Transgenic mice that express Cre recombinase in hypertrophic chondrocytes

In order to investigate the physiological control of hypertrophic chondrocytes which present the terminally differentiated form of chondrocytes, we generated a mouse line expressing the Cre recombinase under the control of the mouse type X collagen (Col10a1) promoter. In situ hybridization analysis demonstrated the expression of Col10a1-Cre transgene in hypertrophic chondrocytes of femur at postnatal day 2 (P2). In order to test the excision activity of the Cre recombinase, the Col10a1-Cre transgenic line was crossed with the mouse strain carrying the Smad4 conditional alleles (Smad4co/co) and the reporter line ROSA26. Multiple tissue PCR of Col10a1-Cre;Smad4co/+ mice revealed the restricted Cre activity in tissues containing hypertrophic chondrocytes. LacZ staining revealed that the Cre activity was observed in the cartilage primordia of ribs at E14.5 and only detected in the lower hypertrophic region of ribs at P1. These data suggest that the Col10a1-Cre mouse line described here could be used to achieve conditional gene targeting in hypertrophic chondrocytes.     

肥大软骨细胞特异性表达Cre重组酶转基因小鼠的建立

肥大软骨细胞是软骨细胞的终末分化形式,在软骨内成骨过程中发挥十分关键的作用。为了研究肥大软骨细胞在骨骼发育过程中的功能,我们构建了在8.2 kb小鼠X型胶原基因（Col10a1）启动子控制下表达Cre重组酶的转基因小鼠品系（Col10a1-8.2-Cre）。采用显微注射法将11.5 kb的转基因片段引入小鼠基因组,共注射受精卵328枚,获得子代鼠51只,经PCR基因型鉴定有3只在基因组上整合有Cre重组酶基因。PCR检测发现Col10a1-8.2-Cre转基因在含有肥大软骨细胞的组织中表达。为了检测Cre重组酶表达的强度和组织特异性,转基因小鼠与ROSA26报告小鼠交配。子代ROSA26;Col10a1-8.2-Cre双转基因小鼠LacZ染色检测的结果显示,Cre重组酶在所有的肥大软骨细胞中表达。原位杂交的结果验证Col10a1-8.2-Cre转基因表达在肥大区的上端。以上结果表明,我们建立的肥大软骨细胞特异性表达Cre重组酶的转基因小鼠品系可以作为一种遗传学工具,介导目的基因在肥大软骨细胞中的敲除。     

Collagen1alpha1 promoter drives the expression of Cre recombinase in osteoblasts of transgenic mice

Osteoblasts participate in bone formation, bone mineralization, osteoclast differentiation and many pathological processes. To study the function of genes in osteoblasts using Cre-LoxP system, we generated a mouse line expressing the Cre recombinase under the control of the rat Collagen1alpha1 (Col1alpha1) promoter (Col1alpha1-Cre). Two founders were identified by genomic PCR from 16 offsprings, and the integration efficiency is 12.5%. In order to determine the tissue distribution and the activity of Cre recombinase in the transgenic mice, the Col1alpha1-Cre transgenic mice were bred with the ROSA26 reporter strain and a mouse strain that carries Smad4 conditional alleles (Smad4(Co/Co)). Multiple tissue PCR of Col1alpha1-Cre;Smad4(Co/+)mice revealed the restricted Cre activity in bone tissues containing osteoblasts and tendon. LacZ staining in the Col1alpha1-Cre;ROSA26 double transgenic mice revealed that the Cre recombinase began to express in the osteoblasts of calvaria at E14.5. Cre activity was observed in the osteoblasts and osteocytes of P10 double transgenic mice. All these data indicated that the Col1alpha1-Cre transgenic mice could serve as a valuable tool for osteoblast lineage analysis and conditional gene knockout in osteoblasts.     

Cre-mediated somatic site-specific recombination in mice.

Conditional mutant mice equipped with heterologous recombination systems (Cre/lox or Flp/frt) are promising for studying tissue-specific gene function and for designing better models of human diseases. The utility of these mice depends on the cell target specificity, on the efficiency and on the control over timing of gene (in)activation. We have explored the utility of adenoviral vectors and transgenic mice expressing Cre under the control of tissue-specific promoters to achieve Cre/lox-mediated somatic recombination of the LacZ reporter gene, using a newly generated flox LacZ mouse strain. When adeno Cre viruses were administered via different routes, recombination and expression of LacZ was detected in a wide range of tissues. Whereas in liverbeta-galactosidase activity was quickly lost by turnover of expressing cells, even though the recombined allele was retained,beta-galactosidase in other tissues persisted for many months. Our data indicate that the flox LacZ transgenic line can be utilized effectively to monitor the level and functionality of Cre protein produced upon infection with adeno Cre virus or upon crossbreeding with different Cre transgenic lines.     

