TRANSGENIC ANIMALS, CELLS AND CELL LINES THEREFROM, AND THEIR USE

Abstract: The provision of cell lines from virtually any cell type of the animal body is greatly facilitated by transgenic non-human eukaryotic animals of the invention in which at least some cells have (i) a differentiation inhibiting sequence chromosomally incorporated under the control of a non-constitutive promotor and/or (ii) a differentiation inhibiting sequence which is itself conditionally active. Said genes are chromosomally incorporated under the control of a promotor such that expression of said sequence is normally held below an effective level, thus allowing normal cell development. However, cells taken from said animal may be prevented from completing differentiation to a non-dividing state in tissue culture by activating expression of said sequence.

Description: T ANSGENIC ANIMALS, CELLS AND CELL LINES THEREFROM, AND THEIR USE This invention relates to "conditionally" transgenic non-human vertebrate animals, e.g. mammals, in which germ and/or somatic cells have chromosomally incorporated therein a nucleic acid sequence or sequences expression of which, during normal animal development, is inhibited but which may be activated in isolated tissue culture. The invention is also concerned with such isolated cultures and their use in producing immortalized cell lines. Such cell lines in turn have many useful applications. The study of physiological function on the cellular level has greatly benefited from the availability of cell lines which allow biochemical experimentation to be conducted on homogenous populations of clonally derived cells. Such cell lines have frequently been derived from tumours of either spontaneous or experimental origin. More recently, it has become possible to use genetic manipulations to create cell lines by the insertion of particular types of genetic information into the cellular genome. The various types of genetic information which allow the generation of cell lines share the property of preventing cells from differentiating into a non-dividing end-stage cell. In Virology 127, 74-82 (1983) , Petit et al described the use of SV40 in the immortalization of rodent embryo fibroblasts, and numerous studies have defined differentiation inhibiting genes by their ability to rescue fibroblasts from entry into a phenotype of senescence, a phenotype normally seen after a limited number of fibroblast divisions. In terms of generation of cell lines, the entry of fibroblasts into such a senescent phenotype is sometimes called "crisis", and the ability to rescue cells from entry into crisis provides a valuable assay system for the identification of genes in this family. As the cells in which this particular type of genetic information is expressed can be established as tissue culture lines which will grow for effectively infinite periods in vitro, this family of genes has been called the establishment genes or immortalizing genes (Land et al, Nature 1983, 304, 596-602, Ruley Nature 1983, 304, 602-606) . As these genes also inhibit end- stage differentiation, they are also sometimes called maturation arrest genes. Current analysis suggests that the differentiation inhibiting genes are frequently members of a family of oncogenes called the nuclear oncogenes. These nuclear oncogeneε include a number of viral oncogenes which as yet have no known cellular counterparts (e.g., SV40 large T antigen, polyoma large T antigen, human papilloma virus E7 antigen) (Jat and Sharp 1986, J. Virology 59, 746-750, Rassoulzadegan ez. al, 1982 Nature 30C, 713-718, 1983 PNAS 80, 4354-4358, Phelps et al, 1988 Cell 53, 539-547, and Petit et al supra) and also a number of genes with known cellular ho ologues (of which myc is the best example) . In addition, some genes in the family of cytoplasmic (or growth-controlling) oncogenes also inhibit differentiation in some cell types. For example, the src gene inhibits differentiation of glial progenitor cells. It is however rare for genes thought to function in the stimulation of cell division also to have the capacity to inhibit cellular differentiation processes. In recent experiments a variety of methods have been used to place immortalizing oncogenes into cells to allow the establishment of cell lines. For example, in Molecular and Cellular Biology Vol 6, p.1204-1217 (April 1986) , Jat et al described a murine retrovirus shuttle vector system and its use to construct recombinant retroviruses for infection of rat Fill cells. Resultant cell lines which expressed SV40 large T were not tu origenic but exhibited efficient microcolony formation (in soft agar) . Moreover, using these recombinant retroviruses, they showed that the SV40 large T antigen by itself is capable of efficiently immortalizing primary fibroblasts without crisis (Jat and Sharp supra) . Various types of transfection strategies exist, and retroviral-mediated gene insertion is the most commonly used strategy for introducing immortalizing genetic elements. These strategies share a number of disadvantages. First, there is at present no means of targeting specific cellular populations. Second, the efficiency of effective gene insertion is low (of the order of 1 in 10 4 cells or less) and therefore requires the use of large numbers of cells in order to establish cell lines with any regularity. Third, effective integration of the genetic element requires the induction of cell division in tissue culture. Fourth, extended growth in tissue culture is required before cells can be used in experimentation, and this growth usually involves a period of time in artificial conditions which impose highly artificial selective pressure on cell populations. It would be of great value to have a method which would allow cell lines to be established from a wide variety of cell types with an efficiency and reliability greater than that available with present technology. In addition, all cells of the body can be divided into the two different classes of precursor cells and end-stage cells. Precursor cells (which include progenitor cells and stem cells) are the cells involved in replenishment of specific cellular populations within the body. They can be restricted to the production of only one type of end-stage cell (e.g., skeletal muscle precursors are thought to give rise to only skeletal muscle), they can be bipotential (e.g., granulocyte- macrophage progenitor cells give rise only to granulocytes and macrophages) , or they can give rise to a multiplicity of cell-types (e.g. , naematopoetic stem cells give rise to all the cells of the bloodstream, and embryonic stem cells can give rise to all of the cell- types of the body) . In contrast, end-stage cells represent cells which have reached a final point in their differentiation pathway, and are no longer capable of generating a multiplicity of cell types or, more importantly, of taking part in replenishment of damaged tissue populations. There are a small number of instances, all of them involving the haematopoetic system, where sufficient knowledge about specific precursor cells exists to permit restoration of normal tissue function by using a primitive form of precursor transplantation therapy to replace both precursor cells and differentiated end-stage cells. This primitive form of precursor therapy is the principle behind the widely-used bone marrow transplant, which works by the emplacement of haematopoetic stem cells (along with other cells) from a donor individual into a recipient suffering from depletion of some or all of their normal haematopoetic populations (frequently as a result of radiotherapy to treat a disseminated malignancy) . The injected haematopoetic stem cells colonize the patient's own bone marrow, and go on to produce megakaryocytes, lymphocytes, macrophages, eosinophils and all of the other diverse cell types derived from this single stem cell population. Although there are many areas of contemporary and future scientific and medical practice where the ability to carry out precursor replacement therapy could be of significant value, efforts in this direction are presently frustrated by the paucity of current knowledge about the identity of the precursor populations which contribute to the normal development of most tissues of the body. Despite the many years of work which has gone into the study of these populations, there are only a limited number of instances where specific precursor populations have been identified and can be manipulated in tissue culture in a manner which might allow them to be subsequently introduced into damaged tissue. Similarly, there are only a limited number of instances in which the identity of the molecular signals which cause precursors to divide or to differentiate along specific pathways is known. Obtaining such knowledge, which could be of great value in causing the body's own precursor populations to more effectively replenish themselves or repair damaged tissue, is hampered enormously by the lack of suitable cellular assay systems and also by the lack of suitable source materials for the purification of these important molecules. In 089/09816, McKay et al described a general method of immortalizing cell lines in which a growth promoting gene is introduced into vertebrate cells. The intention is that function of this gene be controlled by an external factor or factors so that the gene function can be regulated at will. It is stated that precursor cells may then be grown with the gene activated, and the resulting cell population then allowed to differentiate by inactivating the gene by changing the conditions to "non-permissive" conditions. In addition, Almazan et al, at the 18th Annual Meeting of the Society for Neuroscience, Toronto, (November 1988) (see Soc. Neurosci. Abstr. 14(2) 1988 1130) , described the immortalization of an oligodendrocyte precursor cell using a temperature sensitive oncogene-carrying retrovirus. However, although the utilization of existing techniques to insert genes into precursor cells has allowed for the development of particular cell lines with precursor qualities, not only do the problems inherent in the generation of any cell lines apply also to the generation of precursor cell lines, but the establishment of precursor cell lines suffers from the further difficulty that precursors may represent only a small fraction of the cells in any given tissue, and the possibility of successfully introducing the required genetic information into the cells is correspondingly reduced. So-called "transgenic" animals have also been known for some years, i.e. animals having incorporated into the animal genome a foreign gene or genes which may be expressed in their new chromosomal environment to change the characteristics of the animal in a directed manner. Papers on transgenic animals first appeared in the literature in 1982. Thus, Pal iter et al. (Cell, 1982, 29:701-710) microinjected a plasmid containing the mouse metallothionein-I promotor/regulatory region joined to the structural gene of herpes virus thymidine kinase. They showed expression of the hybrid gene in vivo and regulatability of the gene in vivo by heavy metals. Gordon and Ruddle (Prog. Clin. Biol. Res., 1982, 85:111- 124) also demonstrated inheritance of injected DNA sequences. Palmiter et al (Nature, 1982 :611-615) showed that mice transgenic for growth hormone, as regulated by the metallothionien promotor, grew to abnormally large size. In 1983, McKnight et al. (Cell, 1983, 32:335-341), Lacy et al (Cell, 1983, 34:343-358), Palmiter et al. (Science, 1983, 22:809-814) , Brinster et al (Nature, 306:332-336) , and Gordon (J. Exp. Zool., 1983, 228:313- 324) all reported on characterization of transgenic animals without discussion of cell growth in tissue culture. In 1984 the first papers indicating that the expression of oncogenes would disrupt normal development appeared. Brinster et al. (Cell, 1984, 37:367-379) demonstrated that mice expressing SV40 large T antigen under the control of the metallothionien promotor developed tumours of the choroid plexus. Tumours developed long afrer birth, indicating the involvement of more than just the transgene in tumour formation. Cell lines were derived from tumour tissue, but no discussion is offered on attempts to derive cell lines from pre- tumourous tissue or from other tissues. Slightly later, Stewart et al. (Cell, 1984, 38:627-637) reported that mice expressing c-myc under the control of a hormonally inducible mouse mammary tumour virus promotor developed mammary adenocarcinomas. There is no discussion of cell lines. The ability to use cells from transgenic mice in culture was also discussed by Ritchies et al. (Nature, 1984, 312:517-520) , who showed that it was possible to make hybridomas by fusing spleen cells from mice transgenic for the immunoglobulin • kappa gene with normal hybridoma fusion partners. In U.S. Patent 4736866, Leder and Stewart describe transgenic non-human mammals in which the germ and somatic cells contain an activated oncogene sequence. Such animals are, inter alia, useful models for the testing of anti-cancer drugs due to their increased tendency to develop neoplasms, and the lower level of drug dosage that can consequently be employed in such testing. The animals do, however, exhibit a pronounced tendency to develop abnormally, and this virtually eliminates their usefulness as models for normal cell development studies or as sources of therapeutic biological materials. Palmiter et al. (Nature, 1985, 316:457-460} , continuing a previous analysis of the effects of SV40 large T antigen in transgenic mice, showed that the development of choroid plexus tumours required the presence of the SV40 enhancer region, and that different SV40 constructs yielded different types of tumours. Cell lines were isolated from hepatocellular tumours of some mice, and these cells were shown to express T antigen in their nuclei. The authors described production of mice with the ts58 temperature-sensitive derivative of SV40, but stated that this construct was not temperature- sensitive. Despite the widespread use of T antigen in the creation of transgenic mice and the widespread use of the ts58 (=tsA58) mutant in the generation of conditionally immortal cell lines in vitro through the use of, e.g., retroviral-mediated gene insertion, no other work in the prior art (including later work of Palmiter and colleagues) returned to the use of this mutant. It can be said that the general goal in transgenic work reported thus far has been to introduce into the germline genetic information which disrupts normal development, generally of specific tissues. In complete contrast, as will be clear hereinafter, the present invention is concerned with the creation of transgenic animals in which normal development is not disrupted, but from which cells can be harvested, inter alia, in which activation of the transgenes in tissue culture will specifically facilitate the study of cells from potentially all tissues of the body. Thus, for example, even though other laboratories have previously gone so far as to build genetic constructs in which a regulatable promotor (H-2K tl ) has been used to regulate oncogene expression, the possibility that use of such promotors in transgenic animals might allow conditional oncogene expression in vitro has not previously been recognized. The most detailed example of this is the studies linking the H-2K b promotor with the c-myc gene described by Morello et al. (Oncogene Research, 1989, 4:111-125) , who constructed several transgenic strains carrying a fusion gene in which the 5' H-2K b promotor sequences were linked to the human c-myc proto-oncogene in order to determine whethe constitutive c-myc expression was found in all tissues and to ascertain the biological effects of such constitutively enforced yc expression in these transgenic animals. The authors obtained 33 mice which led to the establishment of 5 transgenic strains. The authors reported expression of the H-2/myc construct in most organs analyzed, with maximal expression in the lymphoid organs and minimal expression in the brain and in the liver. The level of expression of H-2K/myc was in parallel with the expression of H-2K. Morello et al. also reported that no pathology was observed over a period of 20 months in four of the H-2K/myc mice, and concluded from this that a second genetic event was necessary for immortalization with this construct. In earlier transgenic work, Efrat and Hanahan (Mol. Cell Biol., 1987, 7:192-198) examined cell-specific activity of a reverse promotor element in two lineages of transgenic mice in which the promotor was used to target T antigen expression to the beta islet cells of the pancreas. Expression of the gene was examined in tumour cells. Also, Efrat et al. (Proc. Natl. Acad. Sci. USA, 1988, 85:9037-9041) examined the behaviour of three pancreatic beta-cell lines established from insulino as derived from transgenic mice carrying a hybrid insulin- promotor SV40 T antigen gene. The beta tumour cells, which were all derived from primary beta-cell tumours, maintained the features of beta cells for 50 passages in culture. The authors concluded that "targeted expression of an oncogene with a cell-specific regulatory element can be used both to immortalize a rare cell type and to provide a selection for the maintenance of its differentiated phenotype". In the present invention, an important contribution to the science is that "targeted expression" is not necessary. As will be clear hereinafter, the present transgenic animals can be storehouses of cell types which can be chosen and taken at will, when desired, for immortalization. In yet other transgenic work, Bieberich et al. (Mol. Cell Biol., 1987, 7:4003-4009) also used transgenic mice to study class I antigen function, and found that skin grafts from transgenic mice were rapidly rejected by mice of the background strain, that the class 1 transgene was inducible by interferon treatment and suppressible by human adenovirus 12 transformation. Also in 1987, Choi et al. (J. Virol., 1987, 61:3013- 3019) examined expression of simian virus 40 early region genes under transcriptional control of the mouse mammary tumor virus long terminal repeat. Cells cultured from the transgenic animals showed expression of the chimeric gene which was inducible by glucocorticoids. Many, but not all, tissues which expressed the simian virus 40 sequences showed premalignant features and developed into tumours. In 1988, Paul et al. (Klin ochenstr. , 1988, 66, Suppl. 