Title of Invention	NUCLEIC ACID ENCODING THYMIDINE KINASE
Abstract	A polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the nucleotides corresponding to the splice donor site nucleotides is replaced by another nucleotide and wherein the nucleotides of the splice acceptor sites are not altered.

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Thymidine Kinase Field of the Invention The present invention relates to a nucleic acid encoding a substantially non-splicing thymidine kinase gene and its use in therapy. Background to the Invention There is considerable interest in the use of metabolic suicide genes in gene therapy. Numerous suicide genes are described in the literature, such as, for example, the genes coding for cytosine deaminase, purine nucleoside phosphorylase or a thymidine kinase. Among these genes, the gene coding for herpes simplex virus type 1 thymidine kinase (HSV-tk) is especially advantageous from a therapeutic standpoint. The HSV-tk gene expresses an enzyme, which when used in combination with the nucleoside analogue ganciclovir, is capable of specifically eliminating dividing cells. Of particular importance is the presence of a propagated toxicity effect ("bystander" effect) which is associated with the use of HSV-tk/ganciclovir. This effect manifests itself in the destruction not only of the cells which have incorporated the thymidine kinase (tk) gene, but also the neighbouring cells. The mechanism of action of the HSV-tk/ganciclovir system, may be outlined as follows: mammalian cells modified to express the TK en2yme implement the first step of phosphorylation of ganciclovir to yield ganciclovir monophosphate. Subsequently, cellular kinases enable this ganciclovir monophosphate to be metabolised successively to diphosphate and then triphosphate. The ganciclovir triphosphate thus generated then produces toxic effects by becoming incorporated in the DNA, and partially inhibits the cellular DNA polymerase alpha, thereby causing DNA synthesis to be stopped and hence leading to cell death (Moolten, 1986; Mullen, 1994). The HSV-tk/ganciclovir system can be used in a large number of therapeutic applications and numerous clinical trials have been implemented in the last decade. Methods of using the HSV-tk gene in gene therapy are disclosed in, for example, WO90/07936, US 5,837,510, US 5,861,290, WO 98/04290, WO 97/37542 and US 5,631,236. One interesting application of the HSV-tk/ganciclovir system is in the prevention of graft-versus-host disease (GvHD), a condition that can interfere with the outcome of allogeneic bone marrow transplantation, the treatment of choice for many hematological malignancies. GvHD occurs when T-cells in the transplanted stem cell graft begin to attack the recipients body. Removal of T-cells from the graft may prevent GvHD but also favours disease recurrence and graft rejection. To counter these effects, allogeneic bone marrow transplant patients can be treated by introducing donor T lymphocytes after a delay following the allogeneic bone marrow transplant Transferring the HSV-tk gene to donor T lymphocytes allows their eradication after ganciclovir administration in case of the emergence of GvHD. In one trial, patients received a T-cell depleted bone marrow transplantation together with increasing doses of donor lymphocytes transduced with the HSV-tk gene (Tiberghien et al., 2001). Circulating HSV-tk-expressing cells could be detected for more than one year after engraftment in all patients. Six out of the twelve patients developed GvHD and received ganciclovir, substantially reducing the number of circulating modified cells (85% average decrease in absolute number). Mutants in the HSV-tk gene have been made which increase its biological activity. Examples of such mutant HSV-tk genes are described in, for example, Kokoris et al (1999), WO 95/30007, US 5,877,010, WO 99/19466 and WO 95/14102. However, a serious problem associated with the thymidine kinase/ganciclovir system is the emergence of ganciclovir resistance in HSV-tk transduced cells. This is of particular importance, since the relative proportion of cells which are resistant to ganciclovir may rapidly increase through the course of treatment. The presence of ganciclovir resistance in a lymphoblastoid human T-cell line transduced with a retroviral vector containing the HSV-tk gene was found to be associated with a 227 base pair deletion in the HSV-tk gene (Fillat et al., 2003). The same deletion was also present in human primary T cells transduced with the vector and in 12 patients who received transduced donor T cells (Garin et al., 2001). WO 01/79502 discloses that the cause of this deletion is believed to be due to the presence of nucleotide sequences in the HSV-tk mRNA which act as splice sites to cause the production of a proportion of virus particles carrying an aberrant form of the gene, the remainder carrying the full length gene. A mutant of the thymidine kinase gene is disclosed in WO 01/79502 and in Chalmers 2001, Molecular Therapy 4:146-148, in which the splice sites are removed, and which reduces the production of the aberrant form of the thymidine kinase gene. However, this mutant is still associated with a detrimental amount of gene splicing. CD34-tk fusion constructs are disclosed in Rettig et al., 2003. These fusion constructs contain modified HSV-tk genes. However, there is no demonstration that the modified genes reduce splicing of the HSV-tk mRNA. Thus, there remains a need for a modified thymidine kinase gene that is not susceptible to gene splicing and which addresses the problems associated with ganciclovir resistance. Summary of the Invention The present invention overcomes the aforementioned problem by providing a HSV-tk gene that expresses a greater proportion of unspliced mRNA over previously disclosed HSV-tk genes. We have identified a splice acceptor site 14 base pairs upstream of the previously identified acceptor site. We show that previous attempts to inhibit splicing by modification of the previously identified donor and acceptor sites can result in activation of this acceptor site and subsequent gene splicing. We surprisingly demonstrate that the modification of the HSV-tk gene at certain specific nucleotides provides a HSV-tk gene that is capable of expressing substantially splice-free mRNA. In particular, we show that mutating at least one of the nucleotides at the splice donor site can substantially prevent splicing. Statements of the Invention According to a first aspect of the present invention there is provided a polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the nucleotides corresponding to splice donor site nucleotides is replaced by another nucleotide and wherein the nucleotides of the splice acceptor sites are not altered. Preferably, at least one of the nucleotides corresponding to splice donor site nucleotides at positions 329 and 330 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide and wherein the nucleotides of the splice acceptor sites are not altered. In one embodiment, the nucleotide at position 330 is changed from T to C. According to a second aspect of the present invention there is provided a polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the nucleotides corresponding to splice donor site nucleotides at positions 329 and 330 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide and wherein the nucleotides of the splice acceptor sites are not altered. In one embodiment both the nucleotides corresponding to splice donor site nucleotides at positions 329 and 330 of the wild type sequence in Figure 1 are replaced by another nucleotide. In one embodiment the nucleotide at position 330 is changed from T to C. According to a third aspect of the present invention there is provided a polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the nucleotides corresponding to the splice donor site nucleotides at positions 329 and 330 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide and wherein at least one of the nucleotides corresponding to splice acceptor site nucleotides at positions 554 and 555 and at least one of the nucleotides corresponding to splice acceptor site nucleotides at positions 662 and 663 of the wild type sequence in Figure 1 is not altered. Preferably both the nucleotides corresponding to positions 554 and 555 are not altered. Preferably both the nucleotides corresponding to positions 662 and 663 are not altered. In one embodiment both the nucleotides corresponding to splice donor site nucleotides at positions 329 and 330 of the wild type sequence in Figure 1 are replaced by another nucleotide. In one embodiment the nucleotide at position 330 is changed from T to C. According to a fourth aspect of the present invention there is provided a polynucleotide comprising the TkMut2 sequence in Figure 1. According to a fifth aspect of the present invention there is provided a polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the splice acceptor nucleotides corresponding to the nucleotide(s) at positions 329 and 330 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide and wherein at least one of the nucleotides corresponding to the splice acceptor site nucleotides at positions 541 and 542 of the wild type sequence in Figure 1 is replaced by another nucleotide. In one embodiment both the nucleotides corresponding to splice donor site nucleotides at positions 329 and 330 of the wild type sequence in Figure 1 are replaced by another nucleotide. In one embodiment the nucleotide corresponding to position 330 is changed from T to C. In another embodiment both the nucleotides corresponding to splice acceptor site nucleotides at positions 541 and 542 of the wild type sequence in Figure 1 are replaced by another nucleotide. In another embodiment the nucleotide corresponding to the nucleotide at position 541 is changed from A to T. In another embodiment the nucleotide corresponding to the nucleotide at position 542 is changed from G to C. In another embodiment at least one of the nucleotides corresponding to the splice acceptor site nucleotides at position 554 and 555 of the wild type sequence in Figure 1 is also replaced by another nucleotide such that there is no splice site present. In one embodiment both the nucleotides corresponding to splice acceptor site nucleotides at positions 541 and 542 of the wild type sequence in Figure 1 are replaced by another nucleotide such that there is no splice site present In another embodiment the nucleotide corresponding to the nucleotide at position 555 is changed from G to A. According to a sixth aspect of the present invention there is provided a polynucleotide comprising the TkMut23 sequence in Figure 1. According to a seventh aspect of the present invention there is provided a polynucleotide comprising the TkMut234 sequence in Figure 1. According to an eighth aspect of the present invention there is provided a polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the splice acceptor site nucleotides corresponding to the nucleotide(s) at positions 329 and 330 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide and wherein the nucleotides corresponding to the splice acceptor site nucleotides at positions 554 and 555 of the wild type sequence in Figure 1 is replaced by another nucleotide. In one embodiment both the nucleotides corresponding to splice donor site nucleotides at positions 329 and 330 of the wild type sequence in Figure 1 are replaced by another nucleotide. In another embodiment both the nucleotides corresponding to splice acceptor site nucleotides at positions 554 and 555 of the wild type sequence in Figure 1 are replaced by another nucleotide. Preferably the nucleotide corresponding to the nucleotide at position 330 is changed from T to C. Preferably the nucleotide corresponding to the nucleotide at position 555 is changed from G to A. According to a ninth aspect of the present invention there is provided a polynucleotide comprising the Tk-Mut24 sequence in Figure 1. According to a tenth aspect of the present invention there is provided a polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the