FZD10 regulates cell proliferation and mediates Wnt1 induced neurogenesis in the developing spinal cord.

Wnt/FZD signalling activity is required for spinal cord development, including the dorsal-ventral patterning of the neural tube, where it affects proliferation and specification of neurons. Wnt ligands initiate canonical, ß -catenin-dependent, signaling by binding to Frizzled receptors. However, in many developmental contexts the cognate FZD receptor for a particular Wnt ligand remains to be identified. Here, we characterized FZD10 expression in the dorsal neural tube where it overlaps with both Wnt1 and Wnt3a, as well as markers of dorsal progenitors and interneurons. We show FZD10 expression is sensitive to Wnt1, but not Wnt3a expression, and FZD10 plays a role in neural tube patterning. Knockdown approaches show that Wnt1 induced ventral expansion of dorsal neural markes, Pax6 and Pax7, requires FZD10. In contrast, Wnt3a induced dorsalization of the neural tube is not affected by FZD10 knockdown. Gain of function experiments show that FZD10 is not sufficient on its own to mediate Wnt1 activity in vivo. Indeed excess FZD10 inhibits the dorsalizing activity of Wnt1. However, addition of the Lrp6 co-receptor dramatically enhances the Wnt1/FZD10 mediated activation of dorsal markers. This suggests that the mechanism by which Wnt1 regulates proliferation and patterning in the neural tube requires both FZD10 and Lrp6.

Evaluation of a receptor gene responsible for maternal blood IgY transfer into egg yolks using bursectomized IgY-depleted chickens.

In avian species, maternal immunoglobulin Y (IgY) is transferred from the blood to the yolks of maturing oocytes; however, the mechanism underlying this transfer is unknown. To gain insight into the mechanisms of maternal IgY transfer into egg yolks, IgY-depleted chickens were generated by removing the bursa of Fabricius (bursectomy) during egg incubation, and their egg production and IgY transport ability into egg yolks were determined. After hatching, blood IgY concentrations of the bursectomized chickens decreased gradually until sexual maturity, whereas those of IgA remained low from an early stage of growth (from at least 2 wk of age). Chickens identified as depleted in IgY through screening of blood IgY and IgA concentrations were raised to sexual maturity. At 20 wk of age, both blood and egg yolk IgY concentrations in the IgY-depleted group were 600-fold lower than those of the control group, whereas egg production did not differ between the groups. Intravenously injected, digoxigenin-labeled IgY uptake into the egg yolk was approximately 2-fold higher in the IgY-depleted chickens than in the controls, suggesting that IgY depletion may enhance IgY uptake in maturing oocytes. DNA microarray analysis of the germinal disc, including the oocyte nucleus, revealed that the expression levels of 73 genes were upregulated more than 1.5-fold in the IgY-depleted group, although we could not identify a convincing candidate gene for the IgY receptor. In conclusion, we successfully raised IgY-depleted chickens presenting a marked reduction in egg yolk IgY. The enhanced uptake of injected IgY into the egg yolks of the IgY-depleted chickens supports the existence of a selective IgY transport mechanism in maturing oocytes and ovarian follicles in avian species.

The profiling of amino acids in crop milk and plasma and mRNA abundance of amino acid transporters and enzymes related to amino acid synthesis in the crop tissue of male and female pigeons during incubation and chick-rearing periods.

The present study was carried out to investigate the changes in amino acid (AA) contents of crop milk and plasma and mRNA abundance of AA transporters and AA synthesis-related enzymes in the crop tissue of male and female pigeons during incubation and chick-rearing periods. Forty-two pairs of adult White King pigeons with 2 fertile eggs per pair were randomly divided into 7 groups by different breeding stages. The AA content of crop milk decreased from day 1 (R1) to day 25 (R25) of chick rearing (P < 0.05). In both male and female adult pigeons, the contents of Thr, Leu, Val, His, Asp, and Pro in plasma increased to maximum levels on R25. Parental sex effect and interaction between stage and sex were observed in the AA contents of pigeon plasma (P < 0.05). For AA transporters, the mRNA abundances of SNAT2, ASCT1, LAT1, and y+LAT2 in the male crops reached the highest value on day 17 of incubation (I17), and the peak mRNA levels of PAT-1, xCT, b0,+AT, and CAT1 were found on R7 (P < 0.05). In females, the abundances of ASCT1, B0AT1, asc-1, and CAT1 mRNA peaked on R1, whereas the maximum levels of LAT1, PAT-1, b0,+AT, and y+LAT2 were observed on R7. For enzymes involved in AA synthesis, the highest gene expressions of glutamate dehydrogenase 1, acetolactate synthase in both parent pigeons, and L-threonine 3-dehydrogenase in female pigeon crops were attained on I17. The expressions of ornithine-δ-aminotransferase, glutamic-oxal(o)acetic transaminase 1, glutamic-oxal(o)acetic transaminase 2, asparagine synthetase, serine hydroxymethyltransferase 2, and glutamic-pyruvic transaminase 2 in both sexes and argininosuccinate lyase and L-threonine 3-dehydrogenase in males were the highest on R1. In conclusion, AA used for pigeon crop milk formation may originate from plasma and intracellular synthesis. The genes involved in AA transport and synthesis varied significantly with sexual effects, indicating that other factors should be considered in future explorations of the mechanism of protein formation in crop milk.

