Drosophila Homeotic Gene Research Articles

Expression of Cdx-2 in the mouse embryo and placenta: possible role in patterning of the extra-embryonic membranes.

Three mouse homologues of the Drosophila homeotic gene Caudal (Cad) have been described. They are currently designated Cdx-1, Cdx-2, and Cdx-4. Cdx-1 and 2 are both strongly expressed in the adult mid- and hindgut, while Cdx-1 and 4 have been shown to be activated in the embryonic primitive streak. Using a polyclonal antibody against a fusion protein containing the amino terminal 109 amino acids of murine Cdx-2, we here describe the topographical location of the gene product from early cleavage to 12.5 days of embryonic development. Cdx-2 expression begins at 3.5 days and is confined to the trophectoderm, being absent from the inner cell mass. Subsequently, staining is located in the extra-embryonic ectoderm adjacent to the epiblast, but sparing the more superficially placed polar, as well as the mural trophoblastic cells. Continuing expression in the fetal membranes involves the chorion, the allantoic bud, and, at even later stages, the spongiotrophoblast. From 8.5 days, Cdx-2 begins to be expressed in embryonic tissues, principally (unlike Cdx-1) in the posterior part of the gut from its earliest formation, as well as in the tail bud and in the caudal part of the neural tube. Cdx-2 is, therefore, transcribed well before any other membrane of the Cad homologue group and of the related Hox-C group; its expression in the extra-embryonic membranes and in the hindgut reflects the phylogenetic relationship between the cloaca and the chorio-allantois and suggests the possibility that homeobox genes may be involved in placental development and/or patterning.

Transcriptional silencing of homeotic genes in drosophila

Homeotic genes are subject to transcriptional silencing, which prevents their expression in inappropriate body regions. Here, we shall focus on Drosophila, as little is known about this process in other organisms. Evidence is accumulating that silencing of Drosophila homeotic genes is conferred by two types of cis- regulatory sequences: initiation (SIL-1) and maintenance (SIL-M) elements. The former contain target sites for transient repressors with a highly localised distribution in the early embryo and the latter for constitutive repressors that are likely to be present in all cells. We discuss how SIL-1 elements may cooperate with SIL-M elements to promote formation of a silencing complex. We propose that this complex consists of specific non-histone proteins, the so-called Polycomb group proteins, and that it is anchored at SIL-M elements and at the promoter.

The role of brahma and related proteins in transcription and development

The differential transcription of Drosophila homeotic genes is maintained by the Polycomb and trithorax groups of regulatory proteins, many of which are thought to modulate chromatin structure. During the past year, studies of a trithorax group member, brahma, and related yeast and human proteins have suggested that they are components of huge complexes that assist DNA-binding regulatory proteins to overcome the repressive effects of chromatin on transcription.

Genetic analysis of embryonic cis-acting regulatory elements of the Drosophila homeotic gene sex combs reduced.

The homeotic gene Sex combs reduced (Scr) of Drosophila melanogaster is expressed in the labial and prothoracic segments of the ectoderm, in parasegments two and three of the CNS, and in the visceral mesoderm of the anterior and posterior midgut. The mutationally defined function of Scr is to specify the identity of the labial and prothoracic segments and to control the development of the gastric caeca. The Scr locus occupies a chromosomal region of approximately 80 kb within the Antennapedia complex (ANT-C). To understand how Scr's spatiotemporal expression pattern is generated in the embryo, we have mapped its transcriptional regulatory elements using three approaches. First, we examined the expression pattern of Scr in embryos containing chromosomal rearrangements that remove potential Scr regulatory elements. Second, we made and analyzed a set of Scr minigene transformants. Third, we analyzed a set of Scr-lacZ enhancer tester constructs. Using more sensitive anti-SCR antisera, we discovered that Scr is expressed in tissues that were not previously thought to accumulate SCR: a stripe of ectodermal cells in the parasegment 2 region of stage 5 embryos, the embryonic salivary glands, and the dorsal ridge. Four DNA fragments that had previously been shown in an analysis of Scr-lacZ reporter constructs to contain putative Scr enhancer elements were found to have functional enhancers; similarly, another Scr fragment was found to contain a functional repressor. Our results suggest that regulation of Scr in the labial segment and the CNS requires the apparently synergistic action of multiple, widely spaced enhancer elements. Regulation in the prothorax also appears to be controlled by multiple enhancers:one complete pattern element and one subpattern element. In contrast, Scr regulation in the visceral mesoderm is controlled by an enhancer(s) located in only one DNA fragment.