nucleotides corresponding to splice acceptor site nucleotides at position 554 and 555 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide. In one embodiment both the nucleotides corresponding to splice donor site nucleotides at positions 554 and 555 of the wild type sequence in Figure 1 are replaced by another nucleotide. In one embodiment the nucleotide corresponding to the nucleotide at position 555 is changed from G to A. In one embodiment at least one of the splice acceptor site nucleotides corresponding to the nucleotides at positions 541 and 542 of the wild type sequence in Figure 1 is also replaced by another nucleotide such that there is no splice site present. In another embodiment both the nucleotides corresponding to splice acceptor site nucleotides at positions 541 and 542 of the wild type sequence in Figure 1 are also replaced by another nucleotide such that there is no splice site present. In another embodiment the nucleotide corresponding to the nucleotide at position 541 is changed from A to T. In another embodiment the nucleotide corresponding to the nucleotide at position 542 is changed from G to C. According to an eleventh aspect of the present invention there is provided a polynucleotide comprising the TkMut4 sequence in Figure 1. According to a twelfth aspect of the present invention there is provided a polynucleotide comprising the TkMut34 sequence in Figure 1. Preferably the replacement nucleotide does not alter the sequence of the polypeptide encoded by said nucleotide sequence. According to a thirteenth aspect of the present invention there is provided a vector comprising a polynucleotide of the present invention. Preferably, the vector is an expression vector. According to a fourteenth aspect of the present invention there is provided a host cell comprising a polynucleotide or a vector of the present invention. According to a fifteenth aspect of the present invention there is provided a pharmaceutical composition comprising a polynucleotide, a vector or a host cell of the present invention, and a pharmaceutically acceptable carrier. According to a sixteenth aspect of the present invention there is provided a kit comprising (i) a polynucleotide, a vector, a host cell or a pharmaceutical composition of the present invention; and (i) a substantially non-toxic agent which is converted by thymidine kinase into a toxic agent. According to a seventeenth aspect of the present invention there is provided a polynucleotide, a vector, a host cell or a pharmaceutical composition of the present invention for use in medicine. According to an eighteenth aspect of the present invention there is provided products containing a polynucleotide, a vector, a host cell or a pharmaceutical composition of the present invention, and a substantially non-toxic agent for simultaneous, separate or sequential use in treating a patient with cells in need of destruction, wherein said substantially non-toxic agent is converted by thymidine kinase to a toxic agent. According to a nineteenth aspect of the present invention there is provided a method of destroying cells comprising (i) introducing into said cells a polynucleotide or a vector of the present invention; and (ii) simultaneously, separately or sequentially contacting said cells with a substantially non-toxic agent which is converted by thymidine kinase to a toxic agent. According to a twentieth aspect of the present invention there is provided a method of destroying cells comprising (i) introducing into said cells a polynucleotide or a vector of the present invention; (ii) allowing said cells to express thymidine kinase; and (ii) contacting said cells with a substantially non-toxic agent which is converted by thymidine kinase to a toxic agent According to a twenty first aspect of the present invention there is provided a method of treating a patient with cells in need of destruction comprising (i) introducing into the patient a polynucleotide, a vector or a pharmaceutical composition of the present invention; (ii) simultaneously, separately or sequentially introducing into the patient a substantially non-toxic agent which is converted by thymidine kinase to a toxic agent According to a twenty second aspect of the present invention there is provided a method of treating a patient with cells in need of destruction comprising (i) introducing into the patient a polynucleotide, a- vector or a pharmaceutical composition of the present invention; (ii) allowing said polynucleotide or vector to be taken up by said cells; (iii) allowing said cells to express thymidine kinase; and (iv) introducing into the patient a substantially non-toxic agent which is converted by thymidine kinase to a toxic agent According to a twenty third aspect of the present invention there is provided a method of treating a patient with cells in need of destruction comprising (i) removing the cells from the patient or donor cells; (ii) introducing into the cells ex vivo a polynucleotide or a vector of the present invention; (iii) introducing the modified cells into the patient; (iv) allowing the cells to express thymidine kinase; and (v) administering to the patient a substantially non-toxic agent which is converted by thymidine kinase into a toxic agent. According to a twenty fourth aspect of the present invention there is provided a method of preventing graft-versus-host disease in a patient comprising: (i) administering to a host T-cells genetically engineered to include a polynucleotide of the present invention or a vector of the present invention; and (ii) administering to said host, prior to the occurrence of graft-versus-host disease, a substantially non-toxic agent in an amount effective to kill said genetically engineered T-cells through interaction of said agent with thymidine kinase. Preferably the substantially non-toxic agent used in the present invention is any one of ganciclovir, acyclovir, triflurothymidine, l-[2-deoxy, 2-fluoro, β-D-arabino furanosyl]-5-iodouracil, ara-A, ara 1, 1-β-D arabino furanosyl thymine, 5-ethyl-2'deoxyurine, 5-iodo-5'-amino-2, 5'-dideoxyuridine, idoxuridine, AZT, AIV, dideoxycytidine, Ara C and bromovinyl deoxyuridine (BVDU). According to a twenty fifth aspect of the present invention there is provided use of a polynucleotide, a vector or a host cell of the present invention in the preparation of a medicament for destroying cells in a patient. According to a twenty sixth aspect of the present invention use of polynucleotide, a vector or a host cell of the present invention in the preparation of a medicament for the treatment of cancer. Detailed description Various preferred features and embodiments of the present invention will now be described by way of non-limiting examples. The practice of the present invention will employ, unless otherwise indicated, conventional techniques of chemistry, molecular biology, microbiology, recombinant DNA and immunology, which are within the capabilities of a person of ordinary skill in the art. Such techniques are explained in the literature. See, for example, J. Sambrook, E. F. Fritsch, and T. Maniatis, 1989, Molecular Cloning: A Laboratoiy Manual, Second Edition, Books 1-3, Cold Spring Harbor Laboratory Press; Ausubel, F. M. et al. (1995 and periodic supplements; Cwrent Protocols in Molecular Biology, ch 9, 13, and 16, John Wiley & Sons, New York, N.Y.); B. Roe, J. Crabtree, and A. Kahn, 1996, DNA Isolation and Sequencing: Essential Techniques, John Wiley & Sons; J. M. Polak and James O'[T. McGee, 1990, In Situ Hybridization: Principles and Practice; Oxford University Press; M. J. Gait (Editor), 1984, Oligonucleotide Synthesis: A Practical Approach, Irl Press; D. M. J. Lilley and J. E. Dahlberg, 1992, Methods of Enzymology: DNA Structure Part A: Synthesis and Physical Analysis of DNA Methods in Enzymology, Academic Press; and E. M. Shevach and W. Strober, 1992 and periodic supplements, Cwrent Protocols in Immunology, John Wiley & Sons, New York, NY. Each of these general texts is herein incorporated by reference. HSV-tk gene As used herein, the term "not altered" means not altered from the wild type sequence (Tkwt). As used herein, the term "replaced by another nucleotide" means replaced by a nucleotide that differs from the wild type sequence (Tk wt). The replacements are made such that the relevant splice donor site or splice acceptor sites are removed. Thymidine kinase mutants of the present invention may be prepared from a wide variety of thymidine kinases. Preferably, the thymidine kinase mutant is derived from Herpesviridae thymidine kinase including for example both primate herpesviruses, and nonprimate herpesviruses such as avian herpesviruses. Representative examples of suitable herpesviruses include Heipes Simplex Virus Type 1 (McKnight et al., 1980), Herpes Simplex Virus Type 2 (Swain and Galloway, 1983), Varicella Zoster Virus (Davidson and Scott, 1986), marmoset herpesvirus (Otsuka and Kit, 1984), feline herpesvirus type 1 (Nunberg et al.„ 1989), pseudorabies virus (Kit and Kit, U.S. Patent No. 4,514,497, 1985), equine herpesvirus type 1 (Robertson and Whalley, 1988), bovine herpesvirus type 1 (Mittal and Field, 1989), turkey herpesvirus (Martin et al., 1989), Marek's disease virus (Scott et al., 1989), herpesvirus saimiri (Honess et al., 1989) and Epstein-Barr virus (Baer et ah, 1984). Such herpesviruses may be readily obtained from commercial sources such as the American Type Culture Collection ("ATCC", RockviUe, Maryland). Deposits of certain of the above-identified herpesviruses may be readily obtained from the ATCC, for example: ATCC No. VR-539 (Herpes simplex type 1); ATCC Nos. VR-734 and VR-540 (Herpes Simplex type 2); ATCC NO. VR-586 (Varicella Zoster Virus); ATCC No. VR-783 (Infectious laryngothracheitis); ATCC Nos. VR-624, VR-9S7, VR-2103, VR-2001, VR-2002, VR-2175, VR-585 (Marek's disease virus); ATCC Nos. VR-584B and VR-584B (turkey herpesvirus); ATCC Nos. VR-631 and VR-842 (bovine herpesvirus type 1); and ATCC Nos. VR-2003, VR-2229 and VR-700 (equine herpesvirus type 1). Herpesviruses may also be readily isolated and identified from naturally occurring sources (e.g., from an infected animal). The thymidine kinase gene may be readily isolated and mutated as described below, in order to construct nucleic acid molecules encoding a thymidine kinase gene comprising one or more mutations which substantially reduce the splicing of the gene, as compared to unmutated thymidine kinase. As utilised herein, it should be understood that "unmutated thymidine kinase" refers to native or wild-type thymidine kinase such as that described by McKnight et al. (NucL Acids Res. 5:5949-5964, 1980). It should be noted that in this application nucleotide positions are referred to by reference to a position in Figure 1. However, when such references are made, it will be understood that the invention is not to be limited to the exact sequence as set out in the figure but includes variants and derivatives thereof. Thus, identification of nucleotide locations in other thymidine kinase sequences are contemplated (i.e., nucleotides at positions which the skilled person would consider correspond to the positions identified in Figure 1). The person skilled in the art can readily align similar sequences and locate the same nucleotide locations. Construction of HSV-tk mutants Thymidine kinase mutants of the present invention may be constructed using a variety of techniques. For example, mutations may be introduced at particular loci by synthesising oligonucleotides containing a mutant sequence, flanked by restriction sites enabling ligation to fragments of the native sequence. Following ligation, the resulting reconstructed sequence encodes a derivative having the desired amino acid insertion, substitution, or deletion. Alternatively, oligonucleotide-directed site-specific (or segment specific) mutagenesis procedures may be employed to provide an altered gene having particular codon altered according to the substitution, deletion, or insertion required. Deletion or truncation derivatives of thymidine kinase mutants may also be