Chondrocytes Transdifferentiate into Osteoblasts in Endochondral Bone during Development,Postnatal Growth and Fracture Healing in Mice

One of the crucial steps in endochondral bone formation is the replacement of a cartilage matrix produced by chondrocytes with bone trabeculae made by osteoblasts. However, the precise sources of osteoblasts responsible for trabecular bone formation have not been fully defined. To investigate whether cells derived from hypertrophic chondrocytes contribute to the osteoblast pool in trabecular bones, we genetically labeled either hypertrophic chondrocytes by Col10a1-Cre or chondrocytes by tamoxifen-induced Agc1-CreERT2 using EGFP, LacZ or Tomato expression. Both Cre drivers were specifically active in chondrocytic cells and not in perichondrium, in periosteum or in any of the osteoblast lineage cells. These in vivo experiments allowed us to follow the fate of cells labeled in Col10a1-Cre or Agc1-CreERT2 -expressing chondrocytes. After the labeling of chondrocytes, both during prenatal development and after birth, abundant labeled non-chondrocytic cells were present in the primary spongiosa. These cells were distributed throughout trabeculae surfaces and later were present in the endosteum, and embedded within the bone matrix. Co-expression studies using osteoblast markers indicated that a proportion of the non-chondrocytic cells derived from chondrocytes labeled by Col10a1-Cre or by Agc1-CreERT2 were functional osteoblasts. Hence, our results show that both chondrocytes prior to initial ossification and growth plate chondrocytes before or after birth have the capacity to undergo transdifferentiation to become osteoblasts. The osteoblasts derived from Col10a1-expressing hypertrophic chondrocytes represent about sixty percent of all mature osteoblasts in endochondral bones of one month old mice. A similar process of chondrocyte to osteoblast transdifferentiation was involved during bone fracture healing in adult mice. Thus, in addition to cells in the periosteum chondrocytes represent a major source of osteoblasts contributing to endochondral bone formation in vivo.     

Characterization of Cre recombinase mouse lines enabling cell type-specific targeting of postnatal intervertebral discs

Cre/loxP technology is an important tool for studying cell type-specific gene functions. Cre recombinase mouse lines, including Agc1-CreER^T2, Col2a1-Cre; Col2a1-CreER^T2, Shh-Cre, Shh-CreER^T2, and Osx-Cre, have been proven to be valuable tools to elucidate the biology of long bones, yet the information for their activity in postnatal intervertebral disc (IVD) tissues was very limited. In this study, we used R26-mTmG fluorescent reporter to systematically analyze cell specificity and targeting efficiency of these six mouse lines in IVD tissues at postnatal growing and adult stages. We found that Agc1-CreER^T2 is effective to direct recombination in all components of IVDs, including annulus fibrosus (AF), nucleus pulposus (NP), and cartilaginous endplate (CEP), upon tamoxifen induction at either 2 weeks or 2 months of ages. Moreover, Col2a1-Cre targets most of the cells in IVDs, except for some cells in the outer AF (OAF) and NP. In contrast, the activity of Col2a1-CreER^T2 is mainly limited to the IAF of IVD tissues at either stage of tamoxifen injection. Similarly, Shh-Cre directs recombination specifically in all NP cells, whereas Shh-CreER^T2 is active only in a few NP cells when tamoxifen is administered at either stage. Finally, Osx-Cre targets cells in the CEP, but not in the NP or AF of IVDs tissues at these two stages. Thus, our data demonstrated that all these Cre lines can direct recombination in IVD tissues at postnatal stages with different cell type specificity and/or targeting efficiency, and can, therefore, serve as valuable tools to dissect cell type-specific gene functions in IVD development and homeostasis.     