Mutations in Paralogous Hox Genes Result in Overlapping Homeotic Transformations of the Axial Skeleton: Evidence for Unique and Redundant Function

Hoxd-4 (previously known as Hox-4.2 and - 5.1) is a mouse homeobox-containing gene homologous to the Drosophila homeotic gene Deformed. During embryogenesis, Hoxd-4 is expressed in the presumptive hindbrain and spinal cord, prevertebrae, and other tissues. In the adult, Hoxd-4 transcripts are expressed predominantly in the testis and kidney, and to a lesser extent in intestine and heart. To understand the role of Hoxd-4 during mouse embryogenesis, we generated Hoxd-4 mutant mice. Mice heterozygous or homozygous for the Hoxd-4 mutation exhibit homeotic transformations of the second cervical vertebrae (C2) to the first cervical vertebrae (C1) and malformations of the neural arches of C1 to C3 and of the basioccipital bone. The phenotype was incompletely penetrant and showed variable expressivity on both an F2 hybrid and 129 inbred genetic background. The mutant phenotype was detected in the cartilaginous skeleton of 14.5-day (E14.5) mutant embryos but no apparent differences were detected in the somites of E9.5 mutant embryos, suggesting that the abnormalities develop after E9.5 perhaps during or after resegmentation of the somites to form the prevertebrae. These results suggest that Hoxd-4 plays a role in conferring position information along the anteroposterior axis in the skeleton. The phenotypic similarities and differences between Hoxd-4 and previously reported Hoxa-4 and Hoxb-4 mutant mice suggest that Hox gene paralogs have both redundant and unique functions.

Identification of Polycomb and trithorax group responsive elements in the regulatory region of the Drosophila homeotic gene Sex combs reduced.

The Drosophila homeotic gene Sex combs reduced (Scr) is necessary for the establishment and maintenance of the morphological identity of the labial and prothoracic segments. In the early embryo, its expression pattern is established through the activity of several gap and segmentation gene products, as well as other transcription factors. Once established, the Polycomb group (Pc-G) and trithorax group (trx-G) gene products maintain the spatial pattern of Scr expression for the remainder of development. We report the identification of DNA fragments in the Scr regulatory region that may be important for its regulation by Polycomb and trithorax group gene products. When DNA fragments containing these regulatory sequences are subcloned into P-element vectors containing a white minigene, transformants containing these constructs exhibit mosaic patterns of pigmentation in the adult eye, indicating that white minigene expression is repressed in a clonally heritable manner. The size of pigmented and nonpigmented clones in the adult eye suggests that the event determining whether a cell in the eye anlagen will express white occurs at least as early as the first larval instar. The amount of white minigene repression is reduced in some Polycomb group mutants, whereas repression is enhanced in flies mutant for a subset of trithorax group loci. The repressor activity of one fragment, normally located in Scr Intron 2, is increased when it is able to homologously pair, a property consistent with genetic data suggesting that Scr exhibits transvection. Another Scr regulatory fragment, normally located 40 kb upstream of the Scr promoter, silences ectopic expression of an Scr-lacZ fusion gene in the embryo and does so in a Polycomb-dependent manner. We propose that the regulatory sequences located within these DNA fragments may normally mediate the regulation of Scr by proteins encoded by members of the Polycomb and trithorax group loci.

Levels of homeotic protein function can determine developmental identity: evidence from low-level expression of the Drosophila homeotic gene proboscipedia under Hsp70 control.

The autonomous selector capacity of the homeotic proboscipedia (pb) gnee of the Drosophila Antennapedia Complex was tested. We introduced into the germline a P element containing a transcriptional fusion of a mini-gene for pb (normally required for formation of the labial and maxillary palps of the mouthparts) and the Hsp70 promoter. Uninduced expression of this Hsp70:pb element (HSPB) directs a novel, fully penetrant dominant transformation of antennae toward maxillary palps. Gene dosage experiments varying the number of HSPB elements indicate that the extent of the dominant transformation depends on the level of PB protein. At the same time, expression from the transgene also rescues recessive pb mutations. Finally, HSPB function may override the dominant antennal transformations caused by Antennapedia (Antp) mutations in a dose-sensitive manner, directing a switch of the antennal disc-derived appendage from ectopic leg to ectopic maxillary palp. This switch correlated with strikingly reduced ANTP protein accumulation when PB concentrations exceeded a genetically defined threshold level. These observations support a crucial role for quantitative aspects of pb function in determining segmental identity, including cross-regulatory events involved in this determination.

