Generation of single-copy transgenic mouse embryos directly from ES cells by tetraploid embryo complementation

Genetically modified mice as a tool for the study of human diseases

Modeling a human disease is an essential part of biomedical research. The recent advances in the field of molecular genetics made it possible to obtain genetically modified animals for the study of various diseases. Not only monogenic disorders but also chromosomal and multifactorial disorders can be mimicked in lab animals due to genetic modification. Even human infectious diseases can be studied in genetically modified animals. An animal model of a disease enables the tracking of its pathogenesis and, more importantly, to test new therapies. In the first part of this paper, we review the most common DNA modification technologies and provide key ideas on specific technology choices according to the task at hand. In the second part, we focus on the application of genetically modified mice in studying human diseases.

Perspectives in Genome-Editing Techniques for Livestock

Genome editing of farm animals has undeniable practical applications. It helps to improve production traits, enhances the economic value of livestock, and increases disease resistance. Gene-modified animals are also used for biomedical research and drug production and demonstrate the potential to be used as xenograft donors for humans. The recent discovery of site-specific nucleases that allow precision genome editing of a single-cell embryo (or embryonic stem cells) and the development of new embryological delivery manipulations have revolutionized the transgenesis field. These relatively new approaches have already proven to be efficient and reliable for genome engineering and have wide potential for use in agriculture. A number of advanced methodologies have been tested in laboratory models and might be considered for application in livestock animals. At the same time, these methods must meet the requirements of safety, efficiency and availability of their application for a wide range of farm animals. This review aims at covering a brief history of livestock animal genome engineering and outlines possible future directions to design optimal and cost-effective tools for transgenesis in farm species.

Single site-specific integration targeting coupled with embryonic stem cell differentiation provides a high-throughput alternative to in vivo enhancer analyses

Comprehensive analysis of cis-regulatory elements is key to understanding the dynamic gene regulatory networks that control embryonic development. While transgenic animals represent the gold standard assay, their generation is costly, entails significant animal usage, and in utero development complicates time-course studies. As an alternative, embryonic stem (ES) cells can readily be differentiated in a process that correlates well with developing embryos. Here, we describe a highly effective platform for enhancer assays using an Hsp68/Venus reporter cassette that targets to the Hprt locus in mouse ES cells. This platform combines the flexibility of Gateway® cloning, live cell trackability of a fluorescent reporter, low background and the advantages of single copy insertion into a defined genomic locus. We demonstrate the successful recapitulation of tissue-specific enhancer activity for two cardiac and two haematopoietic enhancers. In addition, we used this assay to dissect the functionality of the highly conserved Ets/Ets/Gata motif in the Scl+19 enhancer, which revealed that the Gata motif is not required for initiation of enhancer activity. We further confirmed that Gata2 is not required for endothelial activity of the Scl+19 enhancer using Gata2^−/− Scl+19 transgenic embryos. We have therefore established a valuable toolbox to study gene regulatory networks with broad applicability.

Tcf7l1 prepares epiblast cells in the gastrulating mouse embryo for lineage specification

The core gene regulatory network (GRN) in embryonic stem cells (ESCs) integrates activities of the pro-self-renewal factors Oct4 (Pou5f1), Sox2 and Nanog with that of an inhibitor of self-renewal, Tcf7l1 (Tcf3). The inhibitor function of Tcf7l1 causes dependence on extracellular Wnt/β-catenin signaling activity, making its embryonic role within the ESC GRN unclear. By analyzing intact mouse embryos, we demonstrate that the function of Tcf7l1 is necessary for specification of cell lineages to occur concomitantly with the elaboration of a three-dimensional body plan during gastrulation. In Tcf7l1(-/-) embryos, specification of mesoderm is delayed, effectively uncoupling it from the induction of the primitive streak. Tcf7l1 repressor activity is necessary for a rapid switch in the response of pluripotent cells to Wnt/β-catenin stimulation, from one of self-renewal to a mesoderm specification response. These results identify Tcf7l1 as a unique factor that is necessary in pluripotent cells to prepare them for lineage specification. We suggest that the role of Tcf7l1 in mammals is to inhibit the GRN to ensure the coordination of lineage specification with the dynamic cellular events occurring during gastrulation.

Cardiogenic induction of pluripotent stem cells streamlined through a conserved SDF-1/VEGF/BMP2 integrated network

Background

Pluripotent stem cells produce tissue-specific lineages through programmed acquisition of sequential gene expression patterns that function as a blueprint for organ formation. As embryonic stem cells respond concomitantly to diverse signaling pathways during differentiation, extraction of a pro-cardiogenic network would offer a roadmap to streamline cardiac progenitor output.

