Insertion of a bacterial gene into the mouse germ line using an infectious retrovirus vector

Genome editing for disease resistance in livestock

One of the major burdens on the livestock industry is loss of animals and decrease in production efficiency due to disease. Advances in sequencing technology and genome-editing techniques provide the unique opportunity to generate animals with improved traits. In this review we discuss the techniques currently applied to genetic manipulation of livestock species and the efforts in making animals disease resistant or resilient.

Producing recombinant human milk proteins in the milk of livestock species

Recombinant human proteins produced by the mammary glands of genetically modified transgenic livestock mammals represent a special aspect of milk bioactive components. For therapeutic applications, the often complex posttranslational modifications of human proteins should be recapitulated in the recombinant products. Compared to alternative production methods, mammary gland production is a viable option, underlined by a number of transgenic livestock animal models producing abundant biologically active foreign proteins in their milk. Recombinant proteins isolated from milk have reached different phases of clinical trials, with the first marketing approval for human therapeutic applications from the EMEA achieved in 2006.

In vivo expression of a conditional TGF-beta1 transgene: no evidence for TGF-beta1 transgene expression in SM22alpha-tTA transgenic mice

Transforming growth beta-1 (TGF-beta1) appears to play a critical role in the regulation of arterial intimal growth and the development of atherosclerosis. TGF-beta1 is expressed at increased levels in diseased arteries; however, its role in disease development remains controversial. Experiments in which TGF-beta1 is overexpressed in the artery wall of transgenic mice could clarify the role of TGF-beta1 in the development or prevention of vascular disease. However, constitutive overexpression of a TGF-beta1 transgene in the mouse artery wall is embryonically lethal. Therefore, to overexpress TGF-beta1 in the artery wall of adult mice, we generated mice that were transgenic for a conditional, tetracycline operator (tetO)-driven TGF-beta1 allele. These mice were viable, and when crossed with mice expressing a tetracycline-regulated transactivator (tTA) in the heart, expressed the TGF-beta1 transgene in a cardiac-restricted and doxycycline-dependent manner. Nevertheless, breeding of the tetO-TGF-beta1 transgene into three lines of mice transgenic for a smooth muscle-targeted tTA (SM22alpha-tTA mice; reported elsewhere to transactivate tetO-driven alleles in smooth muscle cells of large arteries) did not yield expression of the TGF-beta1 transgene. Moreover, tTA expression was not detected in aortae of the SM22alpha-tTA mice. Transgenic mice that express tTA at high levels in vascular smooth muscle and reliably transactivate tetO-driven transgenes would be useful for deciphering the role of TGF-beta1 (or other proteins) in normal arterial physiology and in the development of arterial disease. Currently available SM22alpha-tTA mice were not useful for this purpose. Generation of higher-expressing lines of SM22alpha-tTA mice appears warranted.

Effective generation of transgenic mice by Bovine papillomavirus type 1 based self-replicating plasmid that is maintained as extrachromosomal genetic element in three generations of animals

The objective of our study was to analyze the efficiency and the properties of the inheritance of the Bovine papillomavirus type 1 (BPV1) replicator-based plasmid used as vector system for generation of transgenic animals. Previously, we have characterized a series of self-replicating plasmid vectors containing all viral factors necessary and sufficient for stable extrachromosomal replication of the BPV1 genome in the tissue culture system. We also demonstrated that the designed replicating vector system has a considerable benefit in the transgene expression, if compared to the regular expression vector. The vector, which showed the highest stability and maintenance function in the tissue culture was chosen for generation of the transgenic mice by pronuclear injections of the circular supercoiled plasmid. This method resulted in successful production of transgenic animals. Transmission efficiency of the vectors into the F(1) generation of animals varied between 0 and 48%, whereas transmission into the F(2) generation was uniformly near 50%. The maintenance of the vector-plasmids in the F(2) generation of transgenic animals as extrachromosomal genetic element was demonstrated by rescue of the plasmid into the Escherichia coli.