Characterization of the cis-regulatory region of the Drosophila homeotic gene Sex combs reduced.

The Drosophila homeotic gene Sex combs reduced (Scr) controls the segmental identity of the labial and prothoracic segments in the embryo and adult. It encodes a sequence-specific transcription factor that controls, in concert with other gene products, differentiative pathways of tissues in which Scr is expressed. During embryogenesis, Scr accumulation is observed in a discrete spatiotemporal pattern that includes the labial and prothoracic ectoderm, the subesophageal ganglion of the ventral nerve cord and the visceral mesoderm of the anterior and posterior midgut. Previous analyses have demonstrated that breakpoint mutations located in a 75-kb interval, including the Scr transcription unit and 50 kb of upstream DNA, cause Scr misexpression during development, presumably because these mutations remove Scr cis-regulatory sequences from the proximity of the Scr promoter. To gain a better understanding of the regulatory interactions necessary for the control of Scr transcription during embryogenesis, we have begun a molecular analysis of the Scr regulatory interval. DNA fragments from this 75-kb region were subcloned into P-element vectors containing either an Scr-lacZ or hsp70-lacZ fusion gene, and patterns of reporter gene expression were assayed in transgenic embryos. Several fragments appear to contain Scr regulatory sequences, as they direct reporter gene expression in patterns similar to those normally observed for Scr, whereas other DNA fragments direct Scr reporter gene expression in developmentally interesting but non-Scr-like patterns during embryogenesis. Scr expression in some tissues appears to be controlled by multiple regulatory elements that are separated, in some cases, by more than 20 kb of intervening DNA. Interestingly, regulatory sequences that direct reporter gene expression in an Scr-like pattern in the anterior and posterior midgut are imbedded in the regulatory region of the segmentation gene fushi tarazu (ftz), which is normally located between 10 and 20 kb 5' of the Scr transcription start site. This analysis provides an entry point for the study of how Scr transcription is regulated at the molecular level.

Control of the Su(var) modulo gene by homeotic genes in drosophila

Biology of the CellVolume 84, Issue 1-2 p. 111-111 Control of the Su(var) modulo gene by homeotic genes in drosophila Laurent Perrin, Laurent Perrin Laboratoire de Génétique et Physiologie du Développement, C.N.R.S., Luminy, case 907, 13288 Marseille Cedex 09, France.Search for more papers by this authorYacine Graba, Yacine Graba Laboratoire de Génétique et Physiologie du Développement, C.N.R.S., Luminy, case 907, 13288 Marseille Cedex 09, France.Search for more papers by this authorJacques Pradel, Jacques Pradel Laboratoire de Génétique et Physiologie du Développement, C.N.R.S., Luminy, case 907, 13288 Marseille Cedex 09, France.Search for more papers by this author Laurent Perrin, Laurent Perrin Laboratoire de Génétique et Physiologie du Développement, C.N.R.S., Luminy, case 907, 13288 Marseille Cedex 09, France.Search for more papers by this authorYacine Graba, Yacine Graba Laboratoire de Génétique et Physiologie du Développement, C.N.R.S., Luminy, case 907, 13288 Marseille Cedex 09, France.Search for more papers by this authorJacques Pradel, Jacques Pradel Laboratoire de Génétique et Physiologie du Développement, C.N.R.S., Luminy, case 907, 13288 Marseille Cedex 09, France.Search for more papers by this author First published: 1995 https://doi.org/10.1016/0248-4900(96)81431-6AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume84, Issue1-21995Pages 111-111 RelatedInformation

Deformed expression in the Drosophila central nervous system is controlled by an autoactivated intronic enhancer.