11:134-139; Exp. Cell Res., 175, 354-365) created permanently growing hepatocyte lines by growing liver cells from mice expressing SV40 virus sequences driven by the mouse metallothionein enhancer sequence. Most hepatocytes in the liver displayed an immortalized phenotype in culture, and became increasingly transformed-like with further growth in culture. Although the initial cells were nonmalignant, they clearly differed from normal cells in that cells did not require addition of epidermal growth factor to chemically defined medium to promote cell division. In vivc, the mice developed hepatocellular carcinomas. MacKay er al. (Kidney In . , 1988, 33:677-684 . established permanent cell lines of cloned glomerular epithlial, mesangial and endothelial cells from a line of mice transgenic for simian virus 40. These mice appeared normal at birth but by 3 to 4 months of age had sclerosis affecting a variable percentage of their glomeruli. The cells derived from these mice maintained features characteristic of their normal counterparts despite their transformed phenotype. Langdon et al. (Oncogene Res., 1988, 3:271-279) studied the growth of E mu-myc transgenic B-lymphoid cells in vitro to examine their progression to lymphomatous characteristics. Results demonstrated that cells initially required bone marrow feeder layers, after which cultures resolved to monoclonal or oligoclonal composition and then only at a later point achieved growth autonomy, thus indicating the importance of multiple events in establishing growth autonomy. In 089/09816, supra, there is a suggestion that it may be possible to introduce the conditionally immortalized cells described therein into animals, and thereby produce transgenic animals in which the growth promoting gene present in said cells is inactive at normal body temperatures. However, the work described does not take up this suggestion, and how such a technique should be successfully applied is simply not described. There is no disclosure of a promotion system or conditional oncogene producing only lov. * levels of expression as are used in the present invention (see below) , nor is there any disclosure of the use of a stable transgenic animal as a source of cells, differentiated or precursor cells (which later may be immortalized in culture) . It would clearly be highly desirable to provide a method for efficiently obtaining cell lines from potentially any tissue of the body. That is a goal which, aside from specific problems noted above, and even with the immense interest in transgenic animals, has not hitherto come remotely close to realization in the art. The present invention provides in one aspect a transgenic non-human eukaryotic animal having germ cells and/or somatic cells into which a differentiation inhibiting sequence has been chromosomally incorporated under the control of a non-constitutive promotor such that expression of said sequence is normally inhibited allowing normal cell development, but precursor cells taken from said animal may be prevented from completing differentiation in tissue culture by providing permissive conditions for said promotor thereby activating expression of said sequence. In connection with these "conditional" transgenic animals, by "non-constitutive promotor" as used herein is meant a promotor system which: (a) can be induced to cause much higher levels of expression of the sequence under its control than occur in the absence of induction, and (b) in tne absence of induction either does not detectably permit expression or permits expression only at a level which does not inhibit normal cell development. It will be appreciated that a promotor system which is "leaky" or which does permit a small amount of expression may be tolerated if the level of expression fails to impede a normal development pattern. As will be apparent, there are various ways in which expression levels may be kept down in the animals but still be induced to rise when desired. "Conditionality" may thus be achieved using multiple means, as in the specific work described herein which uses a form of double conditionality involving a conditional oncogene. Multiple conditionality may, for example, alternatively depend upon the use of a promotor system employing two or more regulatory genetic elements, the regulation of all of which is necessary to cause the higher levels of expression referred to in (a) above to be achieved. In another aspect, the invention relates to a transgenic non-human animal having cells into which a differentiation inhibiting sequence had been chromosomally incorporated under the control of a non- constitutive promotor, wherein expression of said differentiation inhibiting sequence is inhibited in the absence of activation for said non constitutive promotor. Well differentiated cell types can, if desired, be effectively put into tissue culture from such animals, and, in addition, the present invention provides a method for obtaining precursor cells from normal tissue in a manner which greatly improves the potential for understanding not only the identity of these cells but also the biological principles which regulate their development. The present invention, by its achievement of low expression levels in vivo in the absence of induction, enables successful creation of transgenic animals in which development and differentiation of tissues occurs normally in vivo, providing a storehouse of biological material for various purposes. In general, the invention concerns transgenic non- human eukaryotic animals with germ cells and/or somatic cells which contain a differentiation inhibiting DNA sequence which is inactive in most, or all, tissues of the normal animal and which is constructed so as to specifically have the minimum possible effects on normal development of the animal. Activation of the genetic construct, which is capable of preventing terminal differentiation, is preferentially achieved through manipulation of dissected tissue in vitro, although the constructs can also be activated in vivo. Thus, in another aspect, the invention provides a transgenic non-human eukaryotic animal having gerr. cells and/or somatic cells into which a differentiation inhibiting sequence which is itself conditionally active and inducible has been chromosomally incorporated under the control of a promotor such that expression of said sequence is normally held below an effective level thus allowing normal cell development but precursor cells taken from said animal may be prevented from completing differentiation in tissue culture by activating expression of said sequence. In animals of the type defined immediately above, the conditionally active differentiation inhibiting sequence may be TAgts and/or the promotor may be a "weak" non-inducible promotor, e.g. the TK promotor. It will be appreciated by the skilled reader that molecular enegineering of any of a large number of promoters (as reviewed in, for example, Levine and Manley, Cell, 1989, 59:405-408, Abel and Maniatis, Nature, 1989, 341:24-25, and Mitchell and Tjian, Science, 1989, 245:371-378 will enable the creation of weak, non-inducible promotors also suitable for use in the present invention. In a further aspect, the invention includes a cell which has been isolated from an animal as defined above or which has been derived from such an isolated cell and which has chromosomally incorporated therein said differentiation inhibiting sequence expression of which may be activated. The invention also includes a cell line derived froir such a cell and which has been immortalized by activating expression of said differentiation inhibiting sequence. In yet a further aspect, the invention includes a differentiated cell derived either from a cell as defined above by allowing it to differentiate without activation of expression of said differentiation inhibiting sequence or from a cell line as defined above by deactivating expression of said differentiation inhibiting sequence or from a cell as defined above wherein expression of said differentiation inhibiting sequence has been activated but which cell may nonetheless be induced to differentiate by exposure to an external factor and has been so exposed. The invention also includes a method of producing a transgenic non-human eukaryotic animal having cells into which a differentiation inhibiting sequence has been chromosomally incorporated but in a regulable manner such that expression of said sequence may be activated but is normally inhibited allowing normal cell development, comprising effecting chromosomal incorporation of said differentiation inhibiting sequence under the control of a non-constitutive promotor into at least some cells of said animal or effecting chromosomal incorporation of a differentiation inhibiting sequence which is itself conditionally active and inducible into at least some cells of said animal. The technique used to achieve transgenesis is immaterial to the inventive concept, but normally micro- injection at the embryonic stage is the preferred route using procedures well known in the art. Micro-injection can be used at any developmental stage from the unicellular stage to later embryonic stages. Transgenic animals can, however, be generated in a variety of ways, all of which are to be seen as encompassed within this invention. The genetic construct can be inserted into embryonic stem cells, and these genetically manipulated stem cells can be injected into fertilized zygotes at a stage where a small number of cells are present. The embryonic stem cells in some cases become incorporated successfully in the zygote and cells derived from these genetically manipulated cells can differentiate to form many or all of the cell types found in the body. In some animals, such cells will also contribute to the germ line, thus providing a means of making fully transgenic animals as progeny from the initial chimaeras. It has also been suggested that sperm themselves can be used as a vector for creating transgenic animals (although this claim is currently considered controversial) and so the possibility of incorporation as a result of events at a preconception stage should not be discounted. The manner and timing (in animal developmental terms) of achieving chromosomal incorporation in accordance with the invention does net matter, provided that the differentia ion inhibiting sequence ultimately appears in at least some cells of a transgenic animal under the control of a non-constitutive promotor so that the sequence is regulatable. The central feature of this invention is the intent to avoid a level of expression of the experimentally- introduced genetic information such that normal development is impeded. This feature distinguishes the present invention from all other transgenic experimentation thus far reported. This necessary aspect of the present invention is principally achieved by utilizing promotor sequences which must be specifically activated in order to allow appreciable expression of the differentiation inhibiting gene. In the non-limiting Examples provided hereinafter, oncogene function is further limited by using an oncogene sequence which itself is only conditionally active. The invention accordingly envisages the principle of multiple conditionality. This may be achieved, for example, by the use of a plurality of genetic regulatory elements which, used in concert, provide an overall non- constitutive promotor system as defined above, or it may be achieved by the use of one or more genetic regulatory elements and conditional oncogene-like sequences which, again, used in concert, achieve the desired effect thus providing such a system. The differentiation inhibiting sequences relevant to this invention, are capable of inhibiting differentiation of precursor cells. Sucn sequences or genes include oncogenes which encode for proteins which localize to cellular nuclei. A number of these nuclear oncogenes have the ability to immortalize cells (and thus render them capable of growth for indefinite periods without entering a state of terminal differentiation) and also to inhibit the differentiation of precursor cells into non- dividing end-stage cells. Several of these genes are not only of viral origin but also have, at present, no known mammalian counterparts in the normal genome. For example, the normal cellular counterparts of SV40 large T antigen, human papilloma virus E7 protein and polyoma large T antigen are currently unknown, even though it has been shown that all of these viral proteins are able to interact with normal cellular proteins in a manner thought to be related to the function of the viral proteins in neoplastic transformation (Whyte P. et al, 1988 Nature 388, 124-129, De Caprio J.A. et al, 1988 Cell 54, 275-283, and Dyson N. et al, 1989 Science 243, 934- 937) . Some normal cellular proteins which localize to the nucleus, such as c-myc, also are able to immortalize cells and inhibit precursor cell differentiation (Land et al supra, Dotto G.P. et al, 1985 Nature 318, 472-475, and Dmitrovsky E. et al, 1986 Nature 311, 748-750) . In addition, a small number of oncogenes thought to be associated with growth regulation, such as src, have the ability to immortalize and inhibit the differentiation of specific precursor populations. Broadly, it can be stated that at present there are five diverse groups of genes which may function as differentiation inhibiting sequences in this invention. Some of the known genes in each category are listed in the Table below. The first category of genes are members of the nuclear oncogene family, and include genes like SV40 large T antigen and also genes like myc and myb. The second category of genes are those which can be converted by mutation from suppressor genes to immortalizing genes. The one representative of this category known at present is p 53 , which appears to interact in an as yet unknown manner with the family of proteins also involved in modulation of the retinoblastoma gene product activity. The third category of genes are those commonly thought to be involved in control of cell proliferation, but which also appear to be able to inhibit the differentiation of some cell types. The fourth category of genes