Foreign gene expression by in vivo gene electroporation in the quail testis

To investigate whether or not foreign gene expression is attained in the testis of living Japanese quails, a firefly luciferase reporter gene was transfected by in vivo electroporation (EP), and transcriptional activity of different promoters was compared. In addition, the effect of the Epstein-Barr virus self-replication sequence was also tested. The results showed that luciferase activity in the testis reached almost a plateau value at 50 V. Under this EP condition, no difference was found in transcriptional activity between the simian virus 40 (SV40) and miw promoters. The reporter gene expression in the quail testis was observed over 28 days after in vivo gene EP, although the activity gradually decreased, and the presence of the Epstein-Barr virus self-replication sequence in the SV40 promoter did not significantly prolong the luciferase activity. These results suggest that in vivo gene EP confers strong, though transient, foreign gene expression in the Japanese quail, and it may provide a new powerful approach for studies on transcriptional regulation of genes during proliferation and differentiation of spermatogenic cells in the quail testis.

Recent advances in transgenic technology

Techniques that allow modification of the mammalian genome have made a considerable contribution to many areas of biological science. Despite these achievements, challenges remain in two principal areas of transgenic technology, namely gene regulation and efficient transgenic livestock production. Obtaining reliable and sophisticated expression that rivals that of endogenous genes is frequently problematic. Transgenic science has played an important part in increasing understanding of the complex processes that underlie gene regulation, and this in turn has assisted in the design of transgene constructs expressed in a tightly regulated and faithful manner. The production of transgenic livestock is an inefficient process compared to that of laboratory models, and the lack of totipotential embryonic stem (ES) cell lines in farm animal species hampers the development of this area of work. This article highlights recent progress in efficient trans gene expression systems, and the current efforts being made to find alternative means of generating transgenic livestock.

cis-acting elements that mediate the negative regulation of Moloney murine leukemia virus in mouse early embryos

We have addressed the question of the nature of Moloney murine leukemia virus (MoMuLV) repression in mouse embryos by assaying for the transient expression of MoMuLV-derived constructs microinjected into early cleavage embryos. We show that the same cis-acting DNA sequences responsible for the block in MoMuLV expression in embryonal carcinoma cell lines operate in early embryos: (i) the MoMuLV long terminal repeat is nonfunctional, and (ii) the +147 to +163 repressor binding site, or negative regulatory element, negatively regulates the expression from an active promoter.

Transgenic science

The ability to manipulate the genome of the whole animal has, for the past 10 years, provided researchers with an alternative route of inquiry into many complex biological processes. Transgenic animals have numerous applications, encompassing a wide range of different disciplines, but they have proved especially useful in the investigation of gene regulation and gene function within the context of the living animal. This review describes the different techniques which have been used to produce transgenic animals and highlights advances which have been achieved using the transgenic approach.

Activator-dependent and activator-independent defective recombinant retroviruses from bovine leukemia virus

The replication-competent bovine leukemia virus (BLV) has been modified for use as a vector for foreign genes. The gag, pol, env, and pX regions of the virus were replaced by an exogenous nuclear location signal LacZ (nlsLacZ) or SVnlsLacZ gene. Transfection of the ovine cell line FLK-BLV, which expresses all BLV proteins from a wild-type provirus, with this viral DNA resulted in a viral titer of 10(4) CFU/ml. The inclusion of a large portion of the gag region did not significantly increase the titer. Both activator-dependent and activator-independent retroviruses were constructed. In activator-dependent vectors, the expression of the insert was dependent on the presence of the Tax protein, which activated the BLV long terminal repeat. In activator-independent vectors, the expression of the insert was constitutive because of the presence of an internal promoter. Infections with the recombinant retrovirus were inhibited by specific neutralizing antibodies. The structure of the transduced genetic material was not rearranged. BLV vectors encoding a reporter nlsLacZ gene, the product of which can be detected in single cells, greatly simplified studies of their biological properties. Determination of the host range of BLV vectors established that BLV-based recombinant retroviruses are effective in the transduction of genes in a variety of species and cell types.

Micromanipulation of embryos and germ cells: an approach to gene therapy?

Recent advances in mammalian gamete and embryo micromanipulation have stimulated the scientific and medical communities, and to some degree the public at large, to become aware that treatment of genetic disease by direct alteration of the genetic code may soon be possible. Because these micromanipulation techniques result in modification of the genotype at the earliest stages of development, such "gene therapy" affects not only the conceptus itself but also its germ cells. Thus such genetic modifications are heritable and can be transmitted indefinitely to succeeding generations of progeny. In the presentation, both narrow and broad definitions of gene therapy will be considered with respect to the techniques upon which they are based, their potential for treatment of genetic disease, and their current feasibility.