Deformed (Dfd) is a Drosophila homeotic selector gene required for normal development of maxillary segment morphology in the larval and adult head. Consistent with this function, Dfd transcripts are restricted to epidermal, mesodermal and neural cells in the embryonic mandibular and maxillary primordia. Previous studies have identified a far upstream element in Dfd sequences which functions as an epidermal-specific autoregulatory enhancer. In a search through 35 kb of Dfd sequences for additional transcriptional control elements, we have identified a 3.2 kb DNA fragment containing an enhancer that mimics the expression of Dfd in the subesophageal ganglion of the embryonic central nervous system. This Neural autoregulatory enhancer (NAE) maps in the large Dfd intron just upstream of the homeobox exon and requires Dfd protein function for its full activity. A 608 bp NAE subfragment retains regulatory function that is principally localized in the subesophageal ganglion. This small region of the Drosophila melanogaster genome contains numerous blocks of sequence conservation with a comparable region from the Dfd locus of D.hydei. A pair of conserved blocks of NAE sequence match a Dfd protein binding site in the epidermal autoregulatory element, while another conserved sequence motif is repeated multiple times within the 608 bp subelement.

Imaginal disc silencers from Ultrabithorax: evidence for Polycomb response elements

Silencers from the Drosophila homeotic gene Ultrabithorax ( Ubx) require hunchback ( hb) and Polycomb ( Pc) to suppress the of embryonic enhancers outside the Ubx domain. Embryonic silencing is initiated by hb protein which binds to the silencers to repress Ubx, thereby defining the Ubx domain. Here, we study silencing during subsequent development by examining expression patterns in imaginal discs conferred by individual Ubx fragments and pair-wise combinations thereof. We find that fragments which mediate silencing in anterior regions of imaginal discs contain embryonic silencers and hb target sites. One exception to this is a fragment called BXD which is not under hb control itself, but whose silencing activity depends on combination with fragments containing hb protein binding sites. Since silencing by BXD also requires Pc function, this suggests that BXD contains target sites for Pc or for Pc-like proteins. We propose that stable silencing of Ubx is achieved through cooperation between hb and Pc target sites.

Engrailed-mediated repression of Ultrabithorax is necessary for the parasegment 6 identity in Drosophila

The homeotic genes of Drosophila are expressed in overlapping domains along the anterior-to-posterior axis and specify the distinct morphological patterns of each parasegment. Within single parasegments, the levels of homeotic gene expression are often modulated, in part because of cross-regulation by other homeotic gene products. However, the functional significance of different levels of homeotic gene expression is unclear. Here modulations in Ultrabithorax (Ubx) expression within parasegment 6 are examined. Specifically, Ubx is shown to be down-regulated in the posterior compartment of this parasegment by engrailed (en). The significance of Ubx repression by en was demonstrated by characterizing the expression of the Ubx target gene, Distal-less (Dll). In the posterior compartment of parasegment 6, Dll is normally expressed in a small cluster of cells. If Ubx is expressed uniformly via a heat-shock promoter, Dll is inappropriately repressed in these posterior compartment cells. In the anterior compartment of parasegment 6, Dll is normally repressed by high levels of Ubx. However, if en is expressed uniformly via a heat-shock promoter, Ubx is repressed and Dll is derepressed. Because Dll is required for the development of larval sensory structures, these results demonstrate that en-mediated repression of Ubx in the posterior compartment is necessary for the morphology of parasegment 6. Thus, different levels of homeotic gene expression can be important for their segmental patterning functions.

A Polycomb response element in the Ubx gene that determines an epigenetically inherited state of repression.

Segmentation genes provide the signals for the activation and regulation of homeotic genes in Drosophila but cannot maintain the resulting pattern of expression because their activity ceases halfway through embryogenesis. Maintenance of the pattern is due to the Polycomb group of genes (Pc-G) and the trithorax group of genes (trx-G), responsible for the persistence of the active or repressed state of homeotic genes. We have identified a regulatory element in the Ubx gene that responds to Pc-G and trx-G genes. Transposons carrying this element create new binding sites for Pc-G products in the polytene chromosomes. This Pc-G maintenance element (PRE), establishes a repressive complex that keeps enhancers repressed in cells in which they were originally repressed and maintains this state through many cell divisions. The trx-G products stimulate the expression of enhancers in cells in which they were originally active. This mechanism is responsible for the correct regulation of imaginal disc enhancers, which lack themselves antero-posterior positional information. The PRE also causes severe variegation of the mini-white gene present in the transposon, a phenomenon very similar to heterochromatic position-effect variegation. The significance of this mechanism for homeotic gene regulation is discussed.