are typified by a secreted molecule called differentiation inhibiting activity, which seems to work through cell-surface receptors to inhibit the differentiation of embryonic stem cells. Finally, it has recently been discovered that co-operative interactions between mitogens can also inhibit precursor cell differentiation. In studies on the oligodendrocyte-type 2 astrocyte progenitor cell of the rat optic nerve, it has. been found that stimulation of these precursor cells simultaneously with platelet- derived growth factor and basic fibroblaεt growth factor completely inhibits differentiation of precursors into oligodendrocytes and allows 0-2A progenirors to be grown indefinitely in tissue culture in the apparent absence of mutational activation of nuclear oncogenes (Bogler et al., 1990, Proc. Natl. Acad. Sci. USA. 87:6368-6372) . These last two sets of results indicate that it may be possible also to use soluble factors, controlled by inducible, non-constitutive promotors, to inhibit differentiation of cells. In such circumstances, the "differentiation inhibiting sequence" used in the invention is the genetic sequence encoding said factors. TABLE Nuclear oncogenes SV40 large T polyoma large T adenovirus EIA HPV E7 and E6 myc erb A myb - dominant mutations altering tumour suppressor genes some mutants of p53 growth regulatory genes which inhibit differentiation V-src genes which produce differentiation inhibiting agents differentiation inhibiting activity genetic sequences encoding combinations of growth factors which work to inhibit differentiation platelet-derived growth factor + basic fibroblast growth factor. Lists of nuclear oncogenes, which are most frequently the genes having differentiation inhibiting capability, have recently been compiled by Hunter (Cell, 1990, 64:249-270) . The differentiation inhibiting sequences used in the present invention are included in the genome in a manner which limits the potential of the immortalizing gene to function in vivo. For example, a thermolabile form of the SV40 large T antigen (TAg) (Tegtmeyer, 1975 J. Virology 15, 613-618) may be used which is rapidly degraded and thus inactivated at the normal body temperature of the mouse (39.5°C) . In an embodiment of this invention this temperature-sensitive TAg (TAgts) is placed under the control of a promotor which normally controls expression of Class I antigens of the major histocompatability complex (Kimura et al, 1986 Cell 44, 261-272, and Baldwin Jr. A.S. et al, 1987 Mol. Cell Biol. 7, 305-313) . This promotor can be activated by exposure to gamma interferon, but is normally active only at low levels in most tissues of the body of healthy animals. The rationale behind the use of a controllable, non- constitutive promotor iε to inhibit expression of the . gene of interest, except when such expression is desired. Non-constitutive promotors which are usable in the present invention are regulatable by changing the conditions. Under certain conditions (non-permissive) the promotor function is inhibited and normal cellular development takes place. If the conditions are changed (e.g. , in the example referred to above, by exposure to gamma interferon) to be permissive, appreciable expression occurs. Avoiding appreciable expression is critical, as it is known that revertant mutations of a conditional oncogene itself would be likely to cause abnormal development if the oncogene were allowed to be expressed in all tissues at all times, that low levels of activity of a conditional oncogene might become effective if high enough levels of the gene product are expressed (as described in the Example 1 hereinafter) , and expression of functional levels of wild-type differentiation inhibiting sequences in vivo causes tissue transformation (as demonstrated by such other transgenic models as those offered by, e.g., U.S. Patent 4736866) . Revertant mutations which occur by random mutation seem to occur with a frequency of 1 in 10 e cells. As the body of an animal contains -many more than 10 12 cells, this means that many cells of the body will express revertant mutations of the conditional gene. Moreover, experimentation in tissue culture indicates that revertant mutations which involve other cellular control pathways may even occur at frequencies of up to 1 in 10 4 . Thus, every tissue in the body would be expected to carry large numbers of cells which would be expressing, say, a functional oncogene capable of inhibiting normal pathways of differentation. Such a situation is incompatible with normal development. Indeed, prior to the realization of this critical concept, unpublished attempts were made by one of the inventors (P. Jat) and colleagues to create transgenic mice in which TAgts was placed under control of the promotor for beta-actin, which would cause TAgts to be constitutively expressed at a reasonably high level in every cell of the body. Such genetic constructs do not appear to be compatible with survival of the manipulated embryo. Therefore to circumvent the problem of reversion it is essential to use a promotor which dampens expression of TAgts to below an effective level. Another possible promotor system for use in this invention would be one based on a lactose (lac) inducible operon isolated from bacteria. The lac-inducible system is based on having binding sites for a particular protein located between the transcriptional promotor and the transcriptional start sites. Normally, the repressor binds to the operator and sterically hinders transcription. When the inducer substance is present in the cell, it complexes the repressor and prevents it binding to the operator and thereby allows transcription to occur. The specific inducer used is the allo-lactose analogue IPTG, which is non-metabolizeable and does not occur naturally. Lactose can also activate this inducer, but lactose is rapidly metabolized and is present at low levels in almost all tissues. The one bodily fluid known to contain high levels of lactose is milk, raising the possibility that this construct would be induced in the milk-producing cells of lactating females. Unlike Class I antigens, which are expressed by many cells in both a constitutive and inducible manner (resulting in an increased level of expression as compared to the situation before induction) , the lac repressor may yield a still tighter control of expression of the differentiation inhibiting genes used in the present transgenic animals. The lac system is also, of course, known to operate in mammalian cells. The lac-inducible promotor is the one known example of a bacterial promotor which can be activated by a substance which is not likely to cause induction in normal warm-blooded animals, but one can envisage that other analogous bacterial promotor systems will be discovered. In addition, it would be possible to use mutant forms of bacterial promotors, as exemplified by the lap mutation of the lac repressor. Such promotors could be used in the method of this invention. A further example of an inducible promotor is the metallothionein promotor, which is activated by the heavy metals zinc and cadmium. This promotor could also be used in the method of this invention. The MMTV promotor, which is regulated by glucocorticoids, is an inducible promotor which has been used in transgenic experimentation. This promotor, however, suffers from the liability that endogenous glucocorticoid production would activate expression of the differentiation inhibiting construct. At least two advantages of the Class I promotor system can be cited, apart from its general applicability across the animal kingdom (see below) . First, in those tissues in which there is a low level of endogenous Class I antigen expression, the level of activity of this promotor appears normally to be too low to support production of, say, sufficient TAgtsA58 antigen to interfere with normal development. However, addition of an inducer (e.g., gamma interferon) can superinduce activity of this promotor and bring levels of TAgtsA58 antigen to a level where full activity of the differentiation inhibiting gene can be seen. Moreover, the use of this promotor allows expression of, say, TAgtsA58 to be induced in tissues which there is normally no Class I antigen expression (such as the central nervous system) because essentially all cells have functional interferon receptors. A major advantage of the present invention is that the general concepts developed can be applied to all species of warm-blooded animals. For example, in the example referred to above, the Class I promotor normally causes high levels of expression of major histocompatability complex genes when cells are exposed to gamma interferon (Wallach D. et al, Nature 299, Q33- 836) . This pathway of gene activation is already certainly known to occur in humans, bovines, rats, and mice, and seems likely therefore to occur in all warm¬ blooded animals. In addition, as already mentioned, the bacterial lac promotor is known to operate in mammalian cells. The animals of this invention can be used as a source material for the growth, identification, purification and detailed analysis of, inter alia, precursor cells from potentially all tissues of the body (Morston G. et al, Hemopoietic Growth Factors, A. Review Cancer Research 1988, 48, 5624-5637) . Dissected tissue can be placed into tissue culture in conditions which activate the immortalizing/ differentiation inhibiting gene, and precursor cells can then be grown indefinitely. In addition, cells not normally thought of as precursor cells (such as fibroblasts) can also be immortalized by the experimental manipulations which form a part of this invention. As previously indicated, the animals of this invention differ from the animals of U.S. Patent 4736866 ih that they are not suitable for testing of carcinogens or for testing of materials thought to confer protection against the development of neoplasms. Generally, the tissues of the animals of the present invention undergo normal development until the time they are placed in tissue culture. In the case of the present performed work, normal development has been experienced with the exception of the thymuε (which shows a delayed hyperplastic enlargement of the entire organ) , and even within the thymuε the function of the genetic conεtruct which preventε terminal differentiation iε itεelf conditional. Evidence exiεtε that cells which have been subjected to abnormal developmental conditions (aε in U.S. Patent 4736866) do not expreεε the propertieε of normal cells, and that such cells are also predisposed to undergo further mutations which activate oncogenes. Thus, cells harvested from the tissues of animals which express activated oncogenes in vivo may be unreliable aε suitable models for the study of normal cells, and particularly for the study of normal precursor cells. In contrast, cells isolated from the tissues of the animals described in the present invention are expected to have undergone normal development, and when grown in vitro can be expected to be as close to their normal counterparts as it is possible to be once a cell expresses an immortalizing protein. Accordingly, another aspect of the invention provides a method of providing immortalized cells, which method comprises isolating from an animal of the invention precursor or differentiated cells having chromosomally incorporated therein said differentiation inhibiting sequence and subjecting said cells to conditionε in tiεsue culture whereby expression of said differentiation inhibiting sequence is activated. Preventing levels of oncogene expression which might perturb normal development could also theoretically be achieved by manipulation of the promotor system to create still finer tuning mechanisms than exist with a single genetic control element. For example, a theoretically useful promotor syεtem would be that described by Reid et al., Proc. Natl. Acad. Sci., USA., 1989, 86:840-844, in which the thymidine kinase promotor (which iε constitutively expreεsed at low levelε) is put downstream of a short (18 nucleotide) sequence which is sufficient for conferring inducibility with interferon onto the TK promotor. This promotor construct could theoretically be used to drive expression of tsA58 and thus lower the basal constitutive level of expresεion while εtill retaining the interferon inducibility. It is also possible to extend the principle of dual conditionality offered herein, in which the expression of the thermolabile TAgtsA58 mutant of SV40 iε controlled by the promotor elementε of the Claεs I antigen gene. This could be extended by the use of other temperature- sensitive genes with the capacity of inhibiting normal differentiation. In addition, it is possible to build chi eric oncogenes which are rendered conditional by- virtue of containing a hormone receptor sequence which iε essential for oncogene function. Examples of such proteins are discussed by Picard et al. (Cell, 1988, 54:1073-1080) and Eilers et al. (Nature, 1989, 340:66- 68) , who have produced adenovirus E1A protein with a hormone binding domain of the rat glucocorticoid receptor and myc protein with the binding domain of the human oestrogen receptor, respectively. In both instances, the effect of the chimeric protein on host cell function is dependent upon binding of the appropriate hormone to the chimeric protein. In a particular embodiment of the present invention, the differentiation inhibiting εequence or gene uεed iε a temperature sensitive large T antigen derived from simian virus SV40. The utilization of this gene imparts a secondary level of control on gene activity during normal development, in that the protein encoded by this mutant gene iε rapidly degraded at temperatures approximating those of the normal body temperature of a mouse. However, as described elsewhere herein, sufficient evidence exists to indicate that utilization of the temperature sensitive gene in combination with a constitutive (i.e., non-regulatable) promotor would be expected not to be compatible with normal development if the promotor was aε powerful aε, for example, the beta- actin promotor. Thus, other differentiation inhibiting genes can also be used in the present invention, insofar as they are regulatable by a non-constitutive promotor which is inactive in most or all tissues of the normally developing body. Primary cells uεed in the work deεcribed below to illustrate this invention were cells from the skin, thymuε, pancreas, central nervous system, colonic crypts, endothelium, skeletal muscle, and enteric glial cells. However, it is to be understood that the method of the present invention can be used to immortalize virtually any type of cell from the body of an appropriate transgenic animal. Because the particular cellε chosen to exemplify the invention herein are from tiεεueε in which extensive cellular characterization has been carried out, it can be confirmed that the cell lines derived by the method of the present invention express properties expected of normal cells. These cell lines thus provide direct verification of the ability of the present method to be used to produce continuous cell lines of a wide variety of cell types from a wide variety of tissues. In addition, some of the work deεcribed herein demonstrates directly the utility of the animals in generating cultures in which novel cell types are amenable to study. In the accompanying drawings:- Figure 1 is a schematic repreεenration of genetic construct H-2K b tsA58, referred to in the Examples hereinafter; Figure 2 is a growth analysis diagram demonstrating that control of the biological activity of the SV40tsA58 gene occurs in accordance with the principles of the invention; and Figure 3 showε DNA synthesis under various conditions for heart fibroblast cells from a mouse in accordance with the invention and carrying the genetic construct of Figure 1. The skin waε utilized as a principal source of fibroblastε to confirm that the general principles embodied in this invention were correct. Cells from the skin were placed into tissue culture in conditions which activate the promotor used and at temperatures which are permissive for expresεion of the immortalizing function of TAgts. These cells were grown for various lengths of time before being switched to conditions which did not activate promotor function, which were non-permiεεive for TAgts function, or both. As shown in Figure 2 for different transgenic mice, removal of the promotor activating compound (in this case murine gamma interferon) was associated with a reduction in the growth rate of the cells. Growth of cells from most animals was not completely suppressed unless the cells were grown in the absence of gamma interferon and at 39.5°C; this capacity of cells to continue growing, albeit at a reduced rate, in the absence of gamma interferon seems likely to be due to the low level of constitutive expression of Class I antigens in fibroblasts (Israel A. et al, Nature 322, 743-746) . Indeed, it is striking that even the low level of constitutive expression of this promotor which is seen in normal fibroblasts was not associated with the development of any obvious hyperplastic abnormalities of the skin. Figure 2 also showε that expression of a higher copy number of the gene of interest can be associated with continued slow growth of cells even when cells are grown at 39.5°C in the absence of interferon (cultures derived from animals 11 and 36 in the work deεcribed below) . Thiε slow growth is similar to that seen if cells deprived of gamma interferon are grown at 33°C, and is probably due to a breakthrough of activity of the large amounts of T antigen produced (due to the presence of multiple gene copies) prior to its inactivation through degradation at this non-permissive temperature. The entry of fibroblastε into a non-dividing state when activity of the differentiation inhibiting gene is terminated by gamma interferon withdrawal and growth at 39.5°C is similar to that observed in previous studies on the effects of immortalization of fibroblastε with TAgts. In these previous studies (in which TAgts expression was controlled with a constitutively active viral LTR and in which retroviral-mediated gene insertion was used to generate cell lines expreεsing only a single copy of the TAgts gene) , cells grown at 33°C could be grown in tissue culture indefinitely. In contrast, when cells were switched to 39.5°C they rapidly lost the capacity tc undergo further cell division (Jat and Sharp, 1989 Mel.. Cell Biol . 