Ugand binding domain comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, and SEQ ID NO: 40. 25 33. An isolated polynucleotide encoding an ecdysone receptor polypeptide or a retinoid X receptor polypeptide comprising a truncation mutation, wherein the trancation mutation reduces Ugand binding activity of the ecdysone receptor polypeptide or the retinoid X receptor polypeptide. 34. An isolated polynucleotide encoding an ecdysone receptor polypeptide or a 30 retinoid X receptor polypeptide comprising a truncation mutation, wherein the rancation mutation reduces steroid binding activity of the ecdysone receptor polypeptide or the retinoid X receptor polypeptide. 35. An isolated polynucleotide encoding an ecdysone receptor polypeptide or a retinoid X receptor polypeptide comprising a truncation mutation, wherein the trancation mutation reduces non-steroid binding activity of the ecdysone receptor polypeptide or the retinoid X receptor polypeptide. 36. An isolated polynucleotide encoding an ecdysone receptor polypeptide or a retinoid X receptor polypeptide comprising a truncation mutation, wherein the truncation mutation enhances Ugand binding activity of the ecdysone receptor polypeptide or the retinoid X receptor polypeptide. 37. An isolated polynucleotide encoding an ecdysone receptor polypeptide or a retiαoid X receptor polypeptide comprising a truncation mutation, wherein the truncation mutation enhances steroid binding activity of the ecdysone receptor polypeptide or the retinoid X receptor polypeptide. 38. An isolated polynucleotide encoding an ecdysone receptor polypeptide or a retinoid X receptor polypeptide comprising a trancation mutation, wherein the truncation mutation enhances non-steroid binding activity of the ecdysone receptor polypeptide or the retinoid X receptor polypeptide. 39. An isolated polynucleotide encoding a retinoid X receptor polypeptide comprising a truncation mutation, wherein the truncation mutation increases Ugand sensitivity of the retinoid X receptor polypeptide. 40. An isolated polynucleotide encoding a retinoid X receptor polypeptide comprising a trancation mutation, wherein the truncation mutation increases Ugand sensitivity of a heterodimer, wherein the heterodimer comprises said retinoid X receptor polypeptide and a dimerization partner. 41. The isolated polynucleotide according to clai 40, wherein the dimerization partner is an ecdysone receptor polypeptide. 42. An isolated polynucleotide encoding a truncated ecdysone receptor polypeptide, wherein the polynucleotide comprises a nucleic acid sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, and SEQ ID NO: 10. 43. An isolated polypeptide encoded by the isolated polynucleotide according to claim 42. 44. An isolated truncated ecdysone receptor polypeptide comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, and SEQ ID NO: 20. 45. An isolated polynucleotide encoding a truncated retinoid X receptor polypeptide, wherein the polynucleotide comprises a nucleic acid sequence selected from the group consisting of SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, SEQ ID NO: 24, SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, and SEQ ID NO: 30. 46. An isolated polypeptide encoded by the isolated polynucleotide according to claim 45. 47. An isolated truncated retinoid X receptor polypeptide comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 31 , SEQ ID NO: 32, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, and SEQ ID NO: 40. 48. A method of modulating the expression of a gene in a host cell comprising the gene to be modulated comprising the steps of: a) introducing into the host ceU the gene expression modulation system according to claim 1; and b) introducing into the host ceU a ligand that independently combines with the ligand binding domains of the first polypeptide and the second polypeptide; wherein the gene to be expressed is a component of a chimeric gene comprising: i) a response element to which the DNA binding domain from the first polypeptide binds; ii) a promoter that is activated by the transactivation domain of the second polypeptide; and iii) a gene whose expression is to be modulated, whereby a complex is formed comprising the Ugand, the first polypeptide, and the second polypeptide, and whereby the complex modulates expression of the gene in the host cell. 49. The method according to claim 48, wherein the ligand is a compound of the formula: wherein: E is a (C 4 -C6)alkyl containing a tertiary carbon or a cyano(C3-C 5 )alkyl containing a tertiary carbon; R 1 is H, Me, Et, i-Pr, F, formyl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 C1, CH 