IRF7 Is Involved in Both STING and MAVS Mediating IFN-ß Signaling in IRF3-Lacking Chickens.

IFN regulatory factor (IRF) 3 has been identified as the most critical regulator of both RNA and DNA virus-induced IFN production in mammals. However, ambiguity exists in research on chicken IRFs; in particular IRF3 seems to be missing in chickens, making IFN regulation in chickens unclear. In this study, we comprehensively investigated the potential IFN-related IRFs in chickens and showed that IRF7 is the most critical IFN-ß regulator in chickens. With a chicken IRF7 (chIRF7) knockout DF-1 cell line, we conducted a series of experiments to demonstrate that chIRF7 is involved in both chicken STING (chSTING)- and chicken MAVS (chMAVS)-mediated IFN-ß regulation in response to DNA and RNA viral infections, respectively. We further examined the mechanisms of chIRF7 activation by chSTING. We found that chicken TBK1 (chTBK1) is indispensable for chIRF7 activation by chSTING as well as that chSTING interacts with both chIRF7 and chTBK1 to function as a scaffold in chIRF7 activation by chTBK1. More interestingly, we discovered that chSTING mediates the activation of chIRF7 through a conserved SLQxSyS motif. In short, we confirmed that although IRF3 is missing in chickens, they employ IRF7 to reconstitute corresponding IFN signaling to respond to both DNA and RNA viral infections. Additionally, we uncovered a mechanism of chIRF7 activation by chSTING. The results will enrich and deepen our understanding of the regulatory mechanisms of the chicken IFN system.

PDIP38/PolDIP2 controls the DNA damage tolerance pathways by increasing the relative usage of translesion DNA synthesis over template switching.

Replicative DNA polymerases are frequently stalled at damaged template strands. Stalled replication forks are restored by the DNA damage tolerance (DDT) pathways, error-prone translesion DNA synthesis (TLS) to cope with excessive DNA damage, and error-free template switching (TS) by homologous DNA recombination. PDIP38 (Pol-delta interacting protein of 38 kDa), also called Pol δ-interacting protein 2 (PolDIP2), physically associates with TLS DNA polymerases, polymerase η (Polη), Polλ, and PrimPol, and activates them in vitro. It remains unclear whether PDIP38 promotes TLS in vivo, since no method allows for measuring individual TLS events in mammalian cells. We disrupted the PDIP38 gene, generating PDIP38-/- cells from the chicken DT40 and human TK6 B cell lines. These PDIP38-/- cells did not show a significant sensitivity to either UV or H2O2, a phenotype not seen in any TLS-polymerase-deficient DT40 or TK6 mutants. DT40 provides a unique opportunity of examining individual TLS and TS events by the nucleotide sequence analysis of the immunoglobulin variable (Ig V) gene as the cells continuously diversify Ig V by TLS (non-templated Ig V hypermutation) and TS (Ig gene conversion) during in vitro culture. PDIP38-/- cells showed a shift in Ig V diversification from TLS to TS. We measured the relative usage of TLS and TS in TK6 cells at a chemically synthesized UV damage (CPD) integrated into genomic DNA. The loss of PDIP38 also caused an increase in the relative usage of TS. The number of UV-induced sister chromatid exchanges, TS events associated with crossover, was increased a few times in PDIP38-/- human and chicken cells. Collectively, the loss of PDIP38 consistently causes a shift in DDT from TLS to TS without enhancing cellular sensitivity to DNA damage. We propose that PDIP38 controls the relative usage of TLS and TS increasing usage of TLS without changing the overall capability of DDT.

Sox2 is required for olfactory pit formation and olfactory neurogenesis through BMP restriction and Hes5 upregulation.