Specification of a single cell type by a Drosophila homeotic gene

Homeotic genes function in blocks of cells along the body axis to specify diverse developmental pathways. Among the Drosophila homeotic genes, only one is known to be expressed in the endoderm. We show here that expression of this gene, labial ( lab), coincides with copper cells, highly specialized cells of the larval midgut. lab is strictly required for copper cell formation, a requirement that starts in the embryo and extends through larval stages. This implies a function of lab not only in the determination and differentiation of copper cells, but also in the maintenance of their differentiated state. Ectopic expression of lab during embryogenesis reprograms other midgut cells within and outside the lab domain to become copper cell-like. Thus, lab functions in the larval midgut to specify a single cell type.

Homeotic transformations and limb defects in Hox A11 mutant mice.

Hox A11 is one of the expanded set of vertebrate homeo box (Hox) genes with similarities to the Drosophila homeotic gene, Abdominal-B (Abd-B). These Abd-B-type Hox genes have been shown to be expressed in the most caudal regions of the developing vertebrate embryo and in overlapping domains within the developing limbs, suggesting that these genes play important roles in pattern formation in both appendicular and axial regions of the body. In this report whole-mount in situ hybridization in mouse embryos gave a precise description of Hox A11 gene expression in the developing limbs and in the axial domain of the developing body. In addition, we generated a targeted mutation in Hox A11 and characterized the resulting phenotype to begin to dissect developmental functions of the Abd-B subfamily of Hox genes. Hox A11 mutant mice exhibited double homeotic transformations, with the thirteenth thoracic segment posteriorized to form an additional first lumbar vertebra and with the sacral region anteriorized, generating yet another lumbar segment. Furthermore, skeletal malformations were observed in both forelimbs and hindlimbs. In mutant forelimbs, the ulna and radius were misshapen, the pisiform and triangular carpal bones were fused, and abnormal sesamoid bone development occurred. In mutant hindlimbs the tibia and fibula were joined incorrectly and malformed at their distal ends. Also, an enlarged sesamoid developed ventral to the tibiale bone. Both heterozygous and homozygous mice displayed mutant phenotypes adding an additional level of complexity to the Hox code hypothesis.

Cooperative binding of an Ultrabithorax homeodomain protein to nearby and distant DNA sites.

Cooperativity in binding of regulatory proteins to multiple DNA sites can heighten the sensitivity and specificity of the transcriptional response. We report here the cooperative DNA-binding properties of a developmentally active regulatory protein encoded by the Drosophila homeotic gene Ultrabithorax (Ubx). We show that naturally occurring binding sites for the Ubx-encoded protein contain clusters of multiple individual binding site sequences. Such sites can form complexes containing a dozen or more Ubx-encoded protein molecules, with simultaneous cooperative interactions between adjacent and distant DNA sites. The distant mode of interaction involves a DNA looping mechanism; both modes appear to enhance transcriptional activation in a simple yeast assay system. We found that cooperative binding is dependent on sequences outside the homeodomain, and we have identified regions predicted to form coiled coils carboxy terminal to the homeodomains of the Ubx-encoded protein and several other homeotic proteins. On the basis of our findings, we propose a multisite integrative model of homeotic protein action in which functional regulatory elements can be built from a few high-affinity sites, from many lower-affinity sites, or from sites of some intermediate number and affinity. An important corollary of this model is that even small differences in binding of homeotic proteins to individual sites could be summed to yield large overall differences in binding to multiple sites. This model is consistent with reports that homeodomain protein targets contain multiple individual binding site sequences distributed throughout sizable DNA regions. Also consistent is a recent report that sequences carboxy terminal to the Ubx homeodomain can contribute to segmental specificity.

Cooperative Binding of an Ultrabithorax Homeodomain Protein to Nearby and Distant DNA Sites

Cooperativity in binding of regulatory proteins to multiple DNA sites can heighten the sensitivity and specificity of the transcriptional response. We report here the cooperative DNA-binding properties of a developmentally active regulatory protein encoded by the Drosophila homeotic gene Ultrabithorax (Ubx). We show that naturally occurring binding sites for the Ubx-encoded protein contain clusters of multiple individual binding site sequences. Such sites can form complexes containing a dozen or more Ubx-encoded protein molecules, with simultaneous cooperative interactions between adjacent and distant DNA sites. The distant mode of interaction involves a DNA looping mechanism; both modes appear to enhance transcriptional activation in a simple yeast assay system. We found that cooperative binding is dependent on sequences outside the homeodomain, and we have identified regions predicted to form coiled coils carboxy terminal to the homeodomains of the Ubx-encoded protein and several other homeotic proteins. On the basis of our findings, we propose a multisite integrative model of homeotic protein action in which functional regulatory elements can be built from a few high-affinity sites, from many lower-affinity sites, or from sites of some intermediate number and affinity. An important corollary of this model is that even small differences in binding of homeotic proteins to individual sites could be summed to yield large overall differences in binding to multiple sites. This model is consistent with reports that homeodomain protein targets contain multiple individual binding site sequences distributed throughout sizable DNA regions. Also consistent is a recent report that sequences carboxy terminal to the Ubx homeodomain can contribute to segmental specificity.