A novel reporter rat strain that expresses LacZ upon Cre‐mediated recombination

The recent widespread application of Cre/loxP technology has resulted in a new generation of conditional animal models that can better recapitulate many salient features of human disease. These models benefit from the ability to monitor the expression and functionality of Cre protein. We have generated a conditional (Cre/loxP dependent) LacZ reporter rat (termed the LacZ541 rat) to monitor Cre in transgenic rats. When LacZ541 rats were bred with another transgenic rat line expressing Cre recombinase under the control of the CAG promoter, LacZ/Cre double transgenic embryos displayed ubiquitous expression of LacZ, and when LacZ541 rats were bred with transgenic rats expressing Cre/loxP‐dependent oncogenic H‐ or K‐ras, LacZ was expressed in the lesions resulting from the activation of the oncogene. The LacZ541 rat enables evaluation of the performance of Cre‐expressing systems which are based upon transgenic rats or somatic gene transfer vectors and provides efficient and simple lineage marking. genesis 51:268–274. © 2013 Wiley Periodicals, Inc.     

消化道细胞表达Cre重组酶转基因小鼠的功能鉴定

目的:检测白蛋白启动子介导的Cre重组酶转基因小鼠Alb-Cre-2中Cre重组酶的组织分布及其在体内介导基因重组的作用。方法:将Alb-Cre小鼠与Smad4条件基因打靶小鼠交配,利用PCR对Cre重组酶介导重组的组织特异性进行检测;然后,将Alb-Cre-2转基因小鼠与ROSA26报告小鼠交配,利用LacZ染色对双转基因阳性子代小鼠进行检测。结果:PCR结果显示心、肺、胰、脑及消化道中Cre重组酶介导的Smad4基因发生重组;LacZ染色进一步表明Cre重组酶在肝细胞、胃壁细胞、空肠潘氏细胞、回肠杯状细胞、大肠杯状细胞、大肠柱状细胞及空泡细胞中特异性表达,并介导ROSA位点LoxP序列间的重组。结论:Alb-Cre-2转基因小鼠在消化道中具有一定的组织特异性,只在胃壁细胞、空肠潘氏细胞、回肠杯状细胞、大肠杯状细胞,大肠柱状细胞及空泡细胞等细胞类型中特异性表达,并能在体内成功地介导这些消化道上皮细胞基因组上LoxP位点间的重组,是一种研制在消化道特定细胞中特异性基因剔除小鼠的良好工具小鼠。     

BAC constructs in transgenic reporter mouse lines control efficient and specific LacZ expression in hypertrophic chondrocytes under the complete Col10a1 promoter

During endochondral ossification hypertrophic chondrocytes in the growth plate of fetal long bones, ribs and vertebrae play a key role in preparing growth plate cartilage for replacement by bone. In order to establish a reporter gene mouse to facilitate functional analysis of genes expressed in hypertrophic chondrocytes in this process, Col10a1- BAC reporter gene mouse lines were established expressing LacZ specifically in hypertrophic cartilage under the control of the complete Col10a1 gene. For this purpose, a bacterial artificial chromosome (BAC RP23-192A7) containing the entire murine Col10a1 gene together with 200 kb flanking sequences was modified by inserting a LacZ-Neo cassette into the second exon of Col10a1 by homologous recombination in E. coli. Transgenic mice containing between one and seven transgene copies were generated by injection of the purified BAC-Col10a1- lLacZ DNA. X-gal staining of newborns and embryos revealed strong and robust LacZ activity exclusively in hypertrophic cartilage of the fetal and neonatal skeleton of the transgenic offspring. This indicates that expression of the reporter gene in its proper genomic context in the BAC Col10a1 environment is independent of the integration site and reflects authentic Col10a1 expression in vivo. The Col10a1 specific BAC recombination vector described here will enable the specific analysis of effector gene functions in hypertrophic cartilage during skeletal development, endochondral ossification, and fracture callus healing. Sonja Gebhard and Takako Hattori equally contributed to this work.     

Expression of Cre recombinase in chondrocytes causes abnormal craniofacial and skeletal development

The cranial base synchondroses are growth centers that drive cranial and upper facial growth. The intersphenoid synchondrosis (ISS) and the spheno-occipital synchondrosis (SOS) are two major synchondroses located in the middle of the cranial base and are maintained at early developmental stages to sustain cranial base elongation. In this study, we report unexpected premature ossification of ISS and SOS when Cre recombinase is activated in a chondrocyte-specific manner. We used a Cre transgenic line expressing Aggrecan enhancer-driven, Tetracycline-inducible Cre (ATC), of which expression is controlled by a Col2a1 promoter. Neonatal doxycycline injection or doxycycline diet fed to breeders was used to activate Cre recombinase. The premature ossification of ISS and/or SOS led to a reduction in cranial base length and subsequently a dome-shaped skull. Furthermore, the mice carrying either heterozygous or homozygous conditional deletion of Tsc1 or Fip200 using ATC mice developed similar craniofacial abnormalities, indicating that Cre activity itself but not conditional deletion of Tsc1 or Fip200 gene, is the major contributor of this phenotype. In contrast, the Col2a1-Cre mice carrying Cre expression in both perichondrium and chondrocytes and the mice carrying the conditional deletion of Tsc1 or Fip200 using Col2a1-Cre did not manifest the same skull abnormalities. In addition to the defective craniofacial bone development, our data also showed that the Cre activation in chondrocytes significantly compromised bone acquisition in femur. Our data calls for the consideration of the potential in vivo adverse effects caused by Cre expression in chondrocytes and reinforcement of the importance of including Cre-containing controls to facilitate accurate phenotype interpretation in transgenic research.