Methods and Results

To resolve gene ontology priorities within precursor transcriptomes, cardiogenic subpopulations were here generated according to either growth factor guidance or stage-specific biomarker sorting. Innate expression profiles were independently delineated through unbiased systems biology mapping, and cross-referenced to filter transcriptional noise unmasking a conserved progenitor motif (55 up- and 233 down-regulated genes). The streamlined pool of 288 genes organized into a core biological network that prioritized the “Cardiovascular Development” function. Recursive in silico deconvolution of the cardiogenic neighborhood and associated canonical signaling pathways identified a combination of integrated axes, CXCR4/SDF-1, Flk-1/VEGF and BMP2r/BMP2, predicted to synchronize cardiac specification. In vitro targeting of the resolved triad in embryoid bodies accelerated expression of Nkx2.5, Mef2C and cardiac-MHC, enhanced beating activity, and augmented cardiogenic yield.

Conclusions

Transcriptome-wide dissection of a conserved progenitor profile thus revealed functional highways that coordinate cardiogenic maturation from a pluripotent ground state. Validating the bioinformatics algorithm established a strategy to rationally modulate cell fate, and optimize stem cell-derived cardiogenesis.

Germline competence of mouse ES and iPS cell lines: Chimera technologies and genetic background

In mice, gene targeting by homologous recombination continues to play an essential role in the understanding of functional genomics. This strategy allows precise location of the site of transgene integration and is most commonly used to ablate gene expression ("knock-out"), or to introduce mutant or modified alleles at the locus of interest ("knock-in"). The efficacy of producing live, transgenic mice challenges our understanding of this complex process, and of the factors which influence germline competence of embryonic stem cell lines. Increasingly, evidence indicates that culture conditions and in vitro manipulation can affect the germline-competence of Embryonic Stem cell (ES cell) lines by accumulation of chromosome abnormalities and/or epigenetic alterations of the ES cell genome. The effectiveness of ES cell derivation is greatly strain-dependent and it may also influence the germline transmission capability. Recent technical improvements in the production of germline chimeras have been focused on means of generating ES cells lines with a higher germline potential. There are a number of options for generating chimeras from ES cells (ES chimera mice); however, each method has its advantages and disadvantages. Recent developments in induced pluripotent stem (iPS) cell technology have opened new avenues for generation of animals from genetically modified somatic cells by means of chimera technologies. The aim of this review is to give a brief account of how the factors mentioned above are influencing the germline transmission capacity and the developmental potential of mouse pluripotent stem cell lines. The most recent methods for generating specifically ES and iPS chimera mice, including the advantages and disadvantages of each method are also discussed.

Site controlled transgenic mice validating increased expression from human matrix metalloproteinase (MMP-1) promoter due to a naturally occurring SNP

Matrix metalloproteinases (MMPs) comprise a family of more than 20 members, each with the ability to degrade components of the extracellular matrix. The interstitial collagenases have the unique capacity to degrade the stromal collagens, types I, II and III, the body's most abundant proteins. These collagenases include MMP-1, MMP-8, MMP-13 and MMP-14. MMP-1, with a very broad expression pattern, has major roles in mediating matrix destruction in many diseases. We have described a single nucleotide polymorphism (SNP) in the MMP-1 promoter that augments transcription. This SNP is the presence or absence of an extra guanine (G) at -1607 bp, which creates the sequence 5'-GGAA-3'(2G allele), and which is an ETS binding site. Compared to the 1G allele (5'-GAA-3'), the 2G SNP is associated with enhanced transcription of MMP-1 and increased enzymatic activity. Although murine systems are often used to model human diseases, mice have only distant homologues of human MMP-1. Therefore, we used a technique for the targeted insertion of a single copy of a gene at the HPRT locus to compare expression of the 1G and 2G alleles. We generated transgenic mice with -4372 bp of the human MMP-1 promoter containing either the 1G or 2G SNP in front of the lac Z (E.coli ss-galactosidase) gene. We measured the relative expression of the transgenes in vitro in embryonic stem (ES) cells and in fibroblasts derived from embryonic mice. Our data show modest constitutive expression of ss-galactosidase mRNA and protein from these alleles, with the 2G allele more transcriptionally active than the 1G allele. We conclude that these mice represent a model for integration of a single copy of the human MMP-1 promoter into the murine genome, and could be used to study MMP-1 gene expression in a murine system.