Abdominal-B protein isoforms exhibit distinct cuticular transformations and regulatory activities when ectopically expressed in Drosophila embryos

The Drosophila homeotic gene Abdominal-B includes two genetically distinct elements, a morphogenetic (m) activity and a regulatory (r) activity. The proteins responsible for these activities were ectopically expressed in fly embryos. The larval cuticular transformations which result are consistent with the genetically defined role of each protein during normal embryogenesis. Both ABD-B proteins activat ectopic expression of transcripts encoding the m protein, but the levels of Antennapedia, Ultrabithorax and abdominal-A transcripts are differentially repressed. A structural and functional comparison of the ABD-B proteins and a chimeric DFD/ABD-B protein reaffirms that target specificity is largely determined by the homeodomain region and suggests protein domains outside of the homeodomain influence the activation or repression of target gene expression.

The 3' regulatory region of the Abdominal-B gene: genetic analysis supports a model of reiterated and interchangeable regulatory elements.

The Abdominal-B (Abd-B) gene is one of three genes in the bithorax complex, a cluster of homeotic genes in Drosophila. During embryogenesis Abd-B is expressed in a complex pattern, producing four different transcript classes, each of which exhibits a unique spatial pattern of expression. Proper regulation of the class A transcripts is required for appropriate development of the fifth through eighth abdominal segments and is mediated, in part, by a 60-kb regulatory region located 3' of the gene. We have isolated a new mutation, designated Abd-BCorset, which is caused by a deletion that leaves 15 kb of the 3' regulatory sequences immediately adjacent to the gene, but removes 45 kb of the more distant 3' regulatory elements. This mutation produces an unexpected homeotic segmental transformation of the fourth through seventh abdominal segments, and has been analyzed by genetic and molecular techniques. In situ hybridization to Abd-BCorset embryos shows a uniform and moderate level of the Abd-B class A transcript in the posterior abdomen, rather than the normal graded pattern of expression. Our analysis of the Abd-BCorset mutation has prompted a model of the 3' regulatory region of Abd-B based on reiterated cell type-specific elements controlled by adjacent position-sensitive activating elements. The gradient of Abd-B expression normally observed in the posterior abdomen appears to be achieved by varying the number of reiterated elements that are active in each segment.

The Etl-1 gene encodes a nuclear protein differentially expressed during early mouse development.

Recently, we isolated a novel mouse gene, Etl-1 (Enhancer-trap-locus-1), whose deduced amino acid sequence shows in its C-terminal portion striking homology to the brahma protein (BRM), a transcriptional regulator of homeotic genes in Drosophila, and to SNF2/SWI2, a transcriptional regulator of various genes in Saccharomyces cerevisiae. Here we report the generation of antibodies against the Etl-1 gene product (ETL-1) and describe the subcellular localization as well as the expression and distribution of the ETL-1 protein during mouse pre- and early post-implantation development. ETL-1 is a nuclear protein and is expressed in a biphasic manner during early embryogenesis. Moderate levels of ETL-1 were detected in unfertilized and fertilized eggs but in the latter the protein was not concentrated in the pronuclei and seemed evenly distributed throughout the cytoplasm. In two-cell embryos nuclear ETL-1 protein accumulated transiently and levels decreased during subsequent cleavage development. After the morula stage, ETL-1 levels increased again; in blastocysts high levels of ETL-1 were present in inner cell mass cells whereas trophectoderm cells contained little or no ETL-1. During subsequent development essentially all cell types except parietal endoderm and trophoblast cells contained high levels of ETL-1. Our results imply that nuclear ETL-1 is dispensable for the progression to the two cell stage, and suggest that during cleavage ETL-1 might be needed at the onset of embryonic transcription. In blastocysts ETL-1 function might be specifically required in cells of the inner cell mass and later in most cells of the embryo proper and extraembryonic ectoderm lineage.