     

Capn8 promoter directs the expression of Cre recombinase in gastric pit cells of transgenic mice

Gastric pit cells are high‐turnover epithelial cells of the gastric mucosa. They secrete mucus to protect the gastric epithelium from acid and pepsin. To investigate the genetic mechanisms underlying the physiological functions of gastric pit cells, we generated a transgenic mouse line, namely, Capn8‐Cre, in which the expression of Cre recombinase was controlled by the promoter of the intracellular Ca²⁺‐regulated cysteine protease calpain‐8. To test the tissue distribution and excision activity of Cre recombinase, the Capn8‐Cre transgenic mice were bred with the ROSA26 reporter strain and a mouse strain that carries Smad4 conditional alleles (Smad4^Co/Co). Multiple‐tissue PCR and LacZ staining demonstrated that Capn8‐Cre transgenic mouse expressed Cre recombinase in the gastric pit cells. Cre recombinase activity was also detected in the liver and skin tissues. These data suggest that the Capn8‐Cre mouse line described here could be used to dissect gene function in gastric pit cells. genesis 47:674–679, 2009. © 2009 Wiley‐Liss, Inc.     

Oligodendroglial and pan‐neural crest expression of Cre recombinase directed by Sox10 enhancer

Utilizing a recently identified Sox10 distal enhancer directing Cre expression, we report S4F:Cre, a transgenic mouse line capable of inducing recombination in oligodendroglia and all examined neural crest derived tissues. Assayed using R26R:LacZ reporter mice expression was detected in neural crest derived tissues including the forming facial skeleton, dorsal root ganglia, sympathetic ganglia, enteric nervous system, aortae, and melanoblasts, consistent with Sox10 expression. LacZ reporter expression was also detected in non‐neural crest derived tissues including the oligodendrocytes and the ventral neural tube. This line provides appreciable differences in Cre expression pattern from other transgenic mouse lines that mark neural crest populations, including additional populations defined by the expression of other SoxE proteins. The S4F:Cre transgenic line will thus serve as a powerful tool for lineage tracing, gene function characterization, and genome manipulation in these populations. genesis 47:765–770, 2009. © 2009 Wiley‐Liss, Inc.     

Mast cell-specific Cre/loxP-mediated recombination in vivo

Mast cells are important effectors of type I allergy but also essential regulators of innate and adaptive immune responses. The aim of this study was to develop a Cre recombinase-expressing mouse line that allows mast cell-specific inactivation of genes in vivo. Following a BAC transgenic approach, Cre was expressed under the control of the mast cell protease (Mcpt) 5 promoter. Mcpt5-Cre transgenic mice were crossed to the ROSA26-EYFP Cre excision reporter strain. Efficient Cre-mediated recombination was observed in mast cells from the peritoneal cavity and the skin while only minimal reporter gene expression was detected outside the mast cell compartment. Our results show that the Mcpt5 promoter can drive Cre expression in a mast cell-specific fashion. We expect that our Mcpt5-Cre mice will be a useful tool for the investigation of mast cell biology. Julia Scholten and Karin Hartmann contributed equally to this work. Supported by grants from the German Research Counsil (Deutsche Forschungsgemeinschaft, RO 2133/2-2) to A.R. and K.H. and the Koeln Fortune Program/Faculty of Medicine, University of Cologne, to A.R. and K.H.. The authors have no conflict of interest     