Next generation tools for high-throughput promoter and expression analysis employing single-copy knock-ins at the Hprt1 locus

We have engineered a set of useful tools that facilitate targeted single copy knock-in (KI) at the hypoxanthine guanine phosphoribosyl transferase 1 (Hprt1) locus. We employed fine scale mapping to delineate the precise breakpoint location at the Hprt1(b-m3) locus allowing allele specific PCR assays to be established. Our suite of tools contains four targeting expression vectors and a complementing series of embryonic stem cell lines. Two of these vectors encode enhanced green fluorescent protein (EGFP) driven by the human cytomegalovirus immediate-early enhancer/modified chicken beta-actin (CAG) promoter, whereas the other two permit flexible combinations of a chosen promoter combined with a reporter and/or gene of choice. We have validated our tools as part of the Pleiades Promoter Project (http://www.pleiades.org), with the generation of brain-specific EGFP positive germline mouse strains.

A novel mode of enhancer evolution: the Tal1 stem cell enhancer recruited a MIR element to specifically boost its activity

Altered cis-regulation is thought to underpin much of metazoan evolution, yet the underlying mechanisms remain largely obscure. The stem cell leukemia TAL1 (also known as SCL) transcription factor is essential for the normal development of blood stem cells and we have previously shown that the Tal1 +19 enhancer directs expression to hematopoietic stem cells, hematopoietic progenitors, and to endothelium. Here we demonstrate that an adjacent region 1 kb upstream (+18 element) is in an open chromatin configuration and carries active histone marks but does not function as an enhancer in transgenic mice. Instead, it boosts activity of the +19 enhancer both in stable transfection assays and during differentiation of embryonic stem (ES) cells carrying single-copy reporter constructs targeted to the Hprt locus. The +18 element contains a mammalian interspersed repeat (MIR) which is essential for the +18 function and which was transposed to the Tal1 locus approximately 160 million years ago at the time of the mammalian/marsupial branchpoint. Our data demonstrate a previously unrecognized mechanism whereby enhancer activity is modulated by a transposon exerting a "booster" function which would go undetected by conventional transgenic approaches.

Comparison of tetraploid blastocyst microinjection of outbred Crl:CD1(ICR), hybrid B6D2F1/Tac, and inbred C57BL/6NTac embryos for generation of mice derived from embryonic stem cells

Embryo electrofusion and tetraploid blastocyst microinjection is a modification of the traditional embryonic stem cell (ES cell)-based method to generate targeted mutant mice. Viability of tetraploid embryos is reportedly lower than with diploid embryos, with considerable interstrain variation. Here we assessed fetus and pup viability after ES cell microinjection of tetraploid blastocysts derived from outbred, hybrid, and inbred mice. Two-cell mouse embryos (C57BL/6NTac [B6], n = 788; B6D2F1/Tac [BDF1], n = 1871; Crl:CD1(ICR) [CD1], n = 1308) were electrofused; most resultant tetraploid blastocysts were injected with ES cells and surgically transferred into pseudopregnant recipient mice. Reproductive tracts were examined at midgestation for embryologic studies using B6 and BDF1 blastocysts; implantation sites and viable fetuses were counted. Pregnancies were carried to term for studies of targeted mutant mice using BDF1 and CD1 blastocysts, and pup yield was evaluated. Electrofusion rates of 2-cell embryos did not differ among B6, BDF1, and CD1 mice (overall mean, 92.8% +/- 5.4%). For embryologic studies, 244 B6 blastocysts were surgically transferred and 1 fetus was viable (0.41%), compared with 644 BDF1 blastocysts surgically transferred and 88 viable fetuses (13.7%). For targeted mutant mouse studies, 259 BDF1 blastocysts were surgically transferred yielding 10 pups (3.9%); 569 CD1 blastocysts yielded 44 pups (7.7%).