The transcription factor Sox2 is necessary to maintain pluripotency of embryonic stem cells, and to regulate neural development. Neurogenesis in the vertebrate olfactory epithelium persists from embryonic stages through adulthood. The role Sox2 plays for the development of the olfactory epithelium and neurogenesis within has, however, not been determined. Here, by analysing Sox2 conditional knockout mouse embryos and chick embryos deprived of Sox2 in the olfactory epithelium using CRISPR-Cas9, we show that Sox2 activity is crucial for the induction of the neural progenitor gene Hes5 and for subsequent differentiation of the neuronal lineage. Our results also suggest that Sox2 activity promotes the neurogenic domain in the nasal epithelium by restricting Bmp4 expression. The Sox2-deficient olfactory epithelium displays diminished cell cycle progression and proliferation, a dramatic increase in apoptosis and finally olfactory pit atrophy. Moreover, chromatin immunoprecipitation data show that Sox2 directly binds to the Hes5 promoter in both the PNS and CNS. Taken together, our results indicate that Sox2 is essential to establish, maintain and expand the neuronal progenitor pool by suppressing Bmp4 and upregulating Hes5 expression.

NFIA and GATA3 are crucial regulators of embryonic articular cartilage differentiation.

During appendicular skeletal development, the bi-potential cartilage anlagen gives rise to transient cartilage, which is eventually replaced by bone, and to articular cartilage that caps the ends of individual skeletal elements. While the molecular mechanism that regulates transient cartilage differentiation is relatively well understood, the mechanism of articular cartilage differentiation has only begun to be unraveled. Furthermore, the molecules that coordinate the articular and transient cartilage differentiation processes are poorly understood. Here, we have characterized in chick the regulatory roles of two transcription factors, NFIA and GATA3, in articular cartilage differentiation, maintenance and the coordinated differentiation of articular and transient cartilage. Both NFIA and GATA3 block hypertrophic differentiation. Our results suggest that NFIA is not sufficient but necessary for articular cartilage differentiation. Ectopic activation of GATA3 promotes articular cartilage differentiation, whereas inhibition of GATA3 activity promotes transient cartilage differentiation at the expense of articular cartilage. We propose a novel transcriptional circuitry involved in embryonic articular cartilage differentiation, maintenance and its crosstalk with the transient cartilage differentiation program.

Species difference in ANP32A underlies influenza A virus polymerase host restriction.

Influenza pandemics occur unpredictably when zoonotic influenza viruses with novel antigenicity acquire the ability to transmit amongst humans. Host range breaches are limited by incompatibilities between avian virus components and the human host. Barriers include receptor preference, virion stability and poor activity of the avian virus RNA-dependent RNA polymerase in human cells. Mutants of the heterotrimeric viral polymerase components, particularly PB2 protein, are selected during mammalian adaptation, but their mode of action is unknown. We show that a species-specific difference in host protein ANP32A accounts for the suboptimal function of avian virus polymerase in mammalian cells. Avian ANP32A possesses an additional 33 amino acids between the leucine-rich repeats and carboxy-terminal low-complexity acidic region domains. In mammalian cells, avian ANP32A rescued the suboptimal function of avian virus polymerase to levels similar to mammalian-adapted polymerase. Deletion of the avian-specific sequence from chicken ANP32A abrogated this activity, whereas its insertion into human ANP32A, or closely related ANP32B, supported avian virus polymerase function. Substitutions, such as PB2(E627K), were rapidly selected upon infection of humans with avian H5N1 or H7N9 influenza viruses, adapting the viral polymerase for the shorter mammalian ANP32A. Thus ANP32A represents an essential host partner co-opted to support influenza virus replication and is a candidate host target for novel antivirals.

Life without TTP: apparent absence of an important anti-inflammatory protein in birds.

Both innate and adaptive immunity in birds are different from their mammalian counterparts. Understanding bird immunity is important because of the enormous potential impact of avian infectious diseases, both in their role as food animals and as potential carriers of zoonotic diseases in man. The anti-inflammatory protein tristetraprolin (TTP) is an important component of the mammalian innate immune response, in that it binds to and destabilizes key cytokine mRNAs. TTP knockout mice exhibit a severe systemic inflammatory syndrome, and they are abnormally sensitive to innate immune stimuli such as LPS. TTP orthologs have been found in most vertebrates studied, including frogs. Here, we attempted to identify TTP orthologs in chicken and other birds, using database searches and deep mRNA sequencing. Although sequences encoding the two other widely expressed TTP