骨形成蛋白4条件性RNA干扰小鼠的建立

为研究骨形成蛋白4(Bone morphogenetic protein 4, BMP4)在骨骼发育中的作用, 我们以含有LoxPneo的pBSK/U6载体为骨架, 构建小鼠BMP4条件性RNAi(conditional RNA interference), CRNA; 载体(BMP4CRNAi), 经KpnⅠ和AflⅢ双酶切获取针对bmp4并含neo基因的目的干扰片段, 纯化后的目的片段显微注射入0.5 d的FVB/NJ小鼠受精卵, 并植入同期发情的假孕母鼠中, 获取G0代转基因小鼠; 利用PCR对G0代转基因小鼠基因型进行鉴定, PCR阳性的小鼠与FVB/NJ小鼠交配, 最终获取稳定传代的BMP4CRNAi小鼠。稳定传代的BMP4^CRNAi小鼠与成骨和软骨前体细胞表达Cre的转基因小鼠(Col2a1-Cre)交配, 获取BMP4^Col2a1-CRNAi小鼠。分离BMP4Col2a1-CRNAi小鼠原代软骨细胞, 提取总RNA, 利用半定量RT-PCR检测RNA干扰效率。小鼠基因型鉴定结果表明：成功获得条件性RNAi转基因小鼠; BMP4干扰效率检测结果表明：在软骨细胞中BMP4的干扰效率为81％。以上结果表明, 我们成功制备了BMP4^CRNAi小鼠和BMP4^Col2a1-CRNAi小鼠, BMP4^CRNAi小鼠与不同Cre转基因小鼠交配, 可以研究BMP4在不同细胞、组织和器官的功能, BMP4^Col2a1-CRNAi小鼠的获得为研究BMP4在软骨发育中的作用提供了合适的动物模型。     

Development of Mice with Osteoblast-Specific Connexin43 Gene Deletion

Genetic deficiency of Cx43 in vivo causes skeletal developmental defects, osteoblast dysfunction and perinatal lethality. To determine the role of Cx43 in the adult skeleton, we developed two models of osteoblast-specific Cx43 gene deletion using Cre mediated replacement of a “floxed” Cx43 allele with a LacZ reporter gene. Cre recombinase expression in osteoblasts was driven by either the osteocalcin OG2 promoter or the 2.3 kb fragment of the Colα1(I) promoter. Homozygous Cx43^fl/flmice, in which the Cx43 coding region is flanked by two loxP sites, were crossed with Cre expressing mice in a heterozygous Cx43-null background [Cx43^±; Colα1(I)-Cre or Cx43^±; OG2-Cre]. Cx43 gene ablation was demonstrated in tissues by selective X-gal staining of cells lining the endosteal surface, and in cultured osteoblastic cells from calvaria using different approaches. Although no LacZ expression was observed in proliferating calvaria cells, before osteoblast differentiation begins, post-proliferative cells isolated from conditional knockout mice [Cx43^fl/?; Colα1(I)-Cre or Cx43^fl/?; OG2-Cre] developed strong LacZ expression as they differentiated, in parallel to a progressive disappearance of Cx43 mRNA and protein abundance relative to controls. Selective Cre mediated Cx43 gene inactivation in bone forming cells will be useful to determine the role of Cx43 in adult skeletal homeostasis and overcome the perinatal lethality of the conventional null model.     

Tamoxifen-inducible gene deletion reveals a distinct cell type associated with trabecular bone, and direct regulation of PTHrP expression and chondrocyte morphology by Ihh in growth region cartilage

Indian hedgehog (Ihh) controls multiple aspects of endochondral skeletal development by signaling to both chondrocytes and perichondrial cells. Previous efforts to delineate direct effects of Ihh on chondrocytes by Col2-Cre-mediated ablation of Smoothened (Smo, encoding a transmembrane protein indispensable for Ihh signaling) has been only partially successful, due to the inability to discriminate between chondrocytes and perichondrial cells. Here we report a transgenic line (Col2-Cre) expressing under the control of the Colalpha1(II) promoter an inert form of Cre that is activatable by exogenous tamoxifen (TM); TM administration at proper times during embryogenesis induced Cre activity in chondrocytes but not in the perichondrium. By using this mouse line, we deleted Smo within subsets of chondrocytes without affecting the perichondrium and found that Smo removal led to localized disruption of the expression of parathyroid hormone-related protein (PTHrP) and the morphology of chondrocytes. Unexpectedly, TM invariably induced Cre activity in a subset of cells associated with the trabecular bone surface of long bones. These cells, when genetically marked and cultured in vitro, were capable of producing bone nodules. Expression of the Col2-Cre transgene in these cells likely reflected the endogenous Colalpha1(II) promoter activity as similar cells were found to express the IIA isoform of Colalpha1(II) mRNA endogenously. In summary, the present study has not only provided evidence that Ihh signaling directly controls PTHrP expression and chondrocyte morphology in the growth region cartilage, but has also uncovered a distinct cell type associated with the trabecular bone that appears to possess osteogenic potential.     