Improved cardiac function in infarcted mice after treatment with pluripotent embryonic stem cells

Because pluripotent embryonic stem cells (ESCs) are able to differentiate into any tissue, they are attractive agents for tissue regeneration. Although improvement of cardiac function has been observed after transplantation of pluripotent ESCs, the extent to which these effects reflect ESC-mediated remuscularization, revascularization, or paracrine mechanisms is unknown. Moreover, because ESCs may generate teratomas, the ability to predict the outcome of cellular differentiation, especially when transplanting pluripotent ESCs, is essential; conversely, a requirement to use predifferentiated ESCs would limit their application to highly characterized subsets that are available in limited numbers. In the experiments reported here, we transplanted low numbers of two murine ESC lines, respectively engineered to express a beta-galactosidase gene from either a constitutive (elongation factor) or a cardiac-specific (alpha-myosin heavy chain) promoter, into infarcted mouse myocardium. Although ESC-derived tumors formed within the pericardial space in 21% of injected hearts, lacZ histochemistry revealed that engraftment of ESC was restricted to the ischemic myocardium. Echocardiographic monitoring of ESC-injected hearts that did not form tumors revealed functional improvements by 4 weeks postinfarction, including significant increases in ejection fraction, circumferential fiber shortening velocity, and peak mitral blood flow velocity. These experiments indicate that the infarcted myocardial environment can support engraftment and cardiomyogenic differentiation of pluripotent ESCs, concomitant with partial functional recovery.

Artificial chromosome-based transgenes in the study of genome function

The transfer of large DNA fragments to the mouse genome in the form of bacterial, yeast or phage artificial chromosomes is an important process in the definition of transcription units, the modeling of inherited disease states, the dissection of candidate regions identified by linkage analysis and the construction of in vivo reporter genes. However, as with small recombinant transgenes, the transferred sequences are usually integrated randomly often with accompanying genomic alterations and variable expression of the introduced genes due to the site of integration and/or copy number. Therefore, alternative methods of integrating large genomic transgenes into the genome have been developed to avoid the variables associated with random integration. This review encourages the reader to imagine the large variety of applications where artificial chromosome transgenes can facilitate in vivo and ex vivo studies in the mouse and provides a context for making the necessary decisions regarding the specifics of experimental design.

Stable and uniform gene suppression by site-specific integration of siRNA expression cassette in murine embryonic stem cells

We developed a simple system to introduce small interfering RNA (siRNA) into murine embryonic stem cells (ESCs) and then showed its stable and uniform expression. Using hypoxanthine guanine phosphoribosyl transferase 1 (Hprt)-deficient ESCs as a recipient, we efficiently introduced an siRNA expression cassette into the Hprt locus by homologous recombination, which was easily detected by HAT selection. Nearly all of the HAT-resistant clones exhibited a silenced expression of the exogenous target gene (enhanced green fluorescent protein [EGFP]) or the endogenous target gene (Flk1). Flow cytometry profiles demonstrated that there were no significant differences in level of suppression among individual clones and cells. The suppressing effect by siRNA was maintained for more than 1 month in both undifferentiated and differentiated ESCs, while its persistent expression did not disturb their growth or differentiation potential. The stable and uniform suppression capability of this system will help to screen genes and provide important information regarding cell differentiation in ESCs.

Tissue-specific expression of a BAC transgene targeted to the Hprt locus in mouse embryonic stem cells

The hypoxanthine phosphoribosyltransferase (Hprt) locus has been shown to have minimal influence on transgene expression when used as a surrogate site in the mouse genome. We have developed a method to transfer bacterial artificial chromosomes (BACs) as a single copy into the partially deleted Hprt locus of embryonic stem cells. BACs were modified by Cre/loxP recombination to contain the sequences necessary for homologous recombination into and complementation of the partially deleted Hprt locus. Modified BACs were shown to undergo homologous recombination into the genome intact, to be stably transmitted through the germ line of transgenic mice, and to be expressed in the proper tissue-specific manner. This technology will facilitate many studies in which correct interpretation of data depends on developmentally appropriate transgene expression in the absence of rearrangements or deletions of endogenous DNA.

Simple and efficient production of mice derived from embryonic stem cells aggregated with tetraploid embryos

Six newly derived hybrid mouse embryonic stem (ES) cell lines and two inbred ES cell lines were tested for their ability to produce completely ES cell-derived mice by aggregation of ES cells with tetraploid embryos. Forty-five ES cell-tetraploid pups were generated from six hybrid ES cell lines and no pups from two inbred ES cell lines. These pups were found to have increased embryonic and placental weights than control mice. Twenty-two pups survived to adulthood and produced normal offsprings, and the other 23 pups died of several reasons including respiratory distress, abdomen ulcer-like symptoms, and foster failure. The 22 adult ES cell-tetraploid mice were completely ES cell-derived as judged by coat color and germline transmission, only two of them was found to have tetraploid component in liver, blood, and lung as analyzed by microsatellite loci. Our data suggested that genetic heterozygosity is a crucial factor for postnatal survival of ES cell-tetraploid mice, and tetraploid embryo aggregation using hybrid ES cells is a simple and efficient procedure for immediate generation of targeted mouse mutants from genetically modified ES cell clones, in contrast to the standard protocol, which involves the production of chimeras and several breeding steps.