constructed by utilising convenient restriction endonuclease sites adjacent to the desired deletion. Subsequent to restriction, overhangs may be filled in, and the DNA religated. Exemplary methods of making the alterations set forth above are disclosed by Sambrook et al. (Molecular cloning: A Laboratoiy Manual, 2d Ed., Cold Spring Harbor Laboratory Press, 1989). Thymidine kinase mutants may also be constructed utilising techniques of PCR mutagenesis, chemical mutagenesis, chemical mutagenesis (Drinkwater and Klinedinst, 1986) by forced nucleotide misincorporation (e.g., Liao and Wise, 1990), or by use of randomly mutagenised oligonucleotides (Horwitz et al., 1989). In a preferred embodiment of the present invention, the nucleotides are modified taking note of the genetic code such that a codon is changed to a degenerate codon which codes for the same amino acid residue. In this way, it is possible to make coding regions of the protein of interest which encode wild type protein but which do not contain a functional splice site. Splice sites The proportion of RNA which is removed (or "spliced out") during splicing is typically called an intron, and the two pieces of RNA either side of the intron that are joined by splicing are typically called exons (Figure 2). A splice donor site is a site in RNA which lies at the 5' side of the RNA which is removed during the splicing process and which contains the site which is cut and rejoined to a nucleotide residue within a splice acceptor site. Thus, a splice donor site is the junction between the end of an exon and the start of the intron, typically terminating in the dinucleotide GU. In a preferred embodiment of the present invention, one or both of the terminal GU dinucleotides (or GT dinucleotides in the corresponding DNA sequence) of the splice donor site is/are altered to remove the splice site. A splice acceptor site is a site in RNA which lies at the 3' side of the RNA which is removed during the splicing process and which contains the site which is cut and rejoined to a nucleotide residue within a splice donor site. Thus, a splice acceptor site is the junction between the end of an intron (typically terminating with the dinucleotide AG) and the start of the downstream exon. In a preferred embodiment of the present invention, one or both of the terminal AG dinucleotides of the splice acceptor site is/are altered to remove the splice site. Polynucleotides Polynucleotides used in the invention may comprise DNA or RNA. They may be single-stranded or double-stranded. It will be understood by a skilled person that numerous different polynucleotides can encode the same polypeptide as a result of the degeneracy of the genetic code. In addition, it is to be understood that skilled persons may, using routine techniques, make nucleotide substitutions that do not affect the polypeptide sequence encoded by the polynucleotides used in the invention to reflect the codon usage of any particular host organism in which the polypeptides are to be expressed. The polynucleotides may be modified by any method available in the art. Such modifications may be carried out in order to enhance the in vivo activity or life span of polynucleotides of the invention. The polynucleotides used in the invention may encode fusion proteins for example to aid in cellular secretion of the expressed polypeptides. Where, for example, the HSV-tk polypeptide is desired to be expressed from the cell, it may be desirable to add targeting sequences to target the proteins to particular cell compartments or to secretory pathways. Such targeting sequences have been extensively characterised in the art. Polynucleotides such as DNA polynucleotides may be produced recombinantly, synthetically, or by any means available to those of skill in the art. They may also be cloned by standard techniques. Longer polynucleotides will generally be produced using recombinant means, for example using PCR (polymerase chain reaction) cloning techniques. This will involve making a pair of primers (e.g. of about 15 to 30 nucleotides) flanking a region of the lipid targeting sequence which it is desired to clone, bringing the primers into contact with mRNA or cDNA obtained from an animal or human cell, performing a polymerase chain reaction under conditions which bring about amplification of the desired region, isolating the amplified fragment (e.g. by purifying the reaction mixture on an agarose gel) and recovering the amplified DNA. The primers may be designed to contain suitable restriction enzyme recognition sites so that the amplified DNA can be cloned into a suitable cloning vector. It will be appreciated that the polynucleotide of the invention may contain only a coding region for the mutant thymidine kinase. However, it is preferred if the polynucleotide further comprises, in operable linkage, a portion of nucleic acid that allows for efficient translation of the coding sequence in the target cell. It is further preferred if the polynucleotide (when in a DNA form) further comprises a promoter in operable linkage which allows for the transcription of the coding region and the portion of nucleic acid that allows for efficient translation of the coding region in the target cell Protein As used herein, the term "protein" includes single-chain polypeptide molecules as well as multiple-polypeptide complexes where individual constituent polypeptides are linked by covalent or non-covalent means. As used herein, the terms "polypeptide" and "peptide" refer to a polymer in which the monomers are amino acids and are joined together through peptide or disulfide bonds. The terms subunit and domain may also refer to polypeptides and peptides having biological function. Variants, Derivatives, Analogues. Homologues and Fragments In addition to the specific proteins and nucleotides mentioned herein, the present invention also encompasses the use of variants, derivatives, analogues, homologues and fragments thereof In the context of the present invention, a variant of any given sequence is a sequence in which the specific sequence of residues (whether amino acid or nucleic acid residues) has been modified in such a manner that the polypeptide or polynucleotide in question retains at least one of its endogenous functions. A variant sequence can be obtained by addition, deletion, substitution modification replacement and/or variation of at least one residue present in the naturally-occurring protein. The term "derivative" as used herein, in relation to proteins or polypeptides of the present invention includes any substitution of, variation of, modification of, replacement of, deletion of and/or addition of one (or more) amino acid residues from or to the sequence providing that the resultant protein or polypeptide retains at least one of its endogenous functions. The term "analogue" as used herein, in relation to polypeptides or polynucleotides includes any mimetic, that is, a chemical compound that possesses at least one of the endogenous functions of the polypeptides or polynucleotides which it mimics. Typically, amino acid substitutions may be made, for example from 1,2 or 3 to 10 or 20 substitutions provided that the modified sequence retains the required activity or ability, Amino acid substitutions may include the use of non-naturally occurring analogues. Proteins of use in the present invention may also have deletions, insertions or substitutions of amino acid residues which produce a silent change and result in a functionally equivalent protein. Deliberate amino acid substitutions may be made on the basis of similarity in polarity, charge, solubility, hydrophobicity, hydrophilicity, and/or the amphipathic nature of the residues as long as the transport or modulation function is retained. For example, negatively charged amino acids include aspartic acid and glutamic acid; positively charged amino acids include lysine and arginine; and amino acids with uncharged polar head groups having similar hydrophilicity values include leucine, isoleucine, valine, glycine, alanine, asparagine, glutamine, serine, threonine, phenylalanine, and tyrosine. Conservative substitutions may be made, for example according to the Table below. Amino acids in the same block in the second column and preferably in the same line in the third column may be substituted for each other: "Fragments" are also variants and the term typically refers to a selected region of the polypeptide or polynucleotide that is of interest either functionally or, for example, in an assay. "Fragment" thus refers to an amino acid or nucleic acid sequence that is a portion of a full-length polypeptide or polynucleotide. Such variants may be prepared using standard recombinant DNA techniques such as site-directed mutagenesis. Where insertions are to be made, synthetic DNA encoding the insertion together with 5' and 3' flanking regions corresponding to the naturally-occurring sequence either side of the insertion site. The flanking regions will contain convenient restriction sites corresponding to sites in the naturally-occurring sequence so that the sequence may be cut with the appropriate enzyme(s) and the synthetic DNA ligated into the cut. The DNA is then expressed in accordance with the invention to make the encoded protein. These methods are only illustrative of the numerous standard techniques known in the art for manipulation of DNA sequences and other known techniques may also be used. Polynucleotide variants will preferably comprise codon optimised sequences. Codon optimisation is known in the art as a method of enhancing RNA stability and therefor gene expression. The redundancy of the genetic code means that several different codons may encode the same amino-acid. For example, Leucine, Arginine and Serine are each encoded by six different codons. Different organisms show preferences in their use of the different codons. Viruses such as HIV, for instance, use a large number of rare codons. By changing a nucleotide sequence such that rare codons are replaced by the corresponding commonly used mammalian codons, increased expression of the sequences in mammalian target cells can be achieved. Codon usage tables are known in the art for mammalian cells, as well as for a variety of other organisms. Preferably, at least part of the sequence is codon optimised. Even more preferably, the sequence is codon optimised in its entirety. Vectors As it is well known in the art, a vector is a tool that allows or facilitates the transfer of an entity from one environment to another,. In accordance with the present invention, and by way of example, some vectors used in recombinant DNA techniques allow entities, such as a segment of DNA (such as a heterologous DNA segment, such as a heterologous cDNA segment), to be transferred into a host and/or a target cell for the purpose of replicating the vectors comprising the nucleotide sequences used in the invention and/or expressing the proteins used in the invention. Examples of vectors used in recombinant DNA techniques include but are not limited to plasmids, chromosomes, artificial chromosomes or viruses. Polynucleotides used in the invention are preferably incorporated into a vector. Preferably, a polynucleotide in a vector for use in the invention is operably linked to a control sequence that is capable of providing for the expression of the coding sequence by the host cell, i.e. the vector is an expression vector. The term "operably linked" means that the components described are in a relationship permitting them to function in their intended manner. A regulatory sequence "operably linked" to a coding sequence is ligated in such a way that expression of the coding sequence is achieved under conditions compatible with the control sequences. The control sequences may be modified, for example by the addition of further transcriptional regulatory elements to make the level of transcription directed by the control sequences more responsive to transcriptional modulators. Vectors may be transformed or transfected into a suitable host to provide for expression of the tk gene product. This