Collagenl α1 promoter drives the expression of Cre recombinase in osteoblasts of transgenic mice

Osteoblasts participate in bone formation,bone mineralization,osteoclast differentiation and many pathological processes.To study the function of genes in osteoblasts using Cre-LoxP system,we generated a mouse line expressing the Cre recombinase under the control of the rat Collagenlal (Coilal) promoter(Coilatl-Cre).Two founders were identified by genomic PCR from 16 offsprings.and the integration efficiency is 12.5%.In order tO determine the tissue distribution and the activity of Cre rccombinase in the transgenic mice,the Collal-Cre transgenic mice were bred with the ROSA26 reporter strain and a mouse strain that carries Smad4 conditional alleles (Smad4co/co).Multiple tissue PCR of Collal-Cre;Smad4co/ mice revealed the restricted Cre activity in bone tissues containing osteoblasts and tendon.LacZ staining in the Coilal-Cre;ROSA26 double transgenic mice revealed that the Cre recombinase began to express in the osteoblasts of calvaria at E14.5.Cre activity was observed in the osteoblasts and osteocytes of P10 double transgenic mice.All these data indicated that the Collal-Cre transgenic mice could Serve as a valuabletool for osteoblast lineage analysis and conditional gene knockout in osteoblasts.     

A mouse model with tamoxifen‐inducible thyrocyte‐specific cre recombinase activity

We have created a mouse model expressing tamoxifen‐inducible Cre recombinase (CreER^T2) under the control of the thyroglobulin (Tg) gene promoter to be able to study the role of defined genetic modifications in the regulation of thyroid function. We chose the thyroglobulin promoter, as it is expressed specifically in the thyroid. In order to obtain reliable expression under the control of the Tg promoter, we used a P1 artificial chromosome (PAC) containing a large piece of the Tg promoter. A tamoxifen inducible CreER^T2 construct was selected to avoid the possible consequences of the gene deletion for the development of the thyroid gland, and to study the role of gene deletion in the adult thyroid. Transgenic lines (TgCreER^T2) carrying this construct were generated and analyzed by crossing the TgCreER^T2 mice with the ROSA26LacZ reporter strain. The activity and specificity of the Cre recombinase was tested by staining for β‐galactosidase activity and by immunohistochemistry using an anti‐Cre‐antibody. In the TgCreER^T2xROSA26LacZ reporter line, Cre‐mediated recombination occurred specifically in the thyrocytes only after tamoxifen administration, and no significant staining was observed in controls. The recombination efficiency was nearly complete, since almost all thyrocytes showed X‐gal staining. We could also induce the recombination in utero by giving tamoxifen to the pregnant female. In addition, mice expressing TgCreER^T2 had no obvious histological changes, hormonal alterations, or different response to growth stimuli as compared to controls. These results demonstrate that the TgCreER^T2 mouse line is a powerful tool to study temporally controlled deletion of floxed genes in the thyroid. genesis 52:333–340, 2014. © 2014 Wiley Periodicals, Inc.     

Cre expression in primary spermatocytes: A tool for genetic engineering of the germ line

Transgenic mice were generated expressing a testicular Cre recombinase driven by promoter sequences derived from the gene encoding Synaptonemal Complex Protein 1 (Sycp1), expressed at an early stage of the male meiosis (leptotene to zygotene). Recombination at target LoxP sites was examined during germinal differentiation in mice harboring Sycp1-Cre and a second transgene where LoxP sites flank either the βgeo coding region, the Pgk1 promoter, or a tk-neo cassette inserted into the Rxrα locus. The LoxP-flanked transgenes were stably maintained in the somatic tissues of the double transgenic animals, as well as in the progeny of the females. Mice born after mating the double-transgenic males with normal females showed extensive deletions of the LoxP-flanked sequences. When the males were hemizygous for the Sycp1-Cre transgene, the deletions were observed even in the fraction of the offspring which had not inherited the Cre gene, thus demonstrating that expression occurred in the male parent during spermatogenesis. The high efficiency of excision at the LoxP sites makes the Sycp1-Cre transgenic males suitable for evaluating the role of defined gene functions in the germinal differentiation process. Mol. Reprod. Dev. 51:274–280, 1998. © 1998 Wiley-Liss, Inc.