9 , 1672 -1681 ) . The entry of conditionally immortalized fibroblastε into a non-dividing εtate is of particular interest in respect to the way in which this non-dividing state reflects a normal differentiation pathway of fibroblasts. Normal fibroblasts undergo a limited number of divisions before entering a state of senescence in which the cells show normal metabolic function with the exception of being refractory to further cell division (Hayflick L. et al, 1961 Exp. Cell Res. 25, 285, Todaro et al, 1963 J. Cell Biol. 17, 299-313) . The phenotype expressed by conditionally immortalized fibroblasts when they are switched from permiεεive to non-permiεεive conditionε resembles the state of normal seneεcence εo cloεely as to be indistinguishable. Thus, conditionally immortalized cells can undergo the differentiation events of their normal counterpartε when grown in non-permiεεive conditions. Two of the cell lines developed from the thy us, as an illustration of this invention, are defined aε epithelial cell lines by virtue of expresεion of cytokeratins. The derivation of an epithelial cell line was of particular interest because of the importance of these cells in human cancer, and the derivation of a thy ic epithelial cell line was of further interest due to the putative importance of these cells in the development of the T-Iymphocyte populations of the thymus. Cell lines were derived from the thymus because of the tendency of many of the transgenic mice produced to develop thymic hyperplaεia. The cell lines derived from this tisεue all behaved in a conditional manner in tissue culture, and were growth arrested when grown in the absence of interferon at 39.5°C. The conditionality of these cell lines in vitro indicates that even in this tissue the constitutive levels which occur in vitro are insufficient to cause sufficient levels of TAg to be expressed to be able to interfere with normal processes of differentiation and growth control. Such an observation is consistent with the hypothesis that the generation of thymic hyperplasia in vivo was enhanced by the presence of a hepatitis infection in the mouse colony. Such an infection would cause augmented production of interferon in the thymus, thus driving the levels of T antigen above the threshold of non- effectiveness. These results further support the view that it is necesεary to avoid levelε of TAgts being inappropriately expressed in every cell in body (rather than just thoεe cells exposed to inducer) as a consequence, in this case of diεease in the mouse colony) . A cell line from the central nervous system, as a further illustration of this invention, is of particular interest aε a putative precursor for glial tumours and as a demonstration of the potential usefulnesε of the transgenically derived cell lines aε tools for the stud*y of differentiation control. Thiε cell line expreεses astrocyte specific antigens when grown in certain tisεue culture conditionε, but can be induced to express a fibroblaεt-like phenotype in other tisεue culture conditions. The rationale for the characterization of this CNS line as a putative glioma precursor comes from observations that human gliomaε can be antigenically divided into two categories: cells which express glial fibrillary acidic protein (GFAP) and are clearly derived from astrocytes, and cells which do not expreεs GFAP but instead expreεε fibronectin (FN) . It haε been demonstated that cloning of GFAP-expresεing lineε can lead to the generation of GFAP-negative cell lineε which express fibronectin, thus suggeεting that the fibronectin-expreεsing cells (which do not correlate v.'ith any known CNS glial cell) may be traceable back to a CNS lineage. It is potentially relevant to these observations that some experiments indicate that an apparently rare subεet of GFAP-poεitive aεtrocyteε may also express FN (which is not expressed by most astrocytes) . The cell line described herein can be switched from a GFAP-positive phenotype to a FN-positive GFAP-negative phenotype by growth in fetal calf serum. The ability to manipulate the differentiation of this cell provides a suitable assay syεtem for use in the purification of the specific molecular εignalε which induce differentiation along the FN-positive GFAP- negative pathway. A further cell line prepared by the method of the present invention waε derived from the pancreas. A small percentage of the cells in thiε line spontaneously express insulin in all conditions of growth, and some cells of the line also can be labeled with a monoclonal antibody (A2B5) thought to label islet cellε of the pancreaε. It iε not yet known if the variable expreεsion of these markers within the cell line is due to failure to create appropriate differentiation-inducing microenvironments in the tissue culture conditions. Other matters specifically illustrated by the work described herein (and relevant aspectε of the invention) are specifically referred to in the respective Examples hereinafter. The following non-limiting Examples are given to demonstrate and illustrate the principles of the invention. As used in what follows, "mouse x" means the xth mouse from the first described experiment. EXAMPLE 1 Construction of pH-2KbtsA58 Genetic Construct Recombinant pH-2KbtsA58 (see Figure 1) was constructed by attaching tne ' -promotor element of the H-2Kb gene to the early region coding sequences from the SV40 mutant tsA58. The promotor fragment was isolated as the approximately 4.2kb EcoRl-Nrul fragment from plaεmid pH-2Kb (which waε provided by Dr. Andrew Mellor, MRC London). Thiε plasmid was constructed by cloning the EcoRl fragment which encompasses the genomic sequences encoding the H-2Kb gene from the coεmid 88H8 (Weiεε et al, Nature 1983, 301, 671-674) into plaεmid pBR327. The tsA58 DNA was described by Tegtmeyer 1975 J. Virology 16, 168- 178. In the present Examples, the SV40 tsA58 coding sequences were isolated as an approximately 2.6 kb Bgll-BamHl fragment from pUCSV40tsA58 which waε provided by Dr. Hartmut Land (of the Imperial Cancer Reεearch Fund, London) . Thiε plasmid was constructed by inserting the Kpnl (nucleotide 294) to the BamHl (nucleotide 2533) fragment from tsA58 DNA, which encodeε the T antigen coding sequenceε, into the Kpnl and BamHl εites of pUCl9. The Bgll site was blunt-ended using the Klenow fragment of DNA polymerase I. The two fragments were ligated with an equimolar amount of pucl9 which had been digested with EcoRl and BamHl. The ligated products were transformed into JS4 , a recA~derivative of E. coli MC1061 (Casadaban and Cohen 1980 J. Mol. Biol. 138, 179-207; Sedivy et al 1987 Cell 50, 379-389) and ampicillin colonies isolated. DNA miniprepε were prepared from isolated colonies and analysed by digestion with various restriction endonucleaseε to determine if the promotor fragment had been successfully fused to the T antigen coding sequences. Production of Transgenic Mice containing H2K^-Taqtε fusions The above H-2K b -TAgtsA58 plasmids were digested with EcoRl and Sail to prepare DNA fragments which were free of vector sequenceε. These DNA fragments were isolated on agarose gels and injected into male pronuclei of fertilized one-cell mouse eggs at a concentration of 1-2 ug/ml DNA in TE buffer (lOmM Triε, pH 7.5, 0.2 mM EDTA) . The eggε which survived micro-injection were transferred to pseudopregnant femaleε aε deεcribed in Wagner et al (1981) P.N.A.S. 78, 5016. The eggε were derived from a CBA x C57BL/10 mating. The mice were obtained from the MRC breeding colonieε, and were houεed in an environmentally controlled facility maintained on a 10 hour dark: 14 hour light cycle. The eggε in the foster femaleε were allowed to develop to term. Analysis of Tranεgenic Mice At 7-14 days of age, each pup was analyzed to determine if it carried the transgene. DNA prepared from a small section of tail was initially analyzed on a slot blot. Genomic DNA was isolated from 0.1- 0.15 cm sections of tail by the method described in Sambrook et al "Molecular Cloning" (Cold Spring Harbor 1989) . The resulting nucleic acid pellet was washed once in 80% ethanol, dried and resuspended in 200 ul of 10.0 mM Triε, pH 7.4 , 1 mM EDTA. The presence of the construct was determined by hybridizing the filter with a 32 P-labeled fragment specific for the SV40 large T antigen. The probe was prepared by the random priming method of Feinberg & Vogelstein. The integrity of the TAg gene was verified by Southern blot analysis by digesting 10 ug of tail DNA with BamHl. The digested DNA was fractionated on an 0.8% agarose gel, transferred to Zeta Bind™ (Biorad) and hybridized with an SV40 specific probe using published methods (as described in Sambrook et al) . Blots were probed with a 32 p labeled probe for the TAg gene. All manipulations were carried out using either the manufacturers recommended conditions or by standard protocols as deεcribed in Sambrook et al, supra. The slot blot indicated that 34 out of 88 mice carried the chimeric gene. The number of copieε of this fusion gene present in the mice DNA varied from 1 up to 15 copies per cell. Animals containing the H-2K b -TAgtsA58 fusion gene developed normally with the exception of the development of a thymic hyperplaεia. There appeared to be a diεtinct correlation between the levels of. TAg mRNA expresεed and the rapidity of onset of thymic hyperplasia. That the thymic enlargement seen was not the result of neoplaεtic tranεformation waε indicated by the fact that both lobes of the thymus were equally enlarged, and that injection of even as many aε 10 7 thymuε-derived cells into naive recipient mice (either subcutaneouεly or intraperitoneally) in no case caused tumours in the host mice. In addition, aε deεcribed below, almost all stromal cell lines derived from these enlarged thymi appeared to be conditional for growth in culture. The thymic hyperplasia may have been in part due to the presence of an infection of mouse hepatitis virus in the animal colony, which would be expected to cause interferon production in infected animals. Therefore the hepatitis infection is likely to have been aggravated by the already high levelε of endogenous Clasε I antigen expreεsion in thymic tissue, resulting in expression of . TAgtsA58 at a higher level in vivo than would be the case for other tissues of the body. One of the strains of mice expressing only one copy of the hybrid gene took longer (6 months for heterozygotes and 3-4 months for homozygotes) for the hyperplasia to occur. As described below for cells grown in tissue culture, in cases where large a ountε of TAgtsA58 are transcribed .in association with the presence of multiple genetic copies in the DNA of the mice) , it appears that marginal effects of TAgtsA58 on growth promotion do occur even at the permiεsive temperature. In one of the mice in which only one copy of TAgtsA58 was present in the genome thymic hyperplaεia developed only after several months, and mice were able to breed normally and effectively, transmitting the H2K b -TAgtsA58 fusion to the offspring. Analysiε of Fibroblaεtε from H2K----TAgtsA58 Transgenic Mice The following demonstrates the importance of screening for mice which show low levels of transgene expreεεion in vivo in order that the mice function in accordance with the principles of the invention. Aε already indicated, this method of selection is antithetical to the standard methods of transgenic animal selection, in which high levels of in vivo transgene expresεion are εought εo as to have a maximal probability of disrupting normal in vivo development. To demonstrate that the expresεion of H2K b TAgtsA58 in tissues of transgenic mice allowed the generation of cell lines which were conditionally immortalized, the growth of skin fibroblasts was examined. Fibroblasts were derived from 5 different mice by firεt sacrificing the mouse by cervical dislocation, sterlizing the skin and fur with ethanol, shaving off the fur, and dissecting several square cm of skin. This skin waε then finely minced with a sterile scalpel and digested in 500 units per ml collagenase for 2 h at 37°C in Leibovitz'ε L-15 medium. Trypsin was then added to a final concentration of 3000 units per ml, and tiεsue was incubated for a further 15 min at 37°C. Following thiε incubation, the enzymatic digestion waε terminated by the addition of a εolution of soy bean trypsin inhibitor (1000 unitε per ml) and DNAse (15 units per ml) also prepared in L-15. Tissue waε then brought to a volume of 5 ml and was gently triturated up and down in a sterile plaεtic pipette a total of 20 timeε. Undiεsociated chunks of tissue were allowed to settle out and the cells contained in the supernatant were first washed by centrifugation and then resuspended in Dulbecco's Modified Eagle's Medium containing 2 mM glutamine and 10% fetal calf serum and 100 U/ml of recombinant urine gamma interferon. In all caεeε in which cellε were derived from mice carrying the H2K b - TAgtsA58 the cultures prepared in this way grew effectively in tissue culture flasks. Cells were also prepared and grown in identical ways from identically aged normal controls and as, discussed below, the cellε from normal mice succumbed to senensence after short periods of time. The cultures prepared from the H2K b -TAgtsA58 fusion transgenic mice were grown at 33°C in the presence of 100 U/ml of gamma interferon for between 8 and 12 weeks before being tested for the conditionality of their growth. Long before this time all cells derived from mice which did not harbour the H2K b - TAgtsA58 fusion construct had undergone criεiε and εtopped dividing, as expected for non-immortalized fibroblaεtε. In all fibroblast lines derived from the H2K b TAgtεA58 fuεion tranεgenic mice, the placement of these cells in non-permissive conditions inhibited cell growth. Figure 2 shows the results of a colony-forming assay, in which 1000 cells were plated in a 6 cm dish in DMEM + FCS lacking interferon for 24 h, and then were switched into medium which either contained or did not contain 100 U/ml of murine gamma interferon and allowed to grow for 14 days at either 33°C or 39.5°C, during which time medium waε changed twice- weekly. The 24 h preplating in normal medium insures that the initial plating efficiency