The use of a retroviral vector to identify foetal striatal neurones transplanted into the adult striatum

A retrovirus which encodes beta-galactosidase was used to infect embryonic rat striatal cells before grafting these cells into the lesioned adult rat striatum. Examination of the grafts after long term survival (8 months) revealed that a few small and large cells expressed large amounts of bacterial beta-galactosidase activity. The larger diameter cells were identified as neurones by their size, shape and presence of neuronal processes. The identity of the small diameter cell types was not established.

Transfer of a mutant dihydrofolate reductase gene into pre- and postimplantation mouse embryos by a replication-competent retrovirus vector

In order to explore the potential of retrovirus vectors for efficiently transferring foreign genes into mouse embryos, a replication-competent recombinant Moloney murine leukemia virus (Mo-MLV) vector carrying a mutant dihydrofolate reductase (DHFR) cDNA insert in the U3 region of the viral long terminal repeat was used to infect pre- and postimplantation embryos. When preimplantation mouse embryos were infected with the vector, as expected, the provirus integrated into the embryos and the germ line with the same efficiency as that observed with wild-type Mo-MLV, leading to inactivation of the recombinant virus. In contrast, when postimplantation mouse embryos were microinjected with virus-producing cells, between 90 to 100% of the surviving animals proved to be infected with the virus. The recombinant virus spread as efficiently as wild-type Mo-MLV in the infected embryos, resulting in up to three to five proviral copies per genome in heart, thymus, and brain tissues. Substantial expression of mutant DHFR*-coding viral message was found in all somatic tissues analyzed, the amounts correlating with the proviral copy number in the respective organ. These results suggest that replication-competent vectors are useful for efficient transfer and expression of foreign genes into tissues or whole animals when virus spread is needed.

Regulated expression of muscle-specific genes introduced into mouse embryonal stem cells: inverse correlation with DNA methylation

Pluripotent embryonal stem cell lines (ES) were isolated from cultured normal mouse blastocysts. These cells retained their capacity to differentiate into a great variety of cell types in cell cultures or in tumors formed after subcutaneous injection of the cells into nude mice. A chimeric actin/globin gene containing about two-thirds of the rat skeletal muscle actin gene and 730 bp of its 5' flanking region fused to the 3' end of the human embryonic epsilon-globin gene, was inserted into a plasmid containing a neomycin resistance gene (neor) whose transcription is regulated by the SV40 early control elements. The prokaryotic vector DNA sequences of this plasmid (pAG-Neo) were deleted and the two linked genes were introduced into the ES cells by electroporation. G418-resistant clones were isolated, amplified and injected subcutaneously into nude mice. From the teratocarcinoma-like tumors which developed we isolated myogenic as well as nonmyogenic cell lines. In cell lines derived from three independent transfected ES clones, expression of the actin/globin gene was developmentally regulated in myogenic cells. In contrast, in a number of experiments in which the actin/globin gene or other muscle-specific genes were introduced into the ES cells without the removal of the pBR sequences, no expression could be detected at any stage. Moreover, in the differentiated lines derived from these clones, G418 resistance was lost, and no neor transcripts could be detected. Southern-blot analysis of MSPI- or HpaII-digested DNA revealed extensive methylation in the clones that did not express the foreign DNA, whereas no significant methylation of the inserted DNA was observed in clones which expressed the transfected genes. Examination of the DNA extracted from transgenic mice carrying the same actin/globin gene revealed an inverse correlation between methylation of the exogenous gene and its potential to be expressed in the transgenic strain. However, no tissue-specific differences in methylation, related to the tissue specificity of expression of the exogenous gene, could be detected in these experiments. These results suggest that the process of methylation reported here is causally related to constitutive inactivation of the exogenous genes.

Expression of human adenosine deaminase in murine hematopoietic cells

Multiple replication-defective retrovirus vectors were tested for their ability to transfer and express human adenosine deaminase in vitro and in vivo in a mouse bone marrow transplantation model. High-titer virus production was obtained from vectors by using both a retrovirus long terminal repeat promoter and internal transcriptional units with human c-fos and herpes virus thymidine kinase promoters. After infection of primary murine bone marrow with one of these vectors, human adenosine deaminase was detected in 60 to 85% of spleen colony-forming units and in the blood of 14 of 14 syngeneic marrow transplant recipients. This system offers the opportunity to assess methods for increasing efficiency of gene transfer, for regulation of expression of foreign genes in hematopoietic progenitors, and for long-term measurement of the stability of expression in these cells.

Expression of human adenosine deaminase in murine hematopoietic cells

Multiple replication-defective retrovirus vectors were tested for their ability to transfer and express human adenosine deaminase in vitro and in vivo in a mouse bone marrow transplantation model. High-titer virus production was obtained from vectors by using both a retrovirus long terminal repeat promoter and internal transcriptional units with human c-fos and herpes virus thymidine kinase promoters. After infection of primary murine bone marrow with one of these vectors, human adenosine deaminase was detected in 60 to 85% of spleen colony-forming units and in the blood of 14 of 14 syngeneic marrow transplant recipients. This system offers the opportunity to assess methods for increasing efficiency of gene transfer, for regulation of expression of foreign genes in hematopoietic progenitors, and for long-term measurement of the stability of expression in these cells.

Transgenic animals

The ability to introduce foreign genes into the germ line and the successful expression of the inserted gene in the organism have allowed the genetic manipulation of animals on an unprecedented scale. The information gained from the use of the transgenic technology is relevant to almost any aspect of modern biology including developmental gene regulation, the action of oncogenes, the immune system, and mammalian development. Because specific mutations can be introduced into transgenic mice, it becomes feasible to generate precise animal models for human genetic diseases and to begin a systematic genetic dissection of the mammalian genome.

Construction and characterization of a retroviral vector demonstrating efficient expression of cloned cDNA sequences

We describe the construction and properties of a retroviral expression vector, designated pMV-7, designed to transfer unselected cDNAs and produce their encoded proteins in recipient cells. The vector is flanked by the long terminal repeats (LTRs) of the Moloney murine sarcoma virus (MSV) and contains the selectable drug resistance gene neo under the regulation of the herpes simplex virus (HSV) thymidine kinase (tk) promoter. Unique Eco RI and Hind III sites facilitate the introduction of sequences whose transcription is regulated by the 5' LTR. We have inserted cDNAs encoding: (i) the human lymphocyte antigen T4, (ii) the human lymphocyte antigen T8, and (iii) the murine hypoxanthine-guanine phosphoribosyl transferase (HPRT), into the pMV-7 vector. These constructions were used to transduce recipient cells to the neo+ phenotype. In each case, functional assays demonstrated that 65-92% of the neo+ clones produced the appropriate protein encoded by its corresponding cDNA. These clones were characterized further by analyzing the expression of vector-regulated transcripts. The neo+T4+ clones expressed a single full-length LTR-to-LTR transcript as detected by a T4 probe. The neo+T8+ clones, however, expressed both a full-length LTR-to-LTR transcript and an additional smaller transcript as detected by a T8 probe. This smaller transcript probably resulted from the utilization of cryptic signals which control 3' RNA processing. Furthermore, all of the neo+ clones expressed a transcript that initiated from the tk promoter, contained the neo gene, and used polyadenylation signals provided by the 3' LTR. Thus, the pMV-7 vector is capable of high-efficiency transfer and high-frequency expression of the cDNA-encoded protein.

Introduction and expression of the interleukin 2 receptor (Tac) gene in hematopoietic stem cells with retrovirus vectors

Retrovirus vectors provide an efficient carrier for introducing a gene into hematopoietic stem cells although expression of the inserted gene is not always successful. We constructed and compared three retrovirus vectors which carried cDNA encoding the light chain (Tac) of the interleukin 2 receptor under the control of different promoters; long terminal repeat (LTR) of murine retroviruses, the early promoter of simian virus 40 (SV40) and the promoter of the class I antigen gene of the major histocompatibility complex. We made three constructs containing these promoters. A first construct did not contain any additional promoter but LTR. A second and a third constructs contained the SV40 and the class I antigen gene promoters, respectively, in addition to LTR. The LTR of retrovirus vectors is derived from MoMuLV except that the U3 region of the 3'LTR of the third construct is derived from myeloproliferative sarcoma virus (MPSV). The second and third constructs were used for infection of bone marrow stem cells as the first construct was less efficient in expression of the interleukin 2 receptor in fibroblasts. Hematopoietic stem cells infected with the recombinant viruses were transplanted into lethally irradiated mice, and the expression of the transduced gene in hematopoietic progenitor cells was analyzed. Analysis of RNA isolated from spleen colonies showed that substantial amounts of interleukin 2 receptor mRNA were made by the construct containing the class I gene promoter and MPSV LTR. However, we could not detect any transcripts from the constructs containing MoMuLV LTR and SV40 early region promoter.