process may comprise culturing a host cell transformed with an expression vector as described above under conditions to provide for expression by the vector of a coding sequence encoding the protein, and optionally recovering the expressed protein. The vectors used in the present invention may be for example, plasmid or virus vectors provided with an origin of replication, optionally a promoter for the expression of a polynucleotide and optionally a regulator of the promoter. The vectors may contain one or more selectable marker genes, and/or a traceable marker such as GFP. Vectors may be used, for example, to transfect or transform a host cell. Control sequences operably linked to sequences encoding proteins for use in the invention include promoters/enhancers and other expression regulation signals. These control sequences may be selected to be compatible with the host cell for which the expression vector is designed to be used in. The term "promoter" is well-known in the art and encompasses nucleic acid regions ranging in size and complexity from minimal promoters to promoters including upstream elements and enhancers. The promoter is typically selected from promoters which are functional in mammalian cells, although prokaryotic promoters and promoters functional in other eukaryotic cells may be used. The promoter is typically derived from promoter sequences of viral or eukaryotic genes. For example, it may be a promoter derived from the genome of a cell in which expression is to occur. With respect to eukaryotic promoters, they may be promoters that function in a ubiquitous manner (such as promoters of a-actin, b-actin, tubulin) or, alternatively, a tissue-specific manner (such as promoters of the genes for pyruvate kinase). Viral promoters may also be used, for example the Moloney murine leukaemia virus long terminal repeat (MMLV LTR) promoter, the reus sarcoma virus (RSV) LTR promoter or the human cytomegalovirus (CMV) IE promoter. It may also be advantageous for the promoters to be inducible so that the levels of expression of the heterologous gene can be regulated during the life-time of the cell. Inducible means that the levels of expression obtained using the promoter can be regulated. In addition, any of these promoters may be modified by the addition of further regulatory sequences, for example enhancer sequences. Chimeric promoters may also be used comprising sequence elements from two or more different promoters described above. Preferably, the viral vector preferentially transduces a certain cell type or cell types. Retroviral vectors In one embodiment, the vector used in the present invention is a retrovirus based vector which has been genetically engineered so that it can not replicate and produce progeny infectious virus particles once the virus has entered the target celL There are many retroviruses that are widely used for delivery of genes both in tissue culture conditions and in living organisms. Examples include and are not limited to murine leukemia virus (MLV), human immunodeficiency virus (HIV-1), equine infectious anaemia virus (EIAV), mouse mammary tumour virus (MMTV), Rous sarcoma virus (RSV), Fujinami sarcoma virus (FuSV), Moloney murine leukemia virus (Mo-MLV), FBR murine osteosarcoma virus (FBR MSV), Moloney murine sarcoma virus (Mo-MSV), Abelson murine leukemia virus (A-MLV), Avian myelocytomatosis virus-29 (MC29), and Avian erythroblastosis virus (AEV) and all other retroviridiae including lentiviruses. A detailed list of retroviruses may be found in Coffin et al, 1997, "retroviruses", Cold Spring Harbour Laboratory Press Eds: JM Coffin, SM Hughes, HE Vannus pp 758-763. The basic structure of a retrovirus genome is a 5* LTR and a 3' LTR, between or within which are located a packaging signal to enable the genome to be packaged, a primer binding site, integration sites to enable integration into a host cell genome and gag, pol and env genes encoding the packaging components - these are polypeptides required for the assembly of viral particles. More complex retroviruses have additional features, such as rev and RRE sequences in HIV, which enable the efficient export of RNA transcripts of the integrated provirus from the nucleus to the cytoplasm of an infected target cell. In the provirus, these genes are flanked at both ends by regions called long terminal repeats (LTRs). The LTRs are responsible for proviral integration, and transcription. LTRs also serve as enhancer-promoter sequences and can control the expression of the viral genes. Encapsidation of the retroviral RNAs occurs by virtue of a psi sequence located at the 5' end of the viral genome. The LTRs themselves are identical sequences that can be divided into three elements, which are called U3, R and U5. U3 is derived from the sequence unique to the 35 end of the RNA. R is derived from a sequence repeated at both ends of the RNA and U5 is derived from the sequence unique to the 5' end of the RNA. The sizes of the three elements can vary considerably among different retroviruses. In a defective retroviral vector genome gag, pol and env may be absent or not functional. The R regions at both ends of the RNA are repeated sequences. U5 and U3 represent unique sequences at the 5J and 3' ends of the RNA genome respectively. More preferably, the viral vector is a targeted vector, that is it has a tissue tropism which is altered compared to the native virus, so that the vector is targeted to particular cells. This may be achieved by modifying the retroviral Env protein. Preferably the envelope protein is a non-toxic envelope or an envelope which may be produced in non-toxic amounts within the primary target cell, such as for example a MMLV amphotropic envelope or a modified amphotropic envelope. Preferably the envelope is one which allows transduction of human cells. Examples of suitable env genes include, but are not limited to, VSV-G, a MLV amphotropic env such as the 4070A env, the RD114 feline leukaemia virus env or haemagglutinin (HA) from an influenza virus. The Env protein may be one which is capable of binding to a receptor on a limited number of human cell types and may be an engineered envelope containing targeting moieties. The env and gag-pol coding sequences are transcribed from a promoter and optionally an enhancer active in the chosen packaging cell line and the transcription unit is terminated by a polyadenylation signal. For example, if the packaging cell is a human cell, a suitable promoter-enhancer combination is that from the human cytomegalovirus major immediate early (hCMV-MEE) gene and a polyadenylation signal from SV40 virus may be used. Other suitable promoters and polyadenylation signals are known in the art. MLV Preferably, the retroviral vector used in the present invention is an Murine Leukemia Virus (MLV) vector. Retroviral vectors derived from the amphotropic Moloney murine leukemia virus (MLV-A) are commonly used in clinical protocols worldwide. These viruses use cell surface phosphate transporter receptors for entry and then permanently integrate into proliferating ceil chromosomes. The genes are then maintained for the lifetime of the cell. Gene activity on MLV based constructs are easy to control and can be effective over a long time. Clinical trials conducted with these MLV -based systems have shown them to be well tolerated with no adverse side effects. An example of an MLV vector for use in the present invention is a vector derived from SFCMM-3, which carries both the suicide gene HSV-tk and the marker gene ALNGFR (Verzeletti 98, Human Gene Therapy 9:2243). The original vector used in the preparation of SFCMM-3 is LXSN (Miller et al. Improved retroviral vectors for gene transfer and expression. BioTechniques 7:980-990, 1989) (Genebank accession #28248). LXSN vector was modified by the insertion of the HSV-tk gene into the unique Hpa I site ("blunt cut"), removal of the neo gene by digestion with Hind III and Nae I, and insertion of the cDNA encoding ALNGFR in this site. Lentiviral vector In one embodiment, the vector of the present invention may be a lentiviral vector. Lentivirus vectors are part of a larger group of retroviral vectors. A detailed list of lentiviruses may be found in Coffin et al ("Retroviruses" 1997 Cold Spring Harbour Laboratory Press Eds: JM Coffin, SM Hughes, HE Varmus pp 758-763). In brief, lentiviruses can be divided into primate and non-primate groups. Examples of primate lentiviruses include but are not limited to: the human immunodeficiency virus (HIV), the causative agent of human acquired-immunodeficiency syndrome (AIDS), and the simian immunodeficiency virus (SIV). The non-primate lentiviral group includes the prototype "slow virus" visna/maedi virus (VMV), as well as the related caprine arthritis-encephalitis virus (CAEV), equine infectious anaemia virus (EIAV) and the more recently described feline immunodeficiency virus (FIV) and bovine immunodeficiency virus (BIV). A distinction between the lentivirus family and other types of retroviruses is that lentiviruses have the capability to infect both dividing and non-dividing cells (Lewis, 1992; Lewis and Emerman, 1994). In contrast, other retroviruses - such as MLV - are unable to infect non-dividing or slowly dividing cells such as those that make up, for example, muscle, brain, lung and liver tissue. As lentiviruses are able to transduce terminally differentiated/primary cells, the use of a lentiviral screening strategy allows library selection in a primary target non-dividing or slowly dividing host cell. Adenovirus vectors In another embodiment, the vector of the present invention may be an adenovirus vector. The adenovirus is a double-stranded, linear DNA virus that does not go through an RNA intermediate. There are over 50 different human serotypes of adenovirus divided into 6 subgroups based on the genetic sequence homology. The natural target of adenovirus is the respiratory and gastrointestinal epithelia, generally giving rise to only mild symptoms. Serotypes 2 and 5 (with 95% sequence homology) are most commonly used in adenoviral vector systems and are normally associated with upper respiratory tract infections in the young. Adenoviruses are nonenveloped, regular icosohedrons. A typical adenovirus comprises a 140iun encapsidated DNA virus. The icosahedral symmetry of the virus is composed of 152 capsomeres: 240 hexons and 12 pentons. The core of the particle contains the 36kb linear duplex DNA which is covalently associated at the 5' ends with the Terminal Protein (TP) which acts as a primer for DNA replication. The DNA has inverted terminal repeats (ITR) and the length of these varies with the serotype. The adenovirus is a double stranded DNA nonenveloped virus that is capable of in vivo and in vitro transduction of a broad range of cell types of human and non-human origin. These cells include respiratory airway epithelial cells, hepatocytes, muscle cells, cardiac myocytes, synoviocytes, primary mammary epithelial cells and post-mitotically terminally differentiated cells such as neurons. Adenoviral vectors are also capable of transducing non dividing cells. This is very important for diseases, such as cystic fibrosis, in which the affected cells in the lung epithelium, have a slow turnover rate. In fact, several trials are underway utilising adenovirus-mediated transfer of cystic fibrosis transporter (CFTR) into the lungs of afflicted adult cystic fibrosis patients. Adenoviruses have been used as vectors for gene therapy and for expression of heterologous genes. The large (36 kilobase) genome can accommodate up to 8kb of foreign insert DNA and is able to replicate efficiently in complementing cell lines to produce very high titres of up to 10 . Adenovirus is thus one of the best systems to study the expression of genes in primary non-replicative cells. The expression of viral or foreign genes from the adenovirus genome does not require a replicating cell. Adenoviral vectors enter cells by receptor mediated endocytosis. Once inside the cell, adenovirus vectors rarely integrate into the host chromosome. Instead, it functions