was the same in all cultures. After 14 days the cultures were stained with 2% methylene blue, 50% ethanol:water and the number of colonies obtained was counted. As shown in Figure 2, the growth of cells in fully permissive conditions (i.e., 33°C, 100 U/ml cf murine gamma interferon) was greater than in anv of the non-Dermissive conditionε. Detailed analyεiε of skin fibroblast cultureε for conditionality of growth revealed 3 familieε of cultures, depending upon the ability of cellε to grow in fully permiεεive, εemipermissive and nonpermissive conditionε; permiεsive conditions were defined for theεe purposes as growth at 33°C in the preεence of IFN-gamma, εemipermiεsive conditionε included either growth at 33°C in the absence of IFN-gamma or 39.5°C in the preεence of IFN-gamma and nonpermiεεive conditionε were growth at 39.5°C in the absence of IFN-gamma. In the first family of cultureε, growth waε fully conditional and only occurred in permiεεive conditionε. If cellε were grown at 39.5°C, and/or were grown in the abεence of IFN-gamma, cell division did not occur either in standard growth assays or in colony forming assays. These fibroblasts thus behaved as expected from previouε εtudieε in which rat embryo fibroblaεtε were conditionally immortalized with tsA58TAg by retroviral infection (Jat & Sharp, 1989, Mol. Cell. Biol., 9:3093-3096) . In these previous studies, it has been shown that fibroblasts which are conditionally immortalized using retroviral-mediated gene insertion to create cell lines which express tsA58TAg will continue to proliferate only if maintained in permissive conditions. Upon temperature shift to non-permisεive conditionε, fibroblastε rapidly expreεs the senescent phenotype expressed by normal fibroblasts which have been grown for extended periods in vitro. All cultures derived from different individuals within the H2K b - TAgtsA58 strain of mice yielded identical results. In a second family of cultureε, optimal growth was obtained in fully per isεive conditions, a lesεer degree of growth waε εeen in εemipermiεsive conditions and no growth occurred in nonpermisεive conditions. In the third family, cell growth did not completely cease even when cells were grown under nonpermisεive conditionε, although the best growth was εeen in fully permissive conditions and the slowest growth occurred at the fully nonpermissive conditions. The conditionality of growth obεerved in the fibroblasts derived from transgenic animals was correlated with the levels of tsA58TAg expreεsed by these cells. In all cultures, the level of tsA58TAg was reduced by temperature shiftup and/or by removal of IFN-gamma. Interestingly, when the most conditional cultures (those derived from progeny of mouse H2ts6) were grown at 33°C in the absence of IFN-gamma, a condition where these cells did not grow, low levels of TAg could still be detected. This observation is discuεεed in furtner detail in the next Example. To determine whether cell lines which were conditionally immortal could be rendered fully immortal by the introduction of a constitutively active differentiation inhibiting gene, εome of the fibroblaεtε iεolated from the H2ts6 mice were infected with a retroviruε which expressed a wild- type SV40 T antigen and the neomycin resistance gene (Jat & Sharp, J. Virol., 1986, 59:746-750) . Cells which were successfully infected were selected by growth in the G418 antibiotic. When these cells were then switched to non-permisεive conditionε, the cells continued to grow. These experiments thus demonstrate that a cell line can be converted from a conditional to a non-conditional growth state if it is considered desirable to do so. Analysis of Cell Lines from H2K^-TAgtεA58 Transgenic Mice Analysis of thvmic cell line (1) Thiε work demonεtrateε that even in the εingle instance found where the genetic construct used does disrupt normal development, the cell types which have undergone hyperplastic expansion in vivo remain conditional in their in vitro growth. (2) This work further demonεtrates that thymic epithelial cells derived from H2ts6 mice expresε the family of intermediate filament proteinε normally expreεεed by theεe cellε in vivo, thus making these cells a potentially suitable source for the purification of proteins expreεed in their normal counterpartε. (3) This work alεo demonεtrates that the thymic epithelial cell lines derived from H2ts6 mice have the ability to rosette T-lymphocytes, thus making them a suitable cell-type for the study and potential biochemical diεεection of a normal cellular interaction. Becauεe of the development of thymic hyperplasia in- many of the H2K b -TAgtsA58 fuεion tranεgenic mice it was of importance to characterize the growth of thymic derived cellε. To this end, thymic tissue was prepared for culture in the same manner as for fibroblasts, except that the periods for which enzymes were added were limited to a total of 15 min of collagenase together with a further 15 min in the additional presence of trypsin. These cells were then grown as were the skin fibroblasts until confluent flaskε were obtained, after which clonal cell lines were isolated by limited dilution single cell cloning. The cell lines isolated from the thymuεeε of the H2K b -TAgtεA58 fusion transgenic mice were conditional in their growth, and were growth arrested when grown in the absence of interferon at 39.5°C. The conditionality of theεe cell lineε in vitro indicates that even in this tissue the endogenouε levelε of Class I antigen expression are insufficient to cause sufficient levels of T antigen to be expressed to be able to interfere with normal processes of differentiation and growth control. Such an observation is consistent with the hypothesiε that the reaεon for the generation of thymic hyperplaεia in vivo waε probably due to the presence of a hepatitis infection in the mouse colony. Such an infection would cause augmented production of interferon in the thymuε, thuε driving the levelε of T antigen above the threεhold of non- effectiveness. These resultε further εupport the importance of the uεe of an inducible, non- constitutive promotor, as a potent constitutive promotor would cause levels of TAgts to be inappropriately expressed in every cell in the body rather than just thoεe cells exposed to inducer as a consequence, in this case, of diseaεe in the mouεe colony. Two of the cell lineε derived from the thymus were characterized antigenically, and were found to express cytokeratins by staining the cellε with the LE61 pan- anti keratin monoclonal antibody. Aε keratins are specifically expresεed in epithelial cell populationε, the labelling of theεe cells with the LE61 antibody indicates that these cellε are thymic epithlial cellε. The cytokeratin poεitive thymic epithelial cell lines discuεεed above εpecifically bound T- lymphocytes, as detected in a εtandard rosetting asεay. Stromal cellε were mixed with unfractionated freεhly iεolated thymocyteε from nontransgenic BALB/c mice in a ratio of 1:12. Cells were kept in a small volume (200 microlitreε) and incubated on ice for one hour. The mixture waε then centrifuged at 200 g for 5 minuteε, the pellet gently resuspended in 1 ml of PBS and the number of rosettes was counted using a haemocytometer, with 3 or more thy ocytes attached to a stromal cell counting as a rosette. In this assay, the cytokeratin+ thymic epithelial cell lines efficiently formed rosettes with thymocytes. Analysis of a putative glio a precursor cell To derive cell lines of the central nervous system, cells were disεociated from the cerebral corticeε of mouse 11 by the methodε deεcribed in Noble et al (1984, J. Neuroεci. 4, 1892-1903) . The mouεe waε 3 weeks old at the time of disεection. Cellε were grown in chemically-defined mediur. (Bottenstein and Sato, 1979, Proc. Natl. Acad. Sci. U.S.A. 76, 514- 517) in the presence of 10 ng/ml of platelet-derived growth factor [BB homodimer (supplied by British Biotechnology) and 10 ng/ml PDGF AA homodimer (supplied by Chiron Corporation) ] and 10 ng/ml of basic fibroblast growth factor (supplied by Boehringer-Mannheim) . Cells were grown through an initial passaging and then cloned by limiting dilution. Ten clones were isolated and characterized for antigen expresεion. Of theεe, 1 clone conεisted of cells which all expresεed glial fibrillary acidic protein (GFAP) , a cytoskeletal protein specifically expresεed by aεtrocyteε in the CNS (Bigna i, et al, 1972 Brain Reε 43, 429-435) . GFAP expression was analyzed using an anti-GFAP antiserum purchased from Dakopatts Ltd and appropriate fluorescent second layer antibodies (purchased from Southern Biotechnology) . To examine the differentiation potential of the GFAP-expresεing clone of cellε, cellε were replated on poly-L-lysine coated glaεε coverεlipε and treated with a variety of different substances to induce differentiation. Of particular interest were the effects of fetal calf- serum, which induced cells to develop a GFAP- phenotype. The GFAP- cells did express the extracellular matrix protein fibronectin (FN) , which has been reported as being expressed on only some poorly defined astrocytic subpopulations among the CNS glial cells. Although the parental cell line expressed both GFAP and FN, some experiments have suggeεted that in the abεence of εerum thiε parental cell can take on a GFAP+FN- phenotype. The isolation of a cell with the capacity to be regulated between a GFAP- * - phenotype and a GFAP-FN-i- phenotype is of exciting interest in light of the results of recent studies on antigen expression in human gliomaε. Although morphological classification of human gliomaε haε generally assumed that these cells εhare a close lineage relationship with the normal glial cells of the CNS, extensive antigenic analysis of cells derived from gliomaε haε not been in agreement with the morphological categorization of theεe tumours (Kennedy et al 1987 Neuropath Appl. Neurobiol 13, 327-347) . Most importantly, studies thus far have indicated that gliomas can be slotted into one of two antigenic categories, the first being a GFAP+ phenotype and the second being a GFAP-FN+ phenotype. In work by Kennedy et al, the GFAP-FN+ glioma phenotype was found to occur in almost 90% of cell cultures derived from gliomaε. The origin of these cells remains unclear, although it has been εhown that cloneε derived from a GFAP+ glioma may be GFAP- and FN- (Westphal et al, 1988, Cancer Research 48, 731-740) . The results on human gliomas raise the striking possibility that there exists within the nervous syεtem a precurεor cell with the possibility of differentiating along both astrocytic (GFAP+) and non-glial (GFAP-FN+) pathways. The cell we have isolated repreεents the first time such a cell haε been clearly iεolated from brain in a manner that allows rigorous examination of thiε poεεibility. The congruence between the many studies on antigen expression in glioma cells and the antigenic phenotypes which can be expressed by the cell isolated from mouse 11 suggests clearly that this cell line is a candidate for being a glioma precursor cell. Analysis of pancreatic cellε Pancreatic cellε iεolated from the pancreaε of mouse 11 in the same manner as cortical cells, and grown and cloned in the same manner aε cortical cells, have undergone preliminary characterization in tissue culture. A small proportion of cells in the most closely examined clone express insulin (as recognized by anti-insulin antibodies purchased from ICN) and can be labelled with the monoclonal antibody A2B5 (hybridoma cell line obtained from Dr. Marshall Nirenberg of the National Institute of Health, USA) , both of which are markers for pancreatic islet cellε (Eisenbarth et al, Proc. Natl. Acad. Sci. U.S.A., 79, 5066-5070) . It is retrospectively unfortunate that both the cortical and pancreatic clones were isolated from mouse 11, which turned out from much later analysis to have been one of the least conditional of the transgenic mice. Preliminary resultε do however indicate that the cortical and pancreatic cellε may be more conditional than fibroblasts. More importantly, neither the brain nor pancreas of mouse 11 showed any evidence of gross developmental abnormalitieε, indicating that the levels of TAgtsA58 which may have been expressed in these cells was insufficient to interfere with in vivo differentiation. Alternatively, it may be that placement of these cells in tissue culture conditions caused them to express higher levels of Class I antigen than would be the case in vivo. One of the transgenic mice created v.'ith the H2K b tsA58 construct (Mouse No. 6) has bred successfully to produce several litters, all of which have pups which carry the original genotype. In all the Examples that follow, the transgenic animals used were heterozygous progeny of what iε termed the H2ts6 strain (Mouse No. 6) . EXAMPLE 2 Cells were prepared from the heart of one of the offspring of mouse 6 by the same methods by which fibroblasts were prepared from the skin of other mice. The mouse from which the heart tissue was prepared (called "daughter of 6") exhibited no obvious abnormalitieε in organ εize upon diεsection. Cells from daughter of 6 (which were recognised aε being likely heart-derived fibroblaεtε) were grown for 4 weeks at 33°C in the presence of recombinant murine gamma interferon. For experimental analysiε, cellε were plated onto poly-L-lyεine coated glaεs cover slips at 33°C in DMEM + 10% fetal calf serum overnight. On the following day the cells were switched to growth at 33°C or 39.5°C in the presence or absence of gamma interferon. After 3 days of growth in permiεsive or non-permissive conditions, bromodeoxyuridine was added for 24 h and cells were then fixed (by the protocol supplied by Becton Dickinson) and stained with anti-bromodeoxyuridine antibodieε (purchaεed from Becton Dickinεon) , followed by a rhodamine conjugated second antibody (purchased from Southern Biotechnology) to label nuclei of cells which had engaged in DNA synthesiε during the previous 24 h. As shown in Figure 3, cells grown at 33°C in the presence of interferon had a 20-fold greater syntheεiε of DNA during the labelling period than cells grown at 39.5°C in the abεence of interferon. Cellε grown in semi- permissive conditions εhowed intermediate levels of DNA synthesis fully compatible with survival. EXAMPLE 3 (1) The following work demonstrateε the principle that cellε derived from the H2ts6 mice are conditionally immortal, and undergo a normal pathway of terminal differentiation when switched from permissive to non-permissive conditions. (2) This work further demonstrates that it is poεεible to express subfunctional levels of oncogene product, thus confirming the principle that it iε poεεible to expreεε levels of oncogene product which do not interfere with normal developmental pathways. (3) Moreover, this work also demonstrates the need for fine regulation of oncogene activity in order to keep activity below a level which would interfere with normal development. Cultures of skin fibroblasts from H2ts6 mice were prepared as described in Example 1, and displayed the conditionally immortalized phenotype discussed in detail in the previous Example. Western blot analysiε of expression of T antigen in cultures of fibroblasts derived from H2ts6 mice showed a relatively low level of T antigen expression at 33°C in the presence or absence of IFN-gamma although expresεion waε clearly higher in the preεence of IFN-gamma. Thiε obεervation indicated that it might be poεsible to observe dramatic alterations in cell growth as a reεult of small changes in the level of this gene product. To test thiε possibility, a dose-response analyεiε waε effected in which cell growth and colony formation were titred againεt the concentration of IFN-gamma. Fibroblasts derived from progeny of H2ts6 mice showed promotion of cell growth by levels of IFN- gamma as low as 1 U/ml . Analyεiε by colony formation and by cell number analyεiε showed that addition of 100 U/ml of IFN-gamma to these cultures only increased the frequency of colony formation by 3.5 fold in comparison with that seen in the presence of 1 U/ml, and was only 40% increased over that achieved with application of 10 U/ml. The difference in TAg levels at the different doses of IFN-gamma waε not large, with 1 U/ml causing a 2.5- fold increase over basal levelε and 100 U/ml causing approximately a 