2 OH, CH 2 OMe, CH 2 CN, CN, C°CH, 1-proρynyl, 2-propynyl, vinyl, OH, OMe, OEt, cyclopropyl, CF 2 CF 3 , CH=CHCN, allyl, azido, SCN, or SCHF 2 ; R 2 is H, Me, Et, n-Pr, i-Pr, formyl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 C1, CH 2 OH, CH 2 OMe, CH 2 CN, CN, C°CH, 1-propynyl, 2-propynyl, vinyl, Ac, F, CI, OH, OMe, OEt, O-n- Pr, OAc, NMez, NEt 2 , SMe, SEt, SOCF 3 , OCF 2 CF 2 H, COEt, cyclopropyl, CF 2 CF 3 , CH=CHCN, allyl, azido, OCF 3 , OCHF 2 , O-i-Pr, SCN, SCHF 2 , SOMe, NH-CN, or joined with R 3 and the phenyl carbons to which R 2 and R 3 are attached to form an ethylenedioxy, a dihydrofuryl ring with the oxygen adjacent to a phenyl carbon, or a dihydropyryl ring with the oxygen adjacent to a phenyl carbon; R 3 is H, Et, or joined with R 2 and the phenyl carbons to which R 2 and R 3 are attached to form an ethylenedioxy, a dihydrofuryl ring with the oxygen adjacent to a phenyl carbon, or a dihydropyryl ring with the oxygen adjacent to a phenyl carbon; R 4 , R 5 , and R 6 are independently H, Me, Et, F, CI, Br, formyl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 C1, CH 2 OH, CN, C°CH, 1-propynyl, 2-propynyl, vinyl, OMe, OEt, SMe, or SEt. 50. A method of modulating the expression of a gene in a host cell comprising the gene to be modulated comprising the steps of: a) introducing into the host cell the gene expression modulation system of claim 5; and b) introducing into the host ceU a ligand that independently combines with the ligand binding domains of the first polypeptide and the second polypeptide; wherein the gene to be expressed is a component of a chimeric gene comprising: i) a response element to which the DNA binding domain from the first polypeptide binds; ii) a promoter that is activated by the transactivation domain of the second polypeptide; and in) a gene whose expression is to be modulated, whereby a complex is formed comprising the Ugand, the first polypeptide, and the second polypeptide, and whereby the.complex modulates expression of the gene in the host cell. 51. The method according to claim 50, wherein the ligand is a compound of the formula: wherein: E is a (C 4 -C 6 )alkyl containing a tertiary carbon or a cyano(C 3 -Cs)alkyl containing a tertiary carbon; R 1 is H, Me, Et, i-Pr, F, formyl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 C1, CH 2 OH, CH 2 OMe, CH 2 CN, CN, C°CH, 1-proρynyl, 2-propynyl, vinyl, OH, OMe, OEt, cyclopropyl, CF 2 CF 3 , CH=CHCN, allyl, azido, SCN, or SCHF 2 ; R 2 is H, Me, Et, n-Pr, i-Pr, formyl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 C1, CH 2 OH, CH 2 OMe, CH 2 CN, CN, C°CH, 1-proρynyl, 2-propynyl, vinyl, Ac, F, CI, OH, OMe, OEt, O-n- Pr, OAc, NMej, NEt 2 , SMe, SEt, SOCF 3 , OCF 2 CF 2 H, COEt, cyclopropyl, CF 2 CF 3 , CH=CHCN, allyl, azido, OCF 3 , OCHF 2 , O-i-Pr, SCN, SCHF 2 , SOMe, NH-CN, or joined with R 3 and the phenyl carbons to which R 2 and R 3 are attached to form an ethylenedioxy, a dihydrofuryl ring with the oxygen adjacent to a phenyl carbon, or a dihydropyryl ring with the oxygen adjacent to a phenyl carbon; R 3 is H, Et, or joined with R 2 and the phenyl carbons to which R 2 and R 3 are attached to form an ethylenedioxy, a dihydrofuryl ring with the oxygen adjacent to a phenyl carbon, or a dihydropyryl ring with the oxygen adjacent to a phenyl carbon; R 4 , R 5 , and R 6 are independently H, Me, Et, F, CI, Br, formyl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 C1, CH 2 OH, CN, C°CH, 1-propynyl, 2-propynyl, vinyl, OMe, OEt, SMe, or SEt. 52. A method of modulating the expression of a gene in a host cell comprising the gene to be modulated comprising the steps of: a) introducing into the host cell the gene expression modulation system of claim 11; and b) introducing into the host ceU a ligand that independently combines with the ligand binding domains of the first polypeptide and the second polypeptide; wherein the gene to be expressed is a component of a chimeric gene comprising: i) a response element to which the DNA binding domain from the first polypeptide binds; ii) a promoter that is activated by the transactivation domain of the second polypeptide; and iu) a gene whose expression is to be modulated, whereby a complex is formed comprising the Ugand, the first polypeptide, and the second polypeptide, and whereby the complex modulates expression of the gene in the host cell. 53. The method according to claim 52, wherein the ligand is a compound of the formula: wherein: E is a (C 4 -C 6 )alkyl containing a tertiary carbon or a cyano(C 3 -C5)alkyl containing a tertiary carbon; R 1 is H, Me, Et, i-Pr, F, formyl, CF 3 , CHF 2 , CHC1 2> CH 2 F, CH 2 C1, CH 2 OH, CH 2 OMe, CH 2 CN, CN, C°CH, 1-propynyl, 2-propynyl, vinyl, OH, OMe, OEt, cyclopropyl, CF 2 CF 3 , CH=CHCN, allyl, azido, SCN, or SCHF 2 ; R 2 is H, Me, Et, n-Pr, i-Pr, formyl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 C1, CH 2 OH, CH 2 OMe, CH 2 CN, CN, C°CH, 1-propynyl, 2-propynyl, vinyl, Ac, F, CI, OH, OMe, OEt, O-n- Pr, OAc, NMe^, NEt 2 , SMe, SEt, SOCF 3 , OCF 2 CF 2 H, COEt, cyclopropyl, CF 2 CF 3 , CH=CHCN, allyl, azido, OCF 3 , OCHF 2 , O-i-Pr, SCN, SCHF 2 , SOMe, NH-CN, or joined with R 3 and the phenyl carbons to which R 2 and R 3 are attached to form an ethylenedioxy, a dihydrofuryl ring with the oxygen adjacent to a phenyl carbon, or a dihydropyryl ring with the oxygen adjacent to a phenyl carbon; R 3 is H, Et, or joined with R 2 and the phenyl carbons to which R 2 and R 3 are attached to form an ethylenedioxy, a dihydrofuryl ring with the oxygen adjacent to a phenyl carbon, or a dihydropyryl ring with the oxygen adjacent to a phenyl carbon; R 4 , R 5 , and R 6 are independently H, Me, Et, F, CI, Br, formyl, CF 3 , CHF 2 , CHC1 2 , CH 2 F, CH 2 C1, CH 2 OH, CN, C°CH, 1-propynyl, 2-propynyl, vinyl, OMe, OEt, SMe, or SEt. 54. An isolated host ceU into which the gene expression modulation system according to claim 1 has been introduced. 55. The isolated host ceU according to claim 54, wherein the host cell is selected from the group consisting of a bacterial cell, a fungal ceU, a yeast cell, a plant cell, an animal cell, and a mammaUan cell. 56. The isolated host cell according to claim 55, wherein the host cell is a plant ceU, a murine ceU, or a human cell. 57. An isolated host ceU into which the gene expression modulation system according to claim 5 has been introduced. 58. The isolated host ceU according to claim 57, wherein the host cell is selected from the group consisting of a bacterial cell, a fungal ceU, a yeast cell, a plant cell, an animal cell, and a mammaUan cell. 59. The isolated host ceU according to claim 58, wherein the host cell is a plant ceU, a murine ceU, or a human cell. 60. An isolated host ceU into which the gene expression modulation system according to claim 11 has been introduced. 61. The isolated host ceU according to claim 60, wherein the host cell is selected from the group consisting of a bacterial ceU, a fungal ceU, a yeast cell, a plant cell, an animal cell, and a mammaUan ceU. 62. The isolated host cell according to claim 61 , wherein the host cell is a plant ceU, a murine ceU, or a human cell. 63. A non-human organism comprising a host cell into which the gene expression modulation system according to claim 1 has been introduced. 64. The non-human organism according to claim 63 , wherein the aon-human organism is selected from the group consisting of a bacterium, a fungus, a yeast, a plant, an animal, and a mammal. 65. The non-human organism according to claim 64, wherein the non-human organism is selected from the group consisting of a plant, a mouse, a rat, a rabbit, a cat, a dog, a bovine, a goat, a pig, a horse, a sheep, a monkey, and a chimpanzee. 66. A non-human organism comprising a host cell into which the gene expression modulation system according to claim 5 has been introduced. 67. The non-human organism according to claim 66, wherein the non-human organism is selected from the group consisting of a bacterium, a fungus, a yeast, a plant, an animal, and a mammal. 68. The non-human organism according to claim 67, wherein the non-human organism is selected from the group consisting of a plant, a mouse, a rat, a rabbit, a cat, a dog, a bovine, a goat, a pig, a horse, a sheep, a monkey, and a chimpanzee. 69. A non-human organism comprising a host ceU into which the gene expression modulation system according to claim 11 has been introduced. 70. The non-human organism according to claim 69, wherein the non-human organism is selected from the group consisting of a bacterium, a fungus, a yeast, a plant, an animal, and a mammal. 71. The non-human organism according to claim 70, wherein the non-human organism is selected from the group consisting of a plant, a mouse, a rat, a rabbit, a cat, a dog, a bovine, a goat, a pig, a horse, a sheep, a monkey, and a chimpanzee.

Inventors: CRESS DEAN ERVIN, KAPITSKAYA MARIANNA ZINOVJEVNA, PALLI SUBBA REDDY, CRESS, DEAN, ERVIN, KAPITSKAYA, MARIANNA ZINOVJEVNA, PALLI, SUBBA, REDDY
Applicants: CRESS DEAN ERVIN, KAPITSKAYA MARIANNA ZINOVJEVNA, PALLI SUBBA REDDY, ROHM & HAAS, CRESS, DEAN, ERVIN, KAPITSKAYA, MARIANNA ZINOVJEVNA, PALLI, SUBBA, REDDY, ROHM AND HAAS COMPANY
Application year: 2001
Main classification: C07K14
IPC codes: A01H5, A01K67, A61K38, A61K48, C07K14, C12N1, C12N5, C12N15