episomally (independently from the host genome) as a linear genome in the host nucleus. Hence the use of recombinant adenovirus alleviates the problems associated with random integration into the host genome. Pox viral vectors Pox viral vectors may be used in accordance with the present invention, as large fragments of DNA are easily cloned into their genome and recombinant attenuated vaccinia variants have been described (Meyer, et al, 1991; Smith and Moss, 1983). Examples of pox viral vectors include but are not limited to leporipoxvirus: Upton, et al., 1986, (shope fibroma virus); capripoxvirus: Gershon, et al, 1989, (Kenya sheep-1); orthopoxvirus: Weir, et al., 1983, (vaccinia); Esposito, et aL,1984, (monkeypox and variola virus); Hruby, et al., 1983, (vaccinia); Kilpatrick, et al., 1985, (Yaba monkey tumour virus); avipoxvirus: Binns, et al., (1988) (fowlpox); Boyle, et al, 1987, (fowlpox); Schnitzlein, et al., 1988, (fowlpox, quailpox); entomopox (Lytvyn etal, 1992. Poxvirus vectors are used extensively as expression vehicles for genes of interest in eukaryotic cells. Their ease of cloning and propagation in a variety of host cells has led, in particular, to the widespread use of poxvirus vectors for expression of foreign protein and as deliver}'vehicles for vaccine antigens (Moss, 1991). Vaccinia viral vectors The vector of the present invention may be a vaccinia virus vector such as MVA or NYVAC. Most preferred is the vaccinia strain modified virus ankara (MVA) or a strain derived therefrom. Alternatives to vaccinia vectors include avipox vectors such as fowlpox or canarypox known as ALVAC and strains derived therefrom which can infect and express recombinant proteins in human cells but are unable to replicate. Delivery Systems The invention further provides a delivery system for mutant tk polynucleotide of the present invention. The delivery system of the present invention may be a viral or non-viral delivery system. Non-viral delivery mechanisms include but are not limited to lipid mediated transfection, liposomes, immunoliposomes, lipofectin, cationic. facial amphiphiles (CFAs) and combinations thereof. The polynucleotides may be delivered to the target cell population by any suitable Gene Delivery Vehicle, GDV. This includes but is not restricted to, DNA, formulated in lipid or protein complexes or administered as naked DNA via injection or biolistic delivery, viruses such as retroviruses, adenoviruses, poxvirus, lentiviruses, herpes viruses, vaccinia viruses, adeno associated viruses, murine leukemia viruses, semliki forest viruses and baculoviral viruses. Alternatively, the polynucleotides are delivered by cells such as monocytes, macrophages, lymphocytes or hematopoietic stem cells. In particular a cell-dependent delivery system is used In this system the polynucleotide encoding the TK protein are introduced into one or more cells ex vivo and then introduced into the patient. The agents of the present invention may be administered alone but will generally be administered as a pharmaceutical composition. Host Cells Vectors and polynucleotides of the invention may be introduced into host cells for the purpose of replicating the vectors/polynucleotides and/or expressing the TK encoded by the polynucleotide of the invention. The host cell may be a bacterial cell or a eukaryotic cell, for example a yeast, insect or a mammalian cell. The host cell may be a cell for packaging and propagating a virus, such as retroviral packaging cell lines which are well known in the art. The host cell may be a cell in an animal or patient (whether human or animal) which it is desired to destroy. The polynucleotide and vector of the present invention are useful to target to cells to be destroyed. Cells expressing TK may be contacted with an agent which is substantially non-toxic which is converted to a toxic form by TK. Method of destroying cells In one aspect of the present invention there is provided a method of destroying cells comprising (i) introducing into said cells a polynucleotide or a vector of the present invention; (ii) allowing said cells to express thymidine kinase; and contacting said cells with a substantially non-toxic agent which is converted by thymidine kinase to a toxic agent. The introduction into the cells of the polynucleotide or vector, and the contacting of the cells with the substantially non-toxic agent, may be in any order. The cells to be destroyed may be in vitro, such as cells which are grown in culture, or they may be cells which are part of an animal. Representative examples of cells which it is desired to destroy are T-cells, autoimmune cells, tumor cells, cells which do not express or inappropriately express a particular gene, and cells infected with bacteria, viruses, or other intracellular parasites. Treatment It is to be appreciated that all references herein to treatment include curative, palliative and prophylactic treatment. The treatment of mammals is particularly preferred. Both human and veterinary treatments are within the scope of the present invention. In one aspect of the present invention there is provided a method of treating a patient with cells in need of destruction comprising (i) introducing into the patient a polynucleotide, a vector or a pharmaceutical composition of the present invention; (ii) allowing said polynucleotide or expression vector to be taken up by said cells; (iii) allowing said cells to express thymidine kinase; and (iv) introducing into the patient a substantially non-toxic agent which is converted by thymidine kinase to a toxic agent. In another aspect of the present invention there is provided a method of treating a patient with cells in need of destruction comprising (i) removing the cells from the patient or donor cells; (ii) introducing into the cells ex vivo a polynucleotide or an expression vector of the present invention; (iii) introducing the modified cells into the patient; (iv) allowing the cells to express thymidine kinase; and administering to the patient a substantially non-toxic agent which is converted by thymidine kinase into a toxic agent. Representative examples of diseases which can be treated by polynucleotides, vectors and methods of the present invention include diseases such as cancer, hyperkeratosis, prostate hypertrophy, hyperthyroidism, a wide variety of endocrinopathies, autoimmune diseases, allergies, restenosis, a wide variety of viral diseases such as HIV and AIDS, hepatitis and intracellular parasitic diseases. The polynucleotides, vectors and methods of the present invention may be used in the course of therapy following allogeneic bone marrow transplant. Allogeneic bone marrow transplant is the treatment of choice for many hematologic malignancies, such as leukemia, lymphoma and multiple myeloma. Transplantation of allogeneic bone marrow, particularly when employed with high dose chemoradiotherapy, has been shown to produce superior results compared to autologous or syngeneic transplants. In performing allogeneic bone marrow transplant alloreactive T lymphocytes may be removed from the graft to prevent graft versus host disease (GvHD). GvHD occurs when T-cells in the transplanted stem cell graft may begin to attack the recipient's body. However, such removal of cells can increases the incidence of disease relapse, graft rejection and reactivation of viral infection. To counter these effects, allogeneic bone marrow transplant patients can be treated by introducing donor T lymphocytes after a delay following the allogeneic bone marrow transplant The therapeutic promise of delayed introduction of donor T lymphocytes following allogeneic bone marrow transplant, however, is limited by GvHD, a frequent and potentially lethal complication of the treatment. This problem has been addressed by the administering to a patient T-cells genetically engineered to include a polynucleotide encoding a "suicide gene". The polynucleotides and vectors of the present invention encoding the mutant TK may be been used in this regard. Thus, in one aspect of the present invention there is provided a method of preventing graft-versus-host disease in a patient comprising (i) administering to a host T-cells genetically engineered to include a polynucleotide or vector of the present invention; and (ii) administering to said host, prior to the occurrence of graft-versus-host disease, a substantially non-toxic agent in an amount effective to kill genetically engineered T-cells capable of providing a graft-versus-host effect in said patient through interaction of said agent with thymidine kinase. The cells comprising the polynucleotide or vector encoding the mutant tk gene preferably comprises a marker gene which may be used to monitor the presence of the mutant tk gene. Preferably, the marker gene is encoded by the polynucleotide or expression vector of the present invention. An example of one such marker gene is a modified low affinity nerve growth factor receptor (ALNGFR). Modified LNGFR, when expressed on the surface of transduced cells retains the binding properties of the corresponding unmodified NGF receptor with respect to its ligand, yet cannot effect signal transduction as a result of ligand binding. Examples of specific LNGFR modifications are described in US application No.08/602,791. Pharmaceutical compositions A pharmaceutical composition is a composition that comprises or consists of a therapeutically effective amount of a pharmaceutically active agent. It preferably includes a pharmaceutically acceptable carrier, diluent or excipients (including combinations thereof). Acceptable carriers or diluents for therapeutic use are well known in the pharmaceutical art, and are described, for example, in Remington!s Pharmaceutical Sciences, Mack Publishing Co. (A. R. Gennaro edit. 1985). The choice of pharmaceutical carrier, excipient or diluent can be selected with regard to the intended route of administration and standard pharmaceutical practice. The pharmaceutical compositions may comprise as - or in addition to - the carrier, excipient or diluent any suitable binder(s), lubricant(s), suspending agent(s), coating agent(s), solubilising agent(s). Examples of pharmaceutically acceptable carriers include, for example, water, salt solutions, alcohol, silicone, waxes, petroleum jelly, vegetable oils, polyethylene glycols, propylene glycol, liposomes, sugars, gelatin, lactose, amylose, magnesium stearate, talc, surfactants, silicic acid, viscous paraffin, perfume oil, fatty acid monoglycerides and diglycerides, petroethral fatty acid esters, hydroxymethyl-cellulose, polyvinylpyrrolidone, and the like. Where appropriate, the pharmaceutical compositions can be administered by any one or more of: inhalation, in the form of a suppository or pessary, topically in the form of a lotion, solution, cream, ointment or dusting powder, by use of a skin patch, orally in the form of tablets containing excipients such as starch or lactose, or in capsules or ovules either alone or in admixture with excipients, or in the form of elixirs, solutions or suspensions containing flavouring or colouring agents, or they can be injected parenterally, for example intracavemosally, intravenously, intramuscularly or subcutaneously. For parenteral administration, the compositions may be best used in the form of a sterile aqueous solution which may contain other substances, for example enough salts or monosaccharides to make the solution isotonic with blood. For buccal or sublingual administration the compositions may be administered in the form of tablets or lozenges which can be formulated in a conventional manner. There may be different composition/formulation requirements dependent on the different delivery systems. By way of example, the pharmaceutical composition of the present invention may be formulated to be delivered using a mini-pump or by a mucosal route, for example, as a nasal spray or aerosol for inhalation or ingestable solution, or parenterally in which the composition is formulated by an injectable form, for delivery, by, for example, an intravenous, intramuscular or subcutaneous