6-fold increase over basal levels. EXAMPLE 4 Generation of astrocyte cell lines This Example demonstrateε four aεpectε of the uεe of animals of the invention: (1) Astrocyte cell lines, representing a defined differentiated cell type, were generated from the central nervous εyεtem, a tiεsue where the endogeneous level of class 1 antigen expression is essentially non-exiεtent and where no mRNA production in vivo can be detected. Thus, it is demonstrated that transcription of the oncogene construct in vivo is not a prior requisite to the generation of a cell line. (2) The astrocyte cell lines express the normal cell-type specific marker associated with this cell type, thus demonstrating the potential usefulness of cells derived from the H2ts6 mice as a source of purification of a cell-type specific protein. (3) The astrocyte cell lines produce a mitogenic activity known to be expressed by the normal counterparts of the cellε, thus indicating that the astrocyte cell lineε derived from the H2ts6 mice are capable of promoting the division of another cell type, and thus represent a potential source for the purification of the mitogenic factor. (4) Finally, the astrocyte cell lines were generated by first growing brain cells in normal (non-permisεive) tiεεue culture conditionε, followed by purification of the cell-type of interest, followed by growth of the cells of interest in permissive conditions. This thuε demonstrates that the turning on of oncogene function can occur after cellε have been grown in tiεεue culture for a period of time, thus demonstrating that cells need not even be conditionally immortalized at the time of initial dissection. Cultures of cortical astrocytes were prepared by standard procedures (Noble et al., 1984, J. Neurosci., 4:1892-1903; Noble & Murray, 1984, EMBO J., 3:2243-2247) . Briefly, cortices from newborn H2ts6 mice were diεεociated into single cells by enzymatic digestion of tisεue with 0.25% collagenase in L-15 medium and an equal volume of 0.25% trypsin. Cultures were grown at 37°C in DMEM containing 10% fetal calf serum, 2 mM glutamine and 25 microgram/ml of gentamicin. After 7-10 days cultures were placed on a rotary platform overnight at 37°C, and were rotated at speeds just below thoεe which would cauεe foaming of the medium (i.e., about 60-75 rpm) . Aε described previously (Noble et * al. , 1984) , this procedure produces cultureε which are 95% pure astrocytes, as judged by the expression of the aεtrocyte-specific cytoεkeletal protein GFAP (glial fibrillary acidic protein) in 95% of these cells. After cultures of astrocytes were enriched to >95% purity, clonal cell lines were generated by first shifting cellε to 33°C in the preεence of gamma- interferon. Cellε were then infected with a retroviruε which harbourε genes for bacterial beta- galactosidase and a neomycin resistance gene, using the standard infection protocol deεcribed for the use of thiε viruε (Price et al., Proc. Natl. Acad. Sci., USA, 1987, 85:156-160) . One day after infection, cells were removed from the flask by incubation with 0.25% trypsin (see Noble et al., 1984, supra) and replated in medium containing the G418 antibiotic. Resistant clones of cellε emerged from the selection condition, and 10 random clones were selected for further study. Clonal cell lines were readily generated in this manner and 7 of the 10 cell lines constitutively expressed glial fibrillary acidic protein (GFAP) , a specific marker for astrocytes in the CNS. To examine the ability of the astrocyte cell lines to produce mitogenic activity normally associated with these cells, oligodendrocyte-type-2 astrocyte (0-2A) progenitor cells were plated from the optic nerves of 7 day old rats onto the astrocyte monolayers. In previous experiments (Noble & Murray, 1984) it has been shown that astrocytes, but not meningeal cells or fibroblastε, were able to stimulate division of 0-2A progenitors in vitro, and that the 0-2A progenitors stimulated to divide by astrocyte conditioned medium expreεεed a particular bipolar morphology which iε only εeen when these progenitors are grown in the presence of astrocyte monolayerε, aεtrocyte-conditioned medium or platelet-derived growth factor (the mitogen produced by these astrocyte monolayers) . 0-2A progenitors grown on monolayers of clonal astrocyte cell lineε derived from the H2tε6 strain of transgenic mice were indistinguiεhable from thoεe grown on nontransgenic astrocyteε in their diviεion and expression of the expected bipolar morphology. EXAMPLE 5 Glial precursors of the CNS Thiε Example demonstrates that: (1) It is possible directly to immortalize cells from the central nervous syεtem with the characteristics of novel precursor cells by growth of cells in per iεsive conditions, and that the cells grown in this way display the blockade of differentiation normally associated v.'ith expreεεion of nuclear oncogenes. Thus, thiε Example further demonεtrates the ability to generate immortalized cell cultures from a tissue of the body in which there is no detectable in vivo expression of the transgene. (2) Cell division requires the preεence of appropriate growth factors, thus indicating the potential uεefulneεε of cellε derived from the H2tε6 mice aε aεεay systems useful in growth factor purification. (3) Precursor cells derived from the H2ts6 mice can be induced to undergo differentiation by switching cells from permissive to non-permissive conditions, thus demonstrating the potential usefulness of these cells in allowing the growth of novel precursor cellε. (4) Precursor cells grown according to the methods of the invention also retain the capacity to undergo normal differentiation when grown in permissive conditions if cells are exposed to either defined molecular factors or to medium conditioned from a cellular source. This Example thus further demonstrates the potential suitability of cells derived from the H2tε6 mice for use in assay systems which would enable the purification of factors which induce cellular differentiation. Cortical cellε were dissociated as described in Example 3 , except that cells were derived from embryonic day 18 mice and were grown in chemically defined medium (ingredients as described in Raff et al., 1983, Nature, 303:390-396) containing 10 ng/ml of the AA-homodimer of platelet-derived growth factor (Chiron Corporation) , 5 ng/ml of basic fibroblast growth factor (Chiron Corporation) and 20 U/ml IFN-gamma. Cells could be pasεaged readily, and passaged cells maintained in the indicated conditions showed no evidence of differentiation into defined glial cell types. Cultures contained bipolar cells which could be labeled with the A2B5 monoclonal antibody (Eisenbarth et al., 1979, Proc. Natl. Acad. Sci. , U.S.A. 76:4913-4917) , and looked like the 0-2A progenitorε which have been deεcribed in cultures of optic nerve cellε (e.g., Raff et al., Nature, 1983, 303:390-396; Noble & Murray, 1984, EMBO J., 3:2243-2247) . Cultureε alεo contained a separate group of novel cells which were labeled by antibodies against the vimentin intermediate filament (antibodies from Dako-Patts, Ltd.) and with antibodies against SSEA-1 (εtage-specific embryonic antigen-1, described in Gooi et al., Nature, 1981, 292:156-158) . These novel cellε expressed a very primitive morphology and consisted of small round cells with few processes or cytoplasmic extensions. In previous experiments witn cultures prepared from cortices of embryonic rats, similar cellε have been seen which invariably differentiated into astrocyteε or oligodendrocyteε upon paεsaging, regardleεε of the medium into which paεεaged cellε were replated. In contrast, cellε derived from the H2tε6 mice maintained in the deεcribed growth conditionε could readily be repeatedly passaged without undergoing differentiation. The cortical cells derived from cortices of H2tε6 mice could be induced to differentiate by several in vitro manipulations. In all cases, cultures produced oligodendrocytes (which seemed to be derived from the A2B5+ cells, as the oligodendrocytes were A2B5+SSEA-1") and astrocytes (which appeared to be derived from the SSEA-1 + cells as the astrocytes were frequently SSEA-1 + but always A2B5~) . In both cases cellular morphology altered dramatically. Oligodendrocytes expressed their normal multipolar appearance and could be labelled by monoclonal antibodies against galactocerebroside (Ranscht et al., 1982, Proc. Natl. Acad. Sci., USA., 79:2709-2713) . Cellular morphology also changed dramatically in the case of astrocytic differentiation, and the small primitive-looking SSEA-1 + cells were replaced by SSEA-1 ÷ cells with ' large cell bodies and broad membrane expanses. The cells undergoing astrocytic differentiation not only looked like astrocyteε, but alεo expreεsed GFAP. Firεt, removal of PDGF and FGF resulted in differentiation and cell death, thus demonstrating that the oncogene-expresεing cellε required the continuous presence of appropriate growth factors in order to continue growing. Second, cells which were maintained in the growth medium and in the presence of growth factors also differentiated if IFN-gamma was removed from the medium (thus turning off the TAgts expression) . Thiε reεult demonstrates that expression of TAgts is necessary to prevent differentiation even if cells are grown in conditions which allow oncogene-expresεing cellε to. continue growing in a non-differentiated state. Third, cells grown in the presence of PDGF and bFGF in fully permissive conditions (i.e., 33°C, + IFN- gamma) could be induced to differentiate if exposed to medium conditioned by purified cortical astrocytes (prepared as in Noble et al., 1984, J. Neurosci. 4:1892-1903) , thus demonstrating the potential suitability of the H2ts6 precursor cellε for use in assay systemε for the detection and purification of differentiation inducing agents. Fourth, cells grown in the preεence of PDGF and bFGF in fully permiεsive conditions could be induced to differentiate by exposure to 10 ng/ml transforming growth factor-beta (British Biotechnology) or by exposure to 2 ng/ml of ciliary neurotrophic factor (Synergen) , thus indicating the responsiveness of the precursor cells to defined differentiation inducing agents known to be present in the central nervous system, thus further indicating the potential suitability of the H2ts6 precursor cells for use in assay systems for the detection and purification of differentiation inducing agents. EXAMPLE 6 Endothelial Cells (1) This Example demonstrateε that the H2ts6 mice can be used to generate cell lines of endothelial origin. (2) This Example further demonεtrateε that the endothelial cell cultureε generated from the H2ts6 mice secrete a novel differentiation-promoting activity known to be secreted by normal endothelial cells. Thus, this Example further demonstrateε the potential usefulness of cell lines derived from the H2ts6 as a source material which would allow subsequent purification of a molecule expresεing a unique biological activity. Endothelial cell colonieε were prepared aε follows: Two adult mice (2-3 months of age) were decapitated under CO2 coma. The brains were washed in Leibowitz L-15 medium containing 25 microgram/ml of gentamicin and then placed in fresh L-15. Each brain was placed in a 30 mm petri dish containing a few is of L-15. The cerebellum and other white matter tracts (corpus callosum, optic bulb) were removed by dissection. The meningeal sheath was removed without leaving any traces. The remaining grey matter was chopped finely with a sterile scalpel blade and then forced through a gauge 19 needle once and incubated in 0.1% collagenase:dipaεe (BCL) in L-15 for 60 min at 30°C. The tisεue was spun at 1000 g for 10 min at 4°C and the εupernatant diεcarded. Twenty ml of 25% BSA in L-15 were added and mixed thoroughly but without frothing and εpun at 2000 g for 20 min. The floating layer of tiεsue together with the εupernatant waε removed with care without disturbing the small pellet. The supernatant and the tissue were mixed and εpun again at 2000 g for 20 min. Thiε time the tissue layer and supernatant were discarded and the two pellets were suspended in 10 ml of 0.5% BSA in L-15 and spun at 1000 g for 10 min at 4°C to wash the pellets. The pellet was suspended in 0.1% collagenase:diεpase (in L-15) and incubated at 30°C for two hours. After the incubation, DNAse waε added to a final concentration of 10 microgram/ml and the reεulting capillary-containing tissues were spun at 1000 g for 10 min at 4°C. The pellet was again suspended gently in 1 ml of Ca-Mg-free DMEM and layered onto a 10 ml Percoll gradient and spun at 1000 g for 10 min at 4°C. (A linear gradient of 50% Percoll (Pharmacia) in Ca-Mg-free PBS was prepared in advance by mixing 5 parts isotonic Percoll [9 parts Percoll with 1 part 10X Ca-Mg-free PBS] with 5 parts of IX PBS and spinning at 26,000 g for one hour.) The top half of the tube contained cellular debris and single cells. The bottom half contained red blood cells seen as a red ring, and juεt above thiε ring were the intact capillarieε. Thiε layer waε removed carefully and εuεpended in 15 ml of L-15 and εpun at 1000 g for 20 min at 4°C. The supernatant was discarded and the capillaries suεpended gently in growth media (DMEM with 4.5 g/L glucose supplemented with 2mM gluta ine, 20% plasma derived serum as described by Vogel et al. (Proc. Natl. Acad. Sci., 1978, 75:2810-2814) , 10 IU/ml Heparin (Sigma) , 5 ng/ml basic FGF (Chiron Corporation) and 20 U/ml of IFN-gamma. The capillaries were plated onto Vitrogen (Flow Lab) coated 96 well plates at 50% occupancy and incubated at 7.5% C0 2 . The media was changed after three days and then every two days thereafter. Wells with single capillaries were tagged at day 3 and followed to confluence. Endothelial cellε arising from these capillaries grew as colonies with tight boundaries. These cells could then be passaged by gently trypsinizing cells (0.025% trypsin in Ca-Mg free DMEM containing 2 mM EDTA, 3 mm at 30°C) and replating them in Falcon 75 cm 2 flasks which had been precoated with gelatin by incubating the growth surface of the flasks overnight with 2% (w/v) of gelatin (Difco) made up in sterile tissue culture grade water. Just before use, the gelatin was aspirated and the flasks were washed with medium. Unlike normal capillary endothelial cells derived from brains of mice, these cells could be repeatedly passaged. The endothelial cell cultures were examined to determine if these cellε made a novel differentiation regulating activity produced by their normal counterparts. Differentiation of 0-2A progenitors into type-2 astrocytes requires the presence of at least two appropriate inducing factors, these being ciliary neurotrophic factor and an unknown factor found in the matrix of endothelial or meningeal cultures (Lillien & Raff, 1990, Neuron, 5:111-119) . Our own εtudieε have shown that the factor which co-operates with ciliary neurotrophic factor to induce astrocytic differentiation is secreted by a variety of normal endothelial cellε, but not by other cell types. The endothelial cell lineε produced from H2ts6 transgenic mice are as potent a source of this differentiation εtabilizing activity aε are any of the nontransgeni'c endothelial cells which have thuε far been examined. The assay employed to recognize tne endothelial cell-derived factor which works co-operatively with ciliary neurotrophic factor is to prepare cultures of optic nerve cellε from nerves of 7 day old rats by standard methods (e.g., Raff et al., 1983, Nature, 303:390-396) and grow these cells at a density of 3000-5000 cells per cover slip in the presence of chemically-defined medium (prepared aε in Raff et al. , 1983, Nature, 303:390-396) which haε been conditioned for 24 hourε by confluent cultureε of bovine aortic endothelial cellε. 