Two distinct sequence elements mediate retroviral gene expression in embryonal carcinoma cells

Moloney murine leukemia virus (M-MuLV) and M-MuLV-derived retroviral vectors are not expressed in early mouse embryos or in embryonal carcinoma cells. M-MuLV-derived mutants or M-MuLV-related variants which transduce the neomycin phosphotransferase gene can, however, induce drug resistance in embryonal carcinoma cells with high efficiency. In this study we investigated the sequences critical for retroviral gene expression in two different embryonal carcinoma cell lines, F9 and PCC4. We show that two synergistically acting sequence elements mediate expression in embryonal carcinoma cells. One of these is located within the U3 region of the viral long terminal repeat, and the second one is in the 5' untranslated region of the retrovirus. The latter element, characterized by a single point mutation, affects the level of stable RNA in infected cells, suggesting a regulatory mechanism similar to that of human immunodeficiency virus in human T cells.

Construction of a defective retrovirus containing the human hypoxanthine phosphoribosyltransferase cDNA and its expression in cultured cells and mouse bone marrow

Defective ecotropic and amphotropic retroviral vectors containing the cDNA for human hypoxanthine phosphoribosyltransferase (HPRT) were developed for efficient gene transfer and high-level cellular expression of HPRT. Helper cell clones which produced a high viral titer were generated by a simplified method which minimizes cell culture. We used the pZIP-NeoSV(X) vector containing a human hprt cDNA. Viral titers (1 X 10(3) to 5 X 10(4)/ml) of defective SVX HPRT B, a vector containing both the hprt and neo genes, were increased 3- to 10-fold by cocultivation of the ecotropic psi 2 and amphotropic PA-12 helper cells. Higher viral titers (8 X 10(5) to 7.5 X 10(6] were obtained when nonproducer NIH 3T3 cells or psi 2 cells carrying a single copy of SVX HPRT B were either transfected or infected by Moloney leukemia virus. The SVX HPRT B defective virus partially corrected the HPRT deficiency (4 to 56% of normal) of cultured rodent and human Lesch-Nyhan cells. However, instability of HPRT expression was detected in several infected clones. In these unstable variants, both retention and loss of the SVX HPRT B sequences were observed. In the former category, cells which became HPRT- (6-thioguanine resistant [6TGr]) also became G418s, indicative of a cis-acting down regulation of expression. Both hypoxanthine-aminopterin-thymidine resistance (HATr) and G418r could be regained by counterselection in hypoxanthine-aminopterin-thymidine. In vitro mouse bone marrow experiments indicated low-level expression of the neo gene in in vitro CFU assays. Individual CFU were isolated and pooled, and the human hprt gene was shown to be expressed. These studies demonstrated the applicability of vectors like SVX HPRT B for high-titer production of defective retroviruses required for hematopoietic gene transfer and expression.

Retroviral-mediated transfer of genomic globin genes leads to regulated production of RNA and protein

A high-titer amphotropic retroviral vector containing the neomycin resistance gene and a hybrid gamma-beta genomic human globin gene has been constructed. Mouse erythroleukemia cells infected with this virus were found to contain the full transcriptional unit of the transferred human globin gene by Southern blot analysis. These cells contain normally initiated, spliced, and terminated human globin mRNA. The human globin mRNA level increased 5- to 10-fold upon induction of the mouse erythroleukemia cells. Human globin chains were produced but only in a fraction of the cells as detected by immunofluorescent staining. A similar retrovirus containing a human beta-globin gene was used to transduce mouse erythroleukemia cells resulting in much higher levels of human globin synthesis than detected in mouse erythroleukemia cells transduced with the gamma-beta globin virus.

Modulation of gene expression in multiple hematopoietic cell lineages following retroviral vector gene transfer

Retrovirus vectors offer a simple and highly efficient method for introducing new genes into mammalian cells. Here, we have examined the efficiency of gene transfer into hematopoietic cells with retrovirus vectors carrying the neomycin (neo) resistance gene expressed from different transcriptional regulatory regions. Direct infection of mouse bone marrow cells resulted in high efficiencies of gene transfer into a variety of myeloid progenitor cells, including pluripotent, erythroid, and granulocyte-macrophage colony-forming cells with all the vectors examined. However, the progeny derived from individual pluripotent progenitor cells infected with different vectors differed markedly in the proportion of G418-resistant progenitor cells, as judged by their ability to survive selection in the drug G418. This biological assay suggests that the highest level of expression was observed when the neo gene was expressed from constructs that contained the herpes thymidine kinase promoter rather than the viral long terminal repeat or the simian virus 40 early region promoter. In contrast, neo gene expression was highest in fibroblasts infected with the vector containing the simian virus 40 early region promoter. These results show that high and sustainable levels of gene expression in hematopoietic cells can be obtained with retrovirus vectors containing appropriate transcriptional regulatory regions.

Construction of a defective retrovirus containing the human hypoxanthine phosphoribosyltransferase cDNA and its expression in cultured cells and mouse bone marrow

Defective ecotropic and amphotropic retroviral vectors containing the cDNA for human hypoxanthine phosphoribosyltransferase (HPRT) were developed for efficient gene transfer and high-level cellular expression of HPRT. Helper cell clones which produced a high viral titer were generated by a simplified method which minimizes cell culture. We used the pZIP-NeoSV(X) vector containing a human hprt cDNA. Viral titers (1 X 10(3) to 5 X 10(4)/ml) of defective SVX HPRT B, a vector containing both the hprt and neo genes, were increased 3- to 10-fold by cocultivation of the ecotropic psi 2 and amphotropic PA-12 helper cells. Higher viral titers (8 X 10(5) to 7.5 X 10(6] were obtained when nonproducer NIH 3T3 cells or psi 2 cells carrying a single copy of SVX HPRT B were either transfected or infected by Moloney leukemia virus. The SVX HPRT B defective virus partially corrected the HPRT deficiency (4 to 56% of normal) of cultured rodent and human Lesch-Nyhan cells. However, instability of HPRT expression was detected in several infected clones. In these unstable variants, both retention and loss of the SVX HPRT B sequences were observed. In the former category, cells which became HPRT- (6-thioguanine resistant [6TGr]) also became G418s, indicative of a cis-acting down regulation of expression. Both hypoxanthine-aminopterin-thymidine resistance (HATr) and G418r could be regained by counterselection in hypoxanthine-aminopterin-thymidine. In vitro mouse bone marrow experiments indicated low-level expression of the neo gene in in vitro CFU assays. Individual CFU were isolated and pooled, and the human hprt gene was shown to be expressed. These studies demonstrated the applicability of vectors like SVX HPRT B for high-titer production of defective retroviruses required for hematopoietic gene transfer and expression.

Tissue-specific and ectopic expression of genes introduced into transgenic mice by retroviruses

Recombinant retroviruses containing the complete genomic human beta globin gene (under the control of its own promoter) and the bacterial neomycin phosphotransferase gene (under the control of the normal or enhancerless viral promoter) were used to derive transgenic mouse strains by infection of preimplantation embryos. Expression of the beta globin gene in hematopoietic tissues was observed in all transgenic strains. In addition, one strain showed ectopic expression of beta globin in the same tissues that also expressed high levels of RNA from the viral promoter. It is likely that expression from the long terminal repeat (LTR), in contrast to expression from the internal promoter, is dependent on the site of integration. Thus, retroviral vectors can be used for tissue-specific expression of foreign genes in transgenic mice, as well as for the identification of loci that allow developmental activation of a provirus.

Germ-line transmission of genes introduced into cultured pluripotential cells by retroviral vector

Embryonic stem cells isolated directly from mouse embryos can be cultured for long periods in vitro and subsequently repopulate the germ line in chimaeric mice. During the culture period these embryonic cells are accessible for experimental genetic manipulation. Here we report the use of retroviral vectors to introduce exogenous DNA sequences into a stem-cell line and show that these modified cells contribute extensively to the somatic and germ-cell lineages in chimaeric mice. Compared with current methods for manipulation of the mouse genome, this approach has the advantage that powerful somatic-cell genetic techniques can be used to modify and to select cells with germ-line potential, allowing the derivation of transgenic strains with pre-determined genetic changes. We have by this means inserted many proviral vector sequences that provide new chromosomal molecular markers for linkage studies in the mouse and that also may cause insertional mutations.