Aggregation of embryonic stem cells induces Nanog repression and primitive endoderm differentiation

When embryonic stem cells are allowed to aggregate, the outer layer of the aggregated spheres (referred to as embryoid bodies) differentiates into primitive endoderm. This initial specification of cell lineage facilitates further differentiation of the inner mass of the embryoid bodies. These processes are considered to recapitulate early embryonic development from the blastocyst stage to the egg-cylinder stage. Formation of the primitive endoderm layer in the embryoid bodies was induced solely by aggregation of embryonic stem cells, in the presence of leukemia inhibitory factor/STAT3 and serum/BMP4, which were considered to be sufficient for embryonic stem cell self-renewal. Interestingly, cell aggregation by itself induced Nanog repression at the outer layer, which was essential for aggregation-induced primitive endoderm formation. These data illustrate aggregation-based cell-fate specification during early embryonic development, when downregulation of Nanog plays a crucial role.

Tetraploid development in the mouse

Spontaneous duplication of the mammalian genome occurs in approximately 1% of fertilizations. Although one or more whole genome duplications are believed to have influenced vertebrate evolution, polyploidy of contemporary mammals is generally incompatible with normal development and function of all but a few tissues. The production of tetraploid (4n) embryos has become a common experimental manipulation in the mouse. Although development of tetraploid mice has generally not been observed beyond midgestation, tetraploid:diploid (4n:2n) chimeras are widely used as a method for rescuing extraembryonic defects. The tolerance of tissues to polyploidy appears to be dependent on genetic background. Indeed, the recent discovery of a naturally tetraploid rodent species suggests that, in rare genetic backgrounds, mammalian genome duplications may be compatible with the development of viable and fertile adults. Thus, the range of developmental potentials of tetraploid embryos remains in large part unexplored. Here, we review the biological consequences and experimental utility of tetraploid mammals, in particular the mouse.

Avian precardiac endoderm/mesoderm induces cardiac myocyte differentiation in murine embryonic stem cells

The ability to regenerate damaged myocardium with tissue derived from embryonic stem (ES) cells is currently undergoing extensive investigation. As a prerequisite to transplantation therapy, strategies must be developed to induce ES cells to the cardiac phenotype. Toward this end, cues from mechanisms of embryonic induction have been exploited, based on previous findings that anterior lateral endoderm (precardiac endoderm) from gastrulation-stage chick embryos potently induces cardiac myocyte differentiation in both precardiac and nonprecardiac mesoderm. Hypothesizing that avian precardiac endoderm acting as feeder/inducer cells would induce high percentage conversion of murine ES (mES) cells into cardiac myocytes, it was observed that the majority (approximately 65%) of cocultured ES cell-derived embryoid bodies (EBs) were enriched in cardiac myocytes and exhibited rhythmic contractions. By contrast, mouse EBs cultured alone, or on feeder layers of mouse embryonic fibroblasts or avian nonprecardiac posterior endoderm, contained only 7% to 16% cardiac myocytes while exhibiting a relatively low incidence (<10%) of beating. When mES cells were cocultured with a bilayer of explanted precardiac endoderm/mesoderm, the incidence of rhythmically contractile EBs increased to 100%. To verify that the rhythmically contractile cells were derived from murine ES cells, cell-free medium conditioned by avian precardiac endoderm/mesoderm was used to induce myocyte differentiation in a mES cell-line containing a nuclear LacZ reporter marker gene under control of the cardiac-specific alpha-myosin heavy chain promoter, resulting in rhythmic contractility in 92% of EBs in which the majority of cells (average=86%) were identified as cardiac myocytes. The inductive efficacy of medium conditioned by avian precardiac endoderm/mesoderm may provide an opportunity to biochemically define factors that induce cardiac myocyte differentiation in ES cells. The full text of this article is available online at http://circres.ahajournals.org.