0-2A progenitor cellε grown in thiε manner all turn into type-2 astrocytes within 4 days of in vitro growth, while cells grown in chemically-defined medium which is not conditioned by endothelial cells all turn into oligodendrocytes. The type-2 astrocytes are recognized aε being stellate cellε which are GFAP+ and are alεo labeled with the A2B5 monoclonal antibody. Examination of medium conditioned by the endothelial cell lineε prepared from cerebral corticeε of H2tε6 mice demonεtrateε that these endothelial cells secrete a biological activity indistinguishable from that secreted by nontransgenic endothelial cells in terms of both effect and potency. EXAMPLE 7 Colonic Epithelial Cells (1) Thiε Example demonstrates that the invention allows the direct immortalization of a novel category of epithelial cells which have proven intractable to the application of in vitro methodε of gene inεertion for the generation of immortalized cell lineε. Colons were removed from 14-18 day old H2ts6 mice. The colons were sterilized by washing them in 0.04% sodium hypochlorite (in PBS) . In some cases, crypts were removed from the colons by incubating tissue for 1.5 h in 3mM EDTA + 0.05 mM dithiothreitol. Tissue waε washed with PBS and then shaken by hand which reεults in the dislodging of intact crypts from the surrounding tissue. These crypts, which are seen as a finger-εhaped group of cells are then grown in monolayer culture on a substrate of rat- tail collagen on a medium of Dulbecco's Modified Eagle's Medium containing the defined chemical additives specified in Raff et al. (1983, Nature, 303:390-396) plus 2% fetal calf serum + 20 U/ml of IFN-gamma + 20% conditioned medium (conditioned for 24 h) from tumour line LIM 1863 (Whitehead et al., 1987, Cancer Res., 47:2683-2689). Cryptε εit down and epithelium εtartε to εpread out. With nontransgenic crypts, cells will remain viable on a feeder layer of bovine aortic endothelial cells, although they cannot be passaged (Whitehead et al., 1991, J. Tissue Cult. Methods (in presε) ) . In contrast, cells derived from the transgenic animals can be grown and passaged without a feeder layer and set up in monolayer culture. In other cases, the colons were grown in explant culture. The medium used throughout was as deεcribed above. Cultureε were fed two-three timeε a week, with fresh addition of interferon with each feeding. The crypt cultures gave rise to patches of flat cells with a clear epithelioid morphology, while explant cultures gave rise to mixed cultures containing a number of morphologically distinct cell types. The epithelioid cells derived from the crypts are labeled with two anti-keratin antibodies (LE 61 and LP 34, as cited in Lane, vide infra) , and labeling shows a characteristic fibrillary pattern of cytoplasmic staining. Uεe of these mice has thus provided the means for establishing colonic epithelial cells in culture in a manner that waε previously impossible and from tissues that have previously been impossible to culture for more than 24-48 hours. The major cell types of interest in the explant cultures are discussed in the following Example. EX7ΛMPLE 8 Glial Cells of the Enteric Nervouε Syεtem Explant cultureε of colon, prepared aε described in the previouε Example, contained two cell typeε of particular intereεt. One of theεe cellε waε a εmall processing cell which was labeled with antibodieε againεt GFAP, thuε identifying this cell as one of the glial cellε of the enteric nervouε system. These cells have been readily pasεaged, and should be readily convertable to cell lineε. Although there iε considerable interest in the glial cellε of the enteric nervouε system, no cell lines have been described for this tissue to date. The second cell type had a fibroblast like morphology and did not label with anti-GFAP antibodieε. However, upon treatment with transforming growth factor-beta, these cells are induced to expreεε nestin, an intermediate filament protein thought to be specifically expressed by precursor cells of the central nervous system . (Lendahl, Cell, 1990, 60:585-595). The normal cellular counterpart cf these cellε iε not known, but their induced expreεεion cf neεtin raises the posεibility that these cells are a novel precursor population. EXAMPLE 9 Myoblasts (1) Thiε Example de onεtrateε that cell lines generated by the method of the invention retain their capacity to undergo normal differentiation in vitro. (2) This Example further demonstrates that cell lines derived from the H2ts6 mice have the capacity to undergo normal differentiation in vivo, thus indicating the potential usefulnesε of cell lineε derived from the H2tε6 mice in cell transplantation applications. Clonal myoblast cultures were prepared by direct limiting dilution cloning of cellε derived from dissection of skeletal muscle from hindlegε of neonatal mice into εtandard tissue culture conditions for the growth of muscle precursor cells (as described, e.g., ir. Morgan et al. , 1987, J. Muscle Res. and Cell Motil., 8:386-396) , except that cells were grown at 33°C in the presence of gamma- interferon. Clonal cultures were grown continuously for several weeks, and were passaged repeatedly, before transplantation of cells directly into tne skeletal muεcle maεs of nontransgenic mice bearing a known mutation of the dyεtrophin gene. The dystrophin gene mutation means that this protein shows abnormal localization within the multinucleated yotubes which are formed by fusion and differentiation of muscle precursor cells. The transplanted cells derived from transgenic mice could be readily identified by the fact that they generated skeletal myotubes which expressed normal dyεtrophin. In addition, the muεcle precurεor cellε derived from the H2tε6 transgenic mice were able to fuse into multinucleate myotubes in vivo by either growing cells at high density or turning off oncogene expresεion by growth of cells in non- permissive conditions. Cell lines of the preεent invention, in which expression of the differentiation inhibiting gene is regulated, differ from currently available cell lines in that it is theoretically possible to obtain cell lines from any tissue of the body and select for cell lines of any identity. Thus, the preεent technique differε qualitatively from previouε techniqueε in which genetic information waε tranεfected or infected into cellε in a manner which doeε not allow targeting of particular populationε or reliable immortalization of rare cellε. With thiε new technique, rare cells can be isolated by any means available in the art (e.g., fluoresence- activated cell sorting, density centrifugation, panning, immunoselection with magnetic beads, selective adhesion to defined protein or carbohydrate substrates, etc.) and grown in conditions which support activation of the differentiation inhibiting gene, and thus allow the rare cells to be grown in large quantities. Any use to which such cells can be put thus becomes a real practical possibility for the first time, including (but not limited to) the iεolation of cellular components or substances from such rare cellε. Immortalized cellε or differentiated cellε derived from animalε of the invention have a number of important uses, including, inter alia, the following specific further aspects of the invention: A) The use of either immortalized cells which have been obtained by a method as defined above or of differentiated cells derived therefrom or of cells isolated from an animal of the invention and wherein expression of said differentiation inhibiting sequence has been activated but which cell may nonetheless be induced to differentiate by exposure to an external factor and have been so exposed or of cellε isolated from an animal of the invention which have been grown in vitro and a non-conditional immortalizing gene or genes thereafter inserted in vitro, either as a source of a cell-produced substance, optionally a growth or differentiation factor, or in an assay system in relation to such a substance (in one illustrative embodiment of this use, the cell-produced substance is an antibody) ; B) The use either of immortalized cells which have been obtained by a method as defined above or of differentiated cells derived therefrom or of cells isolated from an animal of the invention and wherein expression of said differentiation inhibiting sequence has been activated but which cell may nonetheless be induced to differentiate by exposure to an external factor and have been so exposed or of cells isolated from an animal of the invention which have been grown in vitro and a non-conditional immortalizing gene or genes thereafter inserted in vitro, in the production of a medicament, either: said medicament being for the treatment or prophylaxis of a condition characteriεtic of a cell deficiency or cell-produced factor deficiency or of a cellular malfunction by cell tranεplantation; or said medicament comprising a cell-produced factor derived from any of the aforeεaid cellε; C) An extremely important aspect is a method of therapy or prophylaxis practised on the human or animal body which comprises administering either: immortalized cells which have been obtained by a method as defined above or differentiated cells derived therefrom or cellε isolated from an animal of the invention wherein expression of said differentiation inhibiting sequence has been activated but which cells mav nonetheless be induced to differentiate by exposure to an external factor and have been so exposed or cells isolated from an animal of the invention which have been grown in vitro and a non-conditional immortalizing gene or genes thereafter inserted in vitro; or a factor derived from any of the aforesaid cellε. Particular embodimentε are methods of transplantation therapy practised on the human or animal body and in which immortalized cellε which have been obtained by a method as defined above or differentiated cells derived therefrom by deactivating expresεion of said differentiation inhibiting sequence or cells isolated from an animal of the invention and wherein expression of said differentiation inhibiting sequence has been activated but which cells may nonethelesε be induced to differentiate by exposure to an external factor and have been so exposed or cells isolated from an animal of the invention which have been grown in vitro and a non-conditional immortalizing gene or genes thereafter inserted in vitro, are transplanted into said body under conditions either allowing differentiation of immortalized cells to occur or preventing expresεion of the differentiation inhibiting sequence in the case of differentiated cells, with consequent compensation for a deficiency of or malfunction in pre-existing cells in said body (in two illustrative embodiments ' of such methods, the transplanted cellε are either insulin-producing cellε from the pancreas of said animal cr precursor cellε therefor, the transplantation therapy being for the treatment or prophylaxis of an inεulin-deficiency disease, or glial cells or glial precursor cells, the transplantation therapy being for the treatment or 05 prophylaxis of a disease or disorder of the nervous system) ; and D) The use of immortalized cells which have been obtained by a method as defined above or of differentiated cellε derived therefrom or of cellε iεolated from an animal of the invention and wherein expression of said differentiation inhibiting sequence has been activated but which cells may nonetheless be induced to differentiate by exposure to an external factor and have been so exposed or of cells isolated from an animal of the invention which have been grown in vitro and a non-conditional immortalizing gene or geneε thereafter inεerted in vitro, in a method of in vitro diagnosis. Cell lines are already routinely used as assay systems in the purification of factors which stimulate cell diviεion and differentiation. Cell lineε can alεo be used as assay systems for the purification of genes which regulate division and differentiation. Many of the established oncogenes were identified by virtue of their ability to convert established cell lines to a neoplastic state. Genes which induce cellε to differentiate into specific cell typeε can alεo be identified by tranεfection of genetic material into suitable recipient cells, as evidenced by recent studies on the identification of genes which control muscle cell differentiation. One could envisage, e.g., using insulin-negative pancreatic cell lines aε εuitable assay systems for the identification of factors or genes which induce some pancreatic cells to produce insulin. In general, the present invention provides a means by which large numbers of differentiated cells or precursor cells can be produced. For example, if a large quantity of a specific type of cell is needed for use in diagnostic methodε, for tranεplantation into an individual or for use as a means of producing a desired product (e.g., a mitogen or differentiation factor) , then appropriate cells can be selected using known techniques. These cells can εubεequently be grown in permissive conditions for a desired amount of time. The cells can be studied in permissive conditions, where induction of at least some aspects of differentiation appears to be possible. In addition, cells can be εwitched to non- permissive conditions compatible v.'ith more extensive differentiation along normal pathways. Moreover, cellε can be genetically manipulated in tiεsue culture so as to express a wild-type differentiation-inhibiting sequence, thus allowing the growth of large numbers of cells (e.g., for purposes of purification of a desired protein produced by the cells) without the continued use of permissive growth conditionε. It iε clear from the principles of the present invention that conditionally immortalized cells obtained from the present transgenic animals can be introduced (in non-permissive conditions) into an individual, in whom they will reside in a non-permissive environment. The introduction of such cells is invaluable for the development of precursor transplantation therapies in which large numbers of precursor cells are transplanted into diseased tissue in order to replenish the populations required for normal function. For example, insulin producing cells derived from the pancreas of a transgenic animal of this invention can be surgically implanted into the pancreas of animals suffering from insulin-deficiency diseases (such aε type I diabetes) . The use of genetically engineered cells to enhance regenerative processes or restore tisεue function haε become of increasing practical interest. For example, Gage and colleagues (Science, 1988, 242, 1575) described the injection of fibroblaεtε genetically engineered to overproduce nerve growth factor into the εite of a fimbria-fornix lesion. These transplanted cells prevented retrograde axonal degeneration for at least 2 weeks in initial studies, and more recently have been shown to promote survival of the cholinergic neuronε for up to 8 weeks (Rosenberg, et al, 1989 Am. Soc. Neurosci. Abs. No. 433.2) . Many tumours are histologically related to cells found in the early stages of tisεue development. The animalε of the preεent invention provide a ready εource of cellε for the identification of potential precursors of tumour cells. In addition to the strategy already discussed in relation to the identification of a putative glioma precursor cell, it will also be possible to use other technologies of gene insertion (e.g. , transfection, electroporation, retroviral-mediated gene insertion) to further manipulate the genome of any of the cell lines isolated from the animals of the present invention. Thus, specific precursors or differentiated cell types can be initially grown in numbers large enough to allow utilization of less efficient methods of gene insertion and these further modified cells can be utilized in the study of neoplastic transformation of defined cell types. Furthermore, the animals of the present invention provide a means of obtaining immortalized cell liheε exhibiting preselected mutations. A further aspect of the invention iε thus use of an animal of the invention aε a parent for crossing with a mutant animal parent which expreεseε a preεelected mutation in producing a descendant mutant animal exhibiting normal cell development and from which immortalizable cells expressing said mutation may be isolated. Preferably, in such a use both parents are homozygous for thoεe of their respective traits which must be exhibited by any descendant as defined above. The invention accordingly includes a further aspect which is cellε, immortalized or immortalizable, expressing said preselected mutation and derived from a descendant as defined and produced above. It will be apparent to the skilled reader that various modifications and alterations may be made to the various embodiments discuεεed and/or deεcribed above without departing from the scope of the present inventive concept.