route. Alternatively, the formulation may be designed to be delivered by both routes. Further preferred features and embodiments of the present invention will now be described by way of non-limiting example and with reference to the accompanying drawings in which: Figure 1 shows the multiple alignment of nucleotide sequences of HSV-1 thymidine kinase gene (HSV-tk) mutants (TkMut2 (SEQ ID NO 2), TkMut23 (SEQ ID NO:3), TkMut24 (SEQ ID NO:4), TkMut34 (SEQ ID NO:5), TkMut4 (SEQ ID NO:6) and TkMut234 (SEQ ID NO: 7). Bases are numbered from the ATG start codon of wild-type HSV-tk sequence. Sequences are aligned against the TK wt sequence (TK wt, SEQ ID NO:l). The position of mutations is indicated by a number above the alignment. Mutations at positions (330), (541 and 542), and (555) are represented by the numbers 2, 3 and 4 respectively. Figure 2 shows the splicing consensus sequence. Figure 3 shows the HSV-tk PCR product of cDNA prepared from the supernatant of echotropic packaging cell line GP+ E86 and from the amphotropic packaging cell line gp+ env Aml2 (Aml2) transfected with SFCMM-3 (Verzelletti, 1998, Human Gene Tlierapy 9:2243-2251) and/or TIG (scSFCMM-3) (Chalmers, 2001) vector. Figure 4 shows PCR amplified HSV-tk in peripheral blood lymphocytes (PBL) transduced with SFCMM-3 and/or TK3 (scSFCMM-3) vector. Figure 5 shows the sequence of the critical portion of the HSV-tk gene in SFCMM-3 versus TK3 (scSFCMM-3). The nucleotide positions different between SFCMM-3 and TK3 (scSFCMM-3) vectors are bordered by a square. The gene segments which are deleted in the spliced forms of SFCMM-3 and TK3 are indicated in italics. The corresponding splicing donor and acceptor sites are indicated. Figure 6 shows a schematic summary of the mutations introduced into the SFCMM-3 vector. Figure 7 shows HSV-tk PCR product of cDNA derived from supematants of E86 cells transiently transfected with recombinant plasmids derived form SFCMM-3 comprising the HSV-tk mutants shown in Figure 1. Figure 8 shows HSV-tk PCR product of cDNA derived from supematants of Aml2 cultures which has been transduced with the supematants collected from the E86 cultures described in Figure 7. Figure 9 shows TKA1/TKB1 HSV-tk PCR product of DNA derived from CEM A301 cells transduced with the supematants collected from the Aml2 cultures described in Figure 8. Figure 10 shows TKY1/TKY2 and TKZ1/TKZ2 HSV-tk PCR products of DNA derived from CEM A301 cells transduced with the supematants collected from the Aml2 cultures described in Figure 8. Figure 11 shows TKA1/TKB1 HSV-tk PCR product of DNA derived from PBLs transduced with the supematants collected from the Ami 2 cultures described in Figure 8. Figure 12 shows TKZ1/TKZ2 HSV-tk PCR product of DNA derived from PBLs transduced with the supematants collected from the Am 12 cultures described in Figure 8. Example 1 - Relative frequency of the spliced form of HSV-tk in SFCMM-3#35 supernatant and transduced cells The relative frequency of the spliced form (percent spliced form versus spliced plus unspliced) was evaluated by two quantitative real-time RT-PCR (RNA from supernatant) or PCR (DNA from transduced lymphocytes), specific for each HSV-tk form, using TaqMan 7700 system. Materials and methods SFCMM-3 retroviral vector The GP+em'Aml2 (Aml2) -derived producer cell line, SFCMM-3 clone 35 (Aml2/SFCMM-3#35), was expanded 5 days in DMEM (Cambrex) medium containing 2 mM glutamine and 10% of irradiated FBS (Hyclone). Retroviral supematants were harvested from confluent flasks in X-VTVO 15 medium (Cambrex) supplemented with 2 mM glutamine after 24 h incubation of the cells at 33°C. Virus-containing supematants were filtered and stored at -80°C until further use. T cell transduction Peripheral blood mononuclear cells were collected from healthy donors and isolated by centrifugation on Lymphoprep (Nycomed). Peripheral blood lymphocytes (PBLs) were stimulated with OKT3 (30 ng/ml) (Orthoclone) and cultured in RPMI (Hyclone) containing 3% of autologus plasma, 2 mM glutamine and supplemented with recombinant human interleukin 2, 600 IU/ml (Proleukin), Transduction of PBLs was performed by centrifugation of retroviral vector particles (supernatant from Aml2/SFCMM-3#35) on OKT3-stimulated PBLs. Two centrifugation cycles were perfonned on the second and the third day after OKT3 stimulation-After the transduction phase, lymphocytes were collected and the percentage of actually transduced cells evaluated by FACS analysis, thanks to the ALNGFR molecule, used as a cell surface marker. The transduced cells were then sorted by antibodies conjugated to magnetic beads (Dynabeads), expanded a few days in culture, and then frozen after a total of 10 days in culture. Samples preparation DNA was purified from 1-2X106 cells by using QIAamp DNA Mini kit (Qiagen) following manufacturer's protocols and yield was determined by measuring the OD260. RNA was purified from 140 pi of supernatant by using QIAamp Viral RNA Mini Kit (Qiagen) according to the manufacturer's protocols. 13.5 µl of purified RNA were first subjected to DNase treatment at 25°C for 20 min, and 95°C for 5 min to inactivate DNase. Half of the DNase-treated RNA was then reverse transcribed at 42°C for 1 h using M-MLV Reverse Transcriptase (Invitrogen) and Oligo dT. Synthesized cDNA was 5-fold diluted before using in the Real time PCR. Quantitation of spliced fonn by real time PCR DNA standard curve has been prepared by subcloning the unspliced and truncated form of HSV-tk into pCR2.1 TOPO vector (Invitrogen). Two different sets of primers and probes have been designed using Primer Express™ 1.5 software (PE Applied Biosystems), able to selectively amplify and detect the unspliced and the spliced form of HSV-tk. Two quantitative real time PCRs were set-up, specific for each HSV-tk form, using TaqMan/ABI PRISM 7700 sequence detection system. To set up a Quantitative Real time PCR specific for the HSV-tk unspliced fonn, primers and probe were designed in the spliced region of HSV-tk gene. Real Time PCR for the unspliced form was performed in a 25 µl reaction mixture containing 100-500 ng of genomic DNA or 10 pi of cDNA prepared as described above, IX TaqMan Universal PCR Master Mix (PE Applied Biosystems) 300 nM of each of the two primers TKwtfor (5'-CGG CGG TGG TAA TGA CAA G-3') and Tkwtrev (5'-GCG TCG GTC ACG GCA TA-3') and 200 nM of TKwt MGB probe (5'- FAM CCA GAT AAC AAT GGG C-3'). A TaqMan probe encompassing the splice junction was designed to selectively detect the HSV-tk spliced form. Quantitative Real time PCR specific for the TK spliced (truncated) form was performed in a 25 µlreaction mixture containing 100-500 ng of genomic DNA or 10 pi of cDNA prepared as described above, IX Master Mix (PE Applied Biosystems) 300 nM of each of the two primers TKSP18 (5- GGA TGA GGG CCA CGA A-3') and TKSP16 (5'- CGA ACA TCT ACA CCA CAC AAC A-3') and 200nM of Taq Man probe PSP10 (51 FAM -CCA GCA CGG CCC TGG TCG-TAMRA 3'). Thermal cycling conditions were as follows: initial activation of UNG at 50°C for 2 min, followed by activation of Taq Gold and inactivation of UNG at 95°C for 15 min. Subsequently, 40 cycles of amplification were performed at 95°C for 15 s and 60°C for 1 min. Both PCRs were performed in parallel in MicroAmp optical 96-well reaction plates (Applied Biosystems) using the ABI Prism 7700 Sequence Detection Systems (Applied Biosystems). Mean baseline fluorescence was calculated from PCR cycles 3 to 15, and Ct was defined as the PCR cycle in which the normalized fluorescence intensity of the reporter dye equaled 0.05. Two standard curves with known copy numbers (from 106 to 4 copies/reaction) were generated in each TaqMan assay by plotting the Ct values against the logarithm of the initial input of DNA amount. Standard dilutions and cDNA samples were analyzed in duplicate and triplicate, respectively. Both PCRs were validated and showed extended dynamic range (6 log), high sensitivity ( Results The results are shown in Table 1. Six clinical grade lots of SFCMM-3 vector supernatant were analysed, as well as nine preparations of transduced lymphocytes. The relative frequency of the spliced form is below 5% in all 6 clinical grade lots of SFCMM-3 supernatant (1.12 +/- 0.75, range 0.65-2.60), as well as in 9 preparations of T lymphocytes transduced with SFCMM-3 supernatant (1.89 +/- 1.22, range 0.84-4.00). Example 2 - Variant of spliced TK in scSFCMM-3 supernatants and transduced cells Materials and methods scSFCMM-3 retroviral vector and producer cell lines Vector DNA TK3 (TK3 Molmed and TK3 are different preparations of the same plasmid scSFCMM-3, described in: Chalmers 2001, Molecular Therapy 4:146-148) was transfected into the echotropic packaging cell line GP+ E-86 (E86) by calcium phosphate coprecipitation. The supernatant obtained from the transient transfection of E86 cells was filtered and then used to infect the Aml2 cell line. The fraction of cells containing TK3 was isolated by using immunomagnetic selection and the resulting Aml2/TK3 bulk culture was expanded. After limiting dilution of the Aml2/TK3 bulk culture, clones #53, #71, and #80 were selected on the basis of high growth capacity and transduction efficiency on T lymphocytes. The Aml2/TK3 clones were expanded in DMEM (Cambrex) containing 2 mM glutamine and 10% irradiated FBS (Hyclone). Retroviral supernatants were harvested in X-VIVO 15 medium (Cambrex) supplemented with 2 mM glutamine after 24 h incubation of the cells at 33°C. Virus-containing supernatants were filtered with 0.22 µm filters and stored at -80°C until further use. T cell transduction Peripheral blood mononuclear cells were collected from several healthy donors and isolated by centrifugation on Lymphoprep (Nycomed). Peripheral blood lymphocytes (PBLs) were stimulated with OKT3 (30 ng/ml) (Orthoclone) and cultured in RPMI (Hyclone) containing 3% of autologus plasma, 2 mM glutamine and supplemented with recombinant human interleukin 2, 600 IU/ml (Proleukin). Transduction of PBLs was performed by centrifugation of retroviral vector particles on OKT3-stimulated PBLs. Two centrifugation cycles were performed on the second and the third day after OKT3 stimulation. After the transduction phase, PBLs were collected and the percentage of actually transduced cells evaluated by FACS analysis, thanks to the ALNGFR molecule, used as a cell surface marker. The transduced cells were then sorted by antibodies conjugated to magnetic beads (Dynabeads), expanded a few days in culture, and pellets were prepared for PCR analysis. Polymerase chain reaction The bulk producer cell line (Aml2/TK3 Bulk 2) as well as single producer cell clones obtained by limiting dilution (Aml2/TK3 #53, #71 and #80) were analysed. RNA was extracted from the culture supernatants containing infectious viral particles. RT-PCR was performed with HTK4+ (5'- TTC TCT AGG CGC CGG AAT TCG TT-3') and HTK2- (5f- ATC CAG GAT AAA GAC GTG CAT GG-3') primers or TKA1 (5'- CGT ACC CGA GCC GAT GAC TT-31) and TKB1 (5'-TGT GTC TGT CCT CCG GAA GG-3r) primers. RT PCR was performed using Titan One tube RT-PCR System (Roche) in a 50 jal reaction mixture containing 10 |il of RNA DNase-treated, IX RT-PCR Buffer with 1.5mM of MgCl2 (PE Applied Biosystems), 200 |iM of each deoxynucleotide (dNTP), 200 nM of each primer, 5 mM of dithiothreitol (DTT), 20 U of RNase Inhibitor, 1 pi of Titan Enzyme mix. Supernatants from Aml2/SFCMM-3 #35 and DNA from SFCMM-3 transduced lymphocytes were used as controls. RT- controls were also run in parallel. The RT-PCR cycling profile consisted of a first reverse transcription step for 30 min at 50°C, a denaturation step for 2 min at 94°C followed by 40 cycles with denaturation for 30 s at 95°C, annealing for 30 s at 60°C and elongation for 1 min 30 s at 68°C, and one final elongation step of 10 min at 68°C. Ten microliters of amplified product were analyzed by agarose gel electrophoresis. Genomic DNA was extracted from PBLs of several donors transduced with SFCMM-3 or TK3 vector. PCR was performed in a 25 µl reaction mixture containing 100-500 ng of genomic DNA, IX PCR Buffer with 1.5 mM of MgCfe (Applied Biosystems), 