Conserved enhancer in the serum response factor promoter controls expression during early coronary vasculogenesis

Serum response factor (SRF) is a transcription factor required for mesoderm formation in the developing mouse embryo that is important for myogenic differentiation, including notably, the differentiation of the proepicardial organ (PEO) into coronary vascular cells during early development. To identify regulatory sequences that control SRF expression during early mouse development, we used a novel transgenic approach to study the role of conserved noncoding DNA sequences (CNCS) in the SRF gene. Embryonic stem (ES) cells containing a targeted single-copy of putative SRF regulatory sequences were used to directly generate transgenic embryos by tetraploid aggregation. Because the ES cell-derived targeted embryos are genetically equivalent, except for the putative regulatory sequence of interest, differences in transgene expression can be attributed directly to these sequences. Using this approach, we identified an E-box/Ets containing 270-bp cis-acting module in the SRF promoter that mediates expression in the PEO. Reporter transgenes containing this module express in derivatives of the PEO that give rise to the coronary vasculature, but do not express in the PEO-derived epicardium. These results are the first reported in vivo analysis of SRF regulatory elements that control expression during early development. Using this reporter module and this approach, it should be possible to begin to elucidate molecular mechanisms involved in the differentiation of coronary vasculature progenitor cells, as well as identify additional SRF regulatory elements important during mammalian development.

Practical uses for ecdysteroids in mammals including humans: an update

Ecdysteroids are widely used as inducers for gene-switch systems based on insect ecdysteroid receptors and genes of interest placed under the control of ecdysteroid-response elements. We review here these systems, which are currently mainly used in vitro with cultured cells in order to analyse the role of a wide array of genes, but which are expected to represent the basis for future gene therapy strategies. Such developments raise several questions, which are addressed in detail. First, the metabolic fate of ecdysteroids in mammals, including humans, is only poorly known, and the rapid catabolism of ecdysteroids may impede their use as in vivo inducers. A second set of questions arose in fact much earlier with the pioneering "heterophylic" studies of Burdette in the early sixties on the pharmacological effects of ecdysteroids on mammals. These and subsequent studies showed a wide range of effects, most of them being beneficial for the organism (e.g. hypoglycaemic, hypocholesterolaemic, anabolic). These effects are reviewed and critically analysed, and some hypotheses are proposed to explain the putative mechanisms involved. All of these pharmacological effects have led to the development of a wide array of ecdysteroid-containing preparations, which are primarily used for their anabolic and/or "adaptogenic" properties on humans (or horses or dogs). In the same way, increasing numbers of patents have been deposited concerning various beneficial effects of ecdysteroids in many medical or cosmetic domains, which make ecdysteroids very attractive candidates for several practical uses. It may be questioned whether all these pharmacological actions are compatible with the development of ecdysteroid-inducible gene switches for gene therapy, and also if ecdysteroids should be classified among doping substances.

Of mice and models: improved animal models for biomedical research

The ability to engineer the mouse genome has profoundly transformed biomedical research. During the last decade, conventional transgenic and gene knockout technologies have become invaluable experimental tools for modeling genetic disorders, assigning functions to genes, evaluating drugs and toxins, and by and large helping to answer fundamental questions in basic and applied research. In addition, the growing demand for more sophisticated murine models has also become increasingly evident. Good state-of-principle knowledge about the enormous potential of second-generation conditional mouse technology will be beneficial for any researcher interested in using these experimental tools. In this review we will focus on practice, pivotal principles, and progress in the rapidly expanding area of conditional mouse technology. The review will also present an internet compilation of available tetracycline-inducible mouse models as tools for biomedical research (http://www.zmg.uni-mainz.de/tetmouse/).

Gene targeting in the mouse: advances in introduction of transgenes into the genome by homologous recombination

The ability to stably introduce genes into the germline of animals provides a powerful means to address the genetic basis of physiology. Introduction of genes to generate transgenic animals has facilitated the development of complex genetic models of disease, as well as the in vivo study of gene function. However, one drawback of traditional transgenic technologies in which genes are microinjected into early-stage embryos is that there is little control over where and in how many copies genes are introduced into the genome. The development of animal transgenic technologies, which take advantage of homologous recombination mechanisms and the manipulation of embryonic stem (ES) cells, allows investigators to target and alter specific loci. In mouse transgenic systems, a plethora of sophisticated gene-targeting strategies now permit investigators to manipulate the genome in ways that essentially allow one to introduce virtually any desired change into the genome. Furthermore, when coupled with systems that allow for conditional gene expression, these gene-targeting strategies allow both temporal and tissue specific control of alterations to the genome. In the present review we briefly discuss some of the more recent gene-targeting strategies that have been developed to address the limitations of traditional animal transgenesis.