Claims: CLAIMS : 1. A tranεgenic non-human eukaryotic animal having germ cellε and/or somatic cells into which a differentiation inhibiting sequence has been chromosomally incorporated under the control of a non- constitutive promotor such that expression of said sequence iε normally inhibited allowing normal cell development but precurεor cells taken from said animal may be prevented from completing differentiation in tissue culture by providing permissive conditions for said promotor thereby activating expression of said sequence. 2. An animal as claimed in claim 1 wherein the said differentiation inhibiting sequence iε selected from nuclear oncogeneε, dominant mutations altering tumour suppresεor genes, growth regulatory geneε which inhibit differentiation, geneε which produce differentiation inhibiting agents, and genetic εequences encoding combinationε of growth factors which work to inhibit differentiation. 3. An animal as claimed in claim 2 wherein the differentiation inhibiting sequence is εelected from SV40 large T, polyoma large T, adenoviruε EIA, HPV E7 , HPV E6, myc, erb A, myb, mutants of p53, v-src, differentiation inhibiting activity, and genetic sequences encoding combinations of platelet-derived grovtr. factor and basic fibroblast growth factor. 4. An animal as claimed in any one of claims 1 to 3 wherein to enable achievement of said inhibition of expression whilst allowing said activation of expression in precursor cells taken from said animal said cells contain a plurality of genetic regulatory elements aε said promotor system- and/or a differentiation inhibiting sequence which is itself conditionally active and inducible. 5. An animal as claimed in claim 4 in which an inducible promotor is provided together with a differentiation inhibiting sequence which is itself conditionally active and inducible. 6. An animal as claimed in claim 5 wherein said differentiation inhibiting sequence iε TAgtε. 7. An animal as claimed in any one of claims 1 to 6 wherein said differentiation inhibiting sequence iε under the control of the promotor which normally controlε expreεεion of Claεε I antigenε of the major histocompatability complex. 8. A transgenic non-human eukaryotic animal having germ cells and/or somatic cells into which a differentiation inhibiting sequence v.'hicn is itself conditionally active and inducible has oeen chromosomal!v incorporated under the control of a promotor εuch that expreεεion of said sequence is normally held below an effective level thus allowing normal cell development but precursor cells taken from said animal may be prevented from completing differentiation in tissue culture by activating expression of said sequence. 9. An animal as claimed in claim 8 wherein εaid conditionally active differentiation inhibiting sequence iε TAgts. 10. An animal as claimed in claim 8 or claim 9 wherein said promotor is the TK promotor. 11. An animal aε claimed in any one of claimε 1 to 10 which is a mouse. 12. A cell which has been isolated from an animal as defined in any one of claimε 1 to 11 or which haε been derived from εuch an iεolated cell and which has chromosomally incorporated therein said differentiation inhibiting sequence expression of which may be activated. 13. A cell line derived from a cell aε defined in claim 12 and which haε been immortalized by activating expression of said differentiation inhibiting sequence. 14. A differentiated ceil derived either from a cell as defined in ciair. 12 by allowing it to differentiate without activating expreεεion of said differentiation inhibiting sequence or from a cell line as defined in claim 13 by deactivating expression of said differentiation inhibiting sequence or from a cell as defined in claim 12 wherein expression of said differentiation inhibiting sequence has been activated but which cell may nonethelesε be induced to differentiate by expoεure to an external factor and haε been εo exposed or from a cell aε defined in claim 12 wherein said cell has been grown in vitro and a non- conditional immortalizing gene or genes thereafter inserted in vitro. 15. A method of producing a transgenic non-human eukaryotic animal having cellε into which a differentiation inhibiting sequence has been chromosomally incorporated but in a regulatable manner such that expresεion of εaid εequence may be activated but is normally inhibited allowing normal cell development, comprising effecting chromosomal incorporation of said differentiation inhibiting sequence under the control of a non-constitutive promotor into at least some cells of said animal or effecting chromosomal incorporation of a differentiation inhibiting εequence which iε itself conditionally active and inducible into at least some cells of said animal. 16. A method aε claimed in claim 15 wherein chromosomal incorporation is effected by a microin ection technique at an embryonic stage of animal development. 17. A method as claimed in claim 15 or claim 16 and further defined by the specific feature(s) of any one or more of claims 2 to 11. 18. A method of providing immortalized cells, which method compriseε iεolating from an animal aε defined in any one of claims 1 to 11 precursor or differentiated cells having chromoεomally incorporated therein εaid differentiation inhibiting εequence and εubjecting εaid cellε to conditionε in tiεsue culture whereby expreεsion of said differentiation inhibiting εequence iε activated. 19. A method aε claimed in claim 18 wherein said immortalized cells are pancreatic cells having the capability after differentiation of producing inεulin, or glial cells or glial cell precursors. 20. The use either of immortalized cells which have been obtained by a method as claimed in claim 18 or claim 19 or of differentiated cellε derived therefrom or of cellε as defined in claim 12 wherein expression of said differentiation inhibiting sequence has been activated but which cellε may nonetheless be induced to differentiate by exposure to an external factor and have been εo exposed or of cells aε defined in claim 12 which have been grown in vitro and a non-conditional immortalizing gene or genes thereafter inserted in vitro, either as a source of a cell-produced subεtance, optionally a growth or diff rentiation factor, or in an assay system in relation to εuch a substance. 21. The use claimed in claim 20 wherein said substance is an antibody. 22. A method of therapy or prophylaxis practised on the human or animal body which comprises administering either: immortalized cells which have been obtained by a method as claimed in claim 18 or claim 19 or differentiated cellε derived therefrom or cellε aε defined in claim 12 wherein expreεsion of said differentiation inhibiting sequence has been activated but which cells may nonetheless be induced to differentiate by exposure to an external factor and have been so exposed or cells aε defined in claim 12 which have been grown in vitro and a non-conditional immortalizing gene or geneε thereafter inserted in vitro; or a factor derived from any of the aforesaid cells. 23. The use either of immortalized cells which have been obtained by a method as claimed in claim 18 or claim 19 or of differentiated cells derived therefrom or of cells as defined in claim 12 wherein expression of said differentiation inhibiting sequence has been activated but which cells may nonetheless be induced to differentiate by exposure to an external factor and have been so exposed or of cells as defined in claim 12 which have been grown in vitro and a non-conditional immortalizing gene or genes thereafter inserted in vitro, in the production of a medicament, either: said medicament being for the treatment or prophylaxis of a condition characteristic of a cell deficiency or cell-produced factor deficiency or of a cellular malfunction by cell transplantation; or said medicament comprising a cell- produced factor derived from any of the aforesaid cells. 24. The use claimed in claim 23 wherein said condition is an insulin-deficiency diεease. 25. A method of transplantation therapy practised on the human or animal body and in which either immortalized cellε which have been obtained by a method aε claimed in claim 18 or claim 19 or differentiated cellε derived therefrom or cellε aε defined in claim 12 wherein expreεεion of εaid differentiation inhibiting sequence has been activated but which cells may nonetheless be induced to differentiate by exposure to an external factor and have been εo exposed or cellε aε defined in claim 12 which have been grown in vitro and a non-conditional immortalizing gene or genes thereafter inserted in vitro are transplanted into said body under conditions either allowing differentiation of immortalized cellε to occur or preventing expreεεion of said sequence in the caεe of differentiated- cellε, with consequent compensation for a deficiency of or malfunction in pre-exist σ cells in said bodv. 26. A method as claimed in claim 25 wherein the transplanted cells are: (a) insulin-producing cellε from the pancreaε of said animal or precursor cellε therefor, the transplantation therapy being for the treatment or prophylaxiε of an inεulin-deficiency diεeaεe; or (b) glial cellε or glial precursor cellε, the transplantation therapy being for the treatment or prophylaxis of a disease or disorder of the nervous syεtem. 27. The uεe either of immortalized cells which have been obtained by a method as claimed in claim 18 or claim 19 or of differentiated cells derived therefrom or of cells as defined in claim 12 wherein expression of said differentiation inhibiting sequence has been activated but which cells may nonetheless be induced to differentiate by exposure to an external factor and have been so exposed or of cells aε defined in claim 12 which have been grown in vitro and a non-conditional immortalizing gene or geneε thereafter inserted in vitro, in a method of in vitro diagnosis. 28. The use of an animal as defined in any one of claims 1 to 11 as a parent for crossing with a mutant animal parent which expresses a preselected mutation in producing a descendant mutant animal exhibiting normal cell development and from which immortalizable cells expressing said mutation may be isolated. 29. The use claimed in claim 28 wherein both parents are homozygous for those of their respective traits which muεt be exhibited by any deεcendant aε defined in claim 28. 30. Cellε, immortalized or immortalizable, expressing said preselected mutation and derived from a descendant aε defined in claim 28 and produced in the uεe claimed in claim 28 or claim 29.

Inventors: JAT PARMJIT SINGH, KIOUSSIS DIMITRIS, NOBLE MARK DAVID, JAT, PARMJIT, SINGH, KIOUSSIS, DIMITRIS, NOBLE, MARK, DAVID
Applicants: LUDWIG INST CANCER RES, MEDICAL RES COUNCIL, LUDWIG INSTITUTE FOR CANCER RESEARCH, MEDICAL RESEARCH COUNCIL
Application year: 1991
Main classification: A61K67
IPC codes: A01K67, A61K48, C12N5, C12N15