200 µM of each dNTP, 1600nM of each primer TK2S (5f- CCA TAG CAA CCG ACG TAC G-3') and TKAS (5'- GAA TCG CGG CCA GCA TAG C-30, The PCR cycling profile consisted of a first denaturation step 15 min at 94°C followed by 38 cycles with denaturation for 1 min of 95°C, annealing for 30s at 65°C and extension for 1 min at 72°C, and one final extension step of 10 min at 72°C. Ten microliters of amplified product were analyzed by agarose gel electrophoresis. On the same samples PCR was performed in a 25µl reaction mixture containing 100-500 ng of genomic DNA, IX PCR Buffer with 1.5 mM of MgCl2 (Applied Biosystems), 200µM of each dNTP, 300 nM of each primer HTK4+ (5'- TTC TCT AGG CGC CGG AAT TCG TT-3') and HTK2- (51- ATC CAG GAT AAA GAC GTG CAT GG-31), 1-25 U of AmpliTaq Gold (Applied Biosystems). The PCR cycling profile consisted of a first denaturation step 15 min at 94°C followed by 40 cycles with denaturation for 30 s at 95°C, annealing for 50 s at 60°C and extension for 1 min at 72°C, and one final extension step of 10 min at 72°C. Ten microliters of amplified product were analyzed by agarose gel electrophoresis. cDNA sequence analysis of the TK3 spliced form The lower band of the product amplified with HTK4+/HTK2- primers from Aml2/TK3#53 supernatant was sequenced by using an automated fluorescent DNA sequence apparatus. Sequence was carried out by PRIMM (Milan, Italy). Results HSV-tkRT-PCRfi-om RNA of culture sapernatants Two PCR products were detected analyzing RNA from SFCMM-3 as well as TK3 vectors, produced by Am 12 and E86 packaging cell lines (Figure 3). The TK3 vector corresponds to the described scSFCMM-3 vector (Chalmers, 2001). The major product (upper band) corresponds to the unspliced form of HSV-tk (1020 base pair (bp) with HTK4+/HTK2- primers; 401 bp with TKA1/TKB1 primers) in all the samples. The size of the minor product (lower band) in the SFCMM-3 samples corresponds to the described spliced form, in which 227 bp are deleted {Gavin 2001, Blood 97:222-129), On the contrary, the size of the minor product in the TK3 samples is slightly larger, suggesting the presence of a different spliced form, in which a smaller fragment is deleted; this difference between SFCMM-3 and TK3 samples is particularly evident with TK2S/TKAS primers. Additional, intermediate products observed with HTK4+/HTK2- primers only are considered as possible PCR artifacts. HSV-tkPCR on DNA from transduced lymphocytes Two PCR products were detected analyzing DNA from SFCMM-3 as well as TK3 transduced PBLs (Figure 4). The major product (upper band) corresponds to the unspliced form of HSV-tk (644 bp with TK2S/TKAS primers; 1020 bp with HTK4+/HTK2- primers). The size of the lower band (corresponding to the described spliced form) in the SFCMM-3 samples is 417 bp with TK2S/TKAS primers and 793 bp with HTK4+/HTK2- primers. Sequence of the critical portion of the HSV-tk gene in SFCMM-3 versus TK3. The sequence of the critical portion of the HSV-tk gene in SFCMM-3 versus TK3 is shown in Figure 5. The gene segments which are deleted in the spliced forms of SFCMM-3 and TK3 are indicated in italics. The corresponding splicing donor and acceptor sites are underlined. The sequence data confirm that different spliced forms are generated in SFCMM-3 (227 bp deletion) and in TK3 (214 bp deletion) samples. As expected in both cases the GU and AG dinucleotides are present at 5' and 3' borders of the deleted sequence, respectively. Example 3 - Elimination of splicing by site direct mutagenesis Materials and Methods Starting from the wild type SFCMM-3 vector, recombinant plasmids were prepared by site-directed mutagenesis with different mutations in the HSV-tk gene (Figure 6). Third-base degenerate changes were introduced thus in all instances the wild type amino acid sequence of HSV-tk enzyme was preserved. Site-directed mutagenesis To generate pcDNA3.1-tk plasmid, SFCMM-3 plasmid was digested with EcoRI and Xhol and the EcoRI/XhoI fragment was ligated to EcoRI/XhoI digested pcDNA3.1 (Invitrogen). From the pcDNA3.1-tk plasmid, all the HSV-tk mutants were generated by site-directed mutagenesis using the QuickChange Site-Directed Mutagenesis kit (Stratagene). The oligonucleotide primers used to introduce the desired mutation were each complementary to opposite strands of the vector. The sequences of sense primers are reported in the following table: a The position of nucleotide mutations are underlined in bold. b The numbers refer to the number of each SFCMM-3 HSV-tk plasmid clone mutant The resulting vectors were sequenced to confirm the corrected nucleotide substitutions. HSV-tk mutated fragment was then removed from pcDNA3.1 by digestion with EcoRI and Xhol, then cloned into SFCMM-3 plasmid to generate SFCMM-3 HSV-tk mutants. After sequence confirmation of SFCMM-3 HSV-tk mutants (Figure 1), one plasmid clone of each type was used to transiently transfect ES6 cells. SFCMM-3 and TK3 plasmids were used as controls. The supernatant was collected from the E86 cultures and used to stably transduce the amphotropic packaging cell line Aml2 as described in Materials and Methods section of Example 2. The resulting cell populations were selected for ALNGFR expression. RNA was extracted from the E86 as well as Aml2 culture supernatants containing infectious retroviral particles. RT-PCR reactions were carried out on RNA from E86 and Aml2 supernatants using TKA1 and TKB1 primers according to the protocol described in Materials and Methods section of Example 2. Transduction and PCR analysis of CEM A301 cell line and PBLs CEM A301 cells were cultured in RPMI 1640 (Hyclone) supplemented with 10% FBS (Hyclone) and 2 mM glutamine. CEM A301 cells were infected with Aml2 culture supernatant containing SFCMM-3 HS V-tk mutants 2 and 234. The day before transduction 5X105 cells/ml were cultured. Transduction was then performed by one centrifugation cycle of retroviral vector particles on cells. After the transduction phase, cells were collected and the percentage of actually transduced cells evaluated by FACS analysis. PBLs from three different donors were transduced (as described in Materials and Methods section of Example 2) with Aml2 supernatant containing SFCMM-3 HSV-tk mutant 2 as well as SFCMM-3. The transduced cells were then sorted for ALNGFR expression and expanded a few days in culture. One pellet for PCR analysis was prepared from the transduced non selected cells as well as from transduced selected cells as described in Materials and Methods section of Example 2. Genomic DNA was extracted from pellet cells and PCR reaction was carried out with TKA1 and TKB1 or TKY1 and TKY2 or TKZ1 and TKZ2 primers (as indicated in the table below) in a 25µl reaction mixture containing 100-500 ng of genomic DNA, IX PCR Buffer with 1.5 mM of MgCl2 (Applied Biosystems), 200 nM of each dNTPs, 300 nM of each primer, 1.25 Units of AmpliTaq Gold (Applied Biosystems). The PCR cycling profile consisted of a first denaturation step for 15 min at 94°C followed by 40 cycles with denaturation for 30 s at 94°C, annealing for 50 s at 60°C and extension for 1 min at 72°C, and one final extension step of 10 min at 72°C. Ten microliters of amplified product were analyzed by agarose gel electrophoresis. Results Splicing properties ofHSV-tk mutants in E86 supernatants The major product (upper band) corresponding to the unspliced form of HSV-tk (401 bp) is detected in all the samples except the negative control (supernatant from non transfected E86 cells, ES6 C-) (Figure 7). The minor product (lower band) is less abundant in RNA from all the mutant vectors, in respect to SFCMM-3 and TK3 vectors, produced by E86 packaging cell lines. Splicing properties of HSV-tk mutants in Ami 2 supernatants The unspliced form of HSV-tk is detected in all the samples except the negative controls (supernatant from non infected cells, Aml2 C-5 and H2O) (Figure 8). A lower band is detected in TkMut4 (mutation 4) and TkMut34 (mutations 3+4) mutants, corresponding to the lower band of TK3 and SFCMM-3 sample, respectively. The intensity of ethidium bromide signal is lower compared to TK3 and SFCMM-3, indicating that the spliced product is less represented in the mutants. This band is not detected in TkMut2 (mutation 2), TkMut234 (mutations 2+3+4) and TkMut24 (mutations 2+4) mutants. Indeed, two very weak bands of approximately 100 and 200 bp are detected in TkMut2, TkMut234 and TkMut24 mutants. Splicing properties of HSV-tk mutants in transduced CEMA301 cells andPBLs Genomic DNA from CEM A301 transduced with TkMut2 and TkMut234 mutants as well as with SFCMM-3 and TK3 supematants was analyzed by PCR using TKAl/TKBl primers. The unspliced form of HSV-tk (401 bp) is detected in all the samples except the negative controls (non transduced cells, CEM NT, and H20) (Figure 9). A lower band is detected in TK3 and SFCMM-3 preselection and postselection sample, corresponding to the spliced form of HSV-tk gene. No lower band is observed in CEM transduced with TkMut2 and TkMut234 mutants, before and after selection. To exclude the possibility of alternative splicing events occurring in different regions of HSV-tk gene, a more extensive analysis of CEM A301 transduced with TkMut2 as well as with SFCMM-3 supematants was done using TKY1/TKY2 and TKZ1/TKZ2 primers. The unspliced form of HSV-tk (542 bp and 1149 bp, respectively) is detected in all the samples (Figure 10). A lower band is detected in SFCMM-3 preselection and postselection sample, corresponding to the spliced form of HSV-tk gene. No lower band is observed in CEM transduced with TkMut2, nor with TKY1/TKY2 neither . with TKZ1/TKZ2 PCR primers. Genomic DNA from PBLs of three different donors (Donl, Don2, Don3) transduced with SFCMM-3 and/or TkMut2 supematants was analyzed by PCR using TKAl/TKBl primers. The unspliced form of HSV-tk (401 bp) is detected in all the samples (Figure 11). A lower band is detected in SFCMM-3 sample, corresponding to the spliced form of HSV-tk gene. No lower band is observed in PBLs transduced with TkMut2. The same result was obtained using TKZ1/TKZ2 primers (Figure 12), which encompasses the full-length HSV-tk gene (1149 bp), thus excluding the possibility that the introduced mutation could generated different splicing variants. Overall these findings indicate that the introduced mutations abolish or at least very significantly decrease any HSV-tk gene spliced form in transduced CEM as well as in transduced lymphocytes. Table 1. Analysis of HSV-tk spliced form on SFCMM-3 supernatants and transduced PBLs All publications mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described methods and system of the present invention will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. Although the present invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in biochemistry and biotechnology or related fields are intended to be within the scope of the following claims. References Moolten, Cancer Res. 46,p5276 (1986) Mullen, Pharmac. Titer. 63, pl99 (1994) Marini et al.} Gene Tlierapy 2, p655 (1995) Kokoris et al., Gene Terapy 6, pl415-1426 (1999) Fillat et al., Current. Gene Tlierapy 3, pl3 2003 Tiberghien et al, Blood 97, p63 (2001) Garin et al., Blood 97, pl22 (2001) Chalmers, Molecidar Tlierapy 4 pl46 (2001) Rettig et al., Molecular Therapy 8, p29 (2003) McKnight et al, Nuc. Acids Res 8, p5949 (1980) Swain and Galloway, J. Virol. 46, pi045 (1983) Davidson and Scott, J. Gen. Virol 67, pi759 (1986) Otsuka and Kit, Virology 135, p316-330, (1984) Nunberg et al., J. Virol. 63 p3240 (1989) Robertson and Whalley, Nuc. Acids Res. 16 pi 1303 (1988) Mittal and Field, /. Virol 70 p2901 (1989) Martin et al., J. Virol. 63 p2847 (1989) Scott et al., J. Gen. Virol. 70 p3055 (1989) Honess et al., J. Gen. Virol. 70p3OO3 (1989) Baer et al., Nature (London) 310 p207(1984) McKnight et al., Nucl Acids Res. 8 p5949 (1980) Drinkwater and Klinedinst, PNAS 83 p3402 (1986) Liao and Wise Gene 88 pi07 (1990) Horwitz et al., Genome 3 pi 12 (1989) Lewis et al., EMBO. J 11 p3053 (1992) Lewis and Emerman J. Virol. 68 p510 (1994) Meyer, et al., J. Gen. Virol. 72 pl031 (1991) Smith and Moss Gene, 25 p21 (1983) Upton, et al., J. Virology 60 p920 (19S6) Gershon, et al., J. Gen. Virol. 70 p525 (1989) Weir, et al., J. Virol. 46 p530 (1983) Esposito, et al.. Virology 135 p561 (1984) Hruby, et al., PNAS, 80 p3411 (1983) Kilpatrick, et al, Virology 143 p399 (1985) Binns, et al., /. Gen. Virol 69 pl275 (1988) Boyle, et al., Virology 156 p355 (1987) Schnitzlein, et al., J. Virological Method, 20p341 (1988) Lytvyn, et al., J. Gen. Virol. 73 p3235 (1992) Moss Science 252 pl662 (1991) Verzeletti, Hum. Gene Therapy 9 p2243 (1998) Chalmers, Mol. Therapy 4 pl46 (2001) Claims: 1. A polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the nucleotides corresponding to the splice donor site nucleotides is replaced by another nucleotide and wherein the nucleotides of the splice acceptor sites are not altered. 2. A polynucleotide according to claim 1 wherein at least one of the nucleotides corresponding to the splice donor site nucleotides at positions 329 and 330 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide. 3. A polynucleotide according to claim 1 or 2 wherein both the nucleotides corresponding to splice donor site nucleotides at positions 329 and 330 of the wild type sequence in Figure 1 are replaced by another nucleotide. 4. A polynucleotide according to claim 2 or 3 wherein the nucleotide corresponding to position 330 is changed from T to C. 5. A polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the nucleotides corresponding to splice donor site nucleotides at positions 329 and 330 of the wild type sequence (Tk wt) in Figure 1 is replaced by another nucleotide and wherein the nucleotides of the splice acceptor sites are not altered. 6. A polynucleotide according to claim 5 wherein both the nucleotides corresponding to positions 329 and 330 are replaced by another nucleotide. 7. A polynucleotide according to claim 5 or 6 wherein the nucleotide corresponding to position 330 is changed from T to C. 8. A polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the nucleotides corresponding to the splice donor site nucleotides at positions 329 and 330 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide and wherein at least one of the nucleotides corresponding to splice acceptor site nucleotides at positions 554 and 555 and at least one of the nucleotides corresponding to splice acceptor site nucleotides at positions 662 and 663 of the wild type sequence in Figure 1 is not altered. 9. A polynucleotide according to claim 8 wherein both the nucleotides corresponding to positions 554 and 555 are not altered. 10. A polynucleotide according to claim S or 9 wherein both nucleotides corresponding to positions 662 and 663 are not altered. 11. A polynucleotide according to any one of claims 8 to 10 wherein both nucleotides corresponding to positions 329 and 330 are replaced by another nucleotide. 12. A polynucleotide according to any one of claims 8 to 11 wherein the nucleotide corresponding to position 330 is changed from T to C. 13. A polynucleotide comprising the TkMut2 sequence in Figure 1. 14. A polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the splice acceptor site nucleotides corresponding to the nucleotide(s) at positions 329 and 330 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide and wherein at least one of the nucleotides corresponding to the splice acceptor site nucleotides at positions 541 and 542 of the wild type sequence in Figure 1 is replaced by another nucleotide. 15. A polynucleotide according to claim 14 wherein both nucleotides corresponding to positions 541 and 542 are replaced by another nucleotide. 16. A polynucleotide according to claim 14 or 15 wherein both the nucleotides corresponding to positions 329 and 330 are replaced by another nucleotide. 17. A polynucleotide according to any one of claims 14 to 16 wherein the nucleotide corresponding to position 330 is changed from T to C. 18. A polynucleotide according to any one of claims 14 to 17 wherein the nucleotide corresponding to position 541 is changed from A to T. 19. A polynucleotide according to any one of claims 14 to 18 wherein the nucleotide corresponding to position 542 is changed from G to C. 20. A polynucleotide according to any one of claims 14 to 19 wherein at least one of the nucleotides corresponding to the splice acceptor site nucleotides at position 554 and 555 of the wild type sequence in Figure 1 is also replaced by another nucleotide such that there is no splice site present. 21. A polynucleotide according to claim 20 wherein the nucleotide corresponding to position 555 is changed from G to A. 22. A polynucleotide comprising the TkMut23 sequence in Figure 1. 23. A polynucleotide comprising the TkMut234 sequence in Figure 1. 24. A polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the splice acceptor site nucleotides corresponding to the nucleotide(s) at positions 329 and 330 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide and wherein at least one of the nucleotides corresponding to the splice acceptor site nucleotides at positions 554 and 555 of the wild type sequence in Figure 1 is replaced by another nucleotide. 25. A polynucleotide according to claim 24 wherein both the nucleotides corresponding to positions 329 and 330 of the wild type sequence (Tk wt) in Figure 1 are replaced by another nucleotide. 26. A polynucleotide according to claim 24 or 25 wherein the nucleotide corresponding to position 330 is changed from T to C. 27. A polynucleotide according to any one of claims 24 to 26 wherein the nucleotide corresponding to position 555 is changed from G to A. 28. A polynucleotide comprising the TkMut24 sequence in Figure 1. 29. A polynucleotide comprising a nucleotide sequence encoding a thymidine kinase wherein at least one of the nucleotides corresponding to splice acceptor site nucleotides at position 554 and 555 of the wild type sequence in Figure 1 (Tk wt) is replaced by another nucleotide. 30. A polynucleotide according to. claim 29 wherein the nucleotide corresponding to position 555 is changed from G to A. 31. A polynucleotide according to claim 29 or 30 wherein at least one of the splice acceptor site nucleotides corresponding to the nucleotides at positions 541 and 542 of the wild type sequence in Figure 1 is also replaced by another nucleotide. 32. A polynucleotide according to claim 31 wherein both the nucleotides corresponding to positions 541 and 542 are replaced by another nucleotide such that there is no splice site present. 33. A polynucleotide according to claim 31 or 32 wherein the nucleotide corresponding to position 541 is changed from A to T. 34. A polynucleotide according to any one of claims 31 to 33 wherein the nucleotide corresponding to position 542 is changed from G to C. 35. A polynucleotide comprising the TkMut4 sequence in Figure 1, 36. A polynucleotide comprising the TkMut34 sequence in Figure 1. 37. A polynucleotide according to any one of claims 1 to 36 wherein the replacement nucleotide does not alter the sequence of the polypeptide encoded by said nucleotide sequence, 38. A vector comprising a polynucleotide as defined in any one of the preceding claims. 39. A host cell comprising a polynucleotide as defined in any one of claims 1 to 37 or a vector as defined in claim 38. 40. A pharmaceutical composition comprising a polynucleotide as defined in any one of claims 1 to 37, a vector as defined in claim 38, or a host cell as defined in claim 39, and a pharmaceutically acceptable carrier. 41. A kit comprising (i) a polynucleotide according to any one of claims 1 to 37; or a vector as defined in claim 38, or host cell as defined in claim 39, or a pharmaceutical composition as defined in claim 40; and (iii) a substantially non-toxic agent which is converted by thymidine kinase into a toxic agent. 42. A polynucleotide according to any one of claims 1 to 37, a vector as defined in claim 38, a host cell as defined in claim 39 or a pharmaceutical composition as defined in claim 40 for use in medicine. 43. Products containing a polynucleotide according to any one of claims 1 to 37, a vector as defined in claim 38, a host cell as defined in claim 39 or a pharmaceutical composition as defined in claim 40, and a substantially nontoxic agent for simultaneous, separate or sequential use in treating a patient with cells in need of destruction, wherein said substantially non-toxic agent is converted by thymidine kinase to a toxic agent. 44. A method of destroying cells comprising (ii) introducing into said cells a polynucleotide as defined in any one of claims 1 to 37, or a vector as defined in claim 38; and (ii) simultaneously, separately or sequentially contacting said cells with a substantially non-toxic agent which is converted by thymidine kinase to a toxic agent. 45. A method of destroying cells comprising (i) introducing into said cells a polynucleotide as defined in any one of claims 1 to 37, or a vector as defined in claim 38; (ii) allowing said cells to express thymidine kinase; and (iii) contacting said cells with a substantially non-toxic agent which is converted by thymidine kinase to a toxic agent. 46. A method of treating a patient with cells in need of destruction comprising (i) introducing into the patient a polynucleotide as defined in any one of claims 1 to 37, a vector as defined in claim 38 or a pharmaceutical composition as defined in claim 40; and (ii) simultaneously, separately or sequentially introducing into the patient a substantially non-toxic agent which is converted by thymidine kinase to a toxic agent. 47. A method of treating a patient with cells in need of destruction comprising (i) introducing into the patient a polynucleotide as defined in any one of claims 1 to 37, a vector as defined in claim 38 or a pharmaceutical composition as defined in claim 40; (ii) allowing said polynucleotide or vector to be taken up by said cells; (iii) allowing said cells to express thymidine kinase; and (iv) introducing into the patient a substantially non-toxic agent which is converted by thymidine kinase to a toxic agent. 48. A method of treating a patient with cells in need of destruction comprising (i) removing the cells from the patient or donor cells; (ii) introducing into the cells ex vivo a polynucleotide as defined in any one of claims 1 to 37, or a vector according to claim 38; (iii) introducing the modified cells into the patient; (iv) allowing the cells to express thymidine kinase; and (v) administering to the patient a substantially non-toxic agent which is converted by thymidine kinase into a toxic agent. 49. A method of preventing graft-versus-host disease in a patient comprising: (i) administering to a host T-cells genetically engineered to include a polynucleotide as defined in any one of claims 1 to 37 or a vector according to claim 38; and (ii) administering to said host, prior to the occurrence of graft-versus-host disease, a substantially non-toxic agent in an amount effective to kill said genetically engineered T-cells through interaction of said agent with thymidine kinase. 50. A product according to claim 43 or a method according to any one of claims 44 to 49 wherein the substantially non-toxic agent is any one of ganciclovir, acyclovir, triflurothymidine, l-[2-deoxy, 2-fluoro, (5-D-arabino furanosyl]-5- iodouracil, ara-A, ara 1, 1-P-D arabino furanosyl thymine, 5-ethyl- 2'deoxyurine, 5-iodo-5'-amino-2, 5'-dideoxyuridine, idoxuridine, AZT, ATV, dideoxycytidine, Ara C and bromovinyl deoxyuridine (BVDU). 51. Use of polynucleotide as defined in any of claims 1 to 37, a vector as defined in claim 38 or a host cell as defined in claim 39 in the preparation of a medicament for destroying cells in a patient. 52. Use of polynucleotide as defined in any of claims 1 to 37, a vector as defined in claim 38 or a host cell as defined in claim 39 in the preparation of a medicament for the treatment of cancer. 53. Use of polynucleotide as defined in any of claims 1 to 37, a vector as defined in claim 38 or a host cell as defined in claim 39 in the preparation of a medicament for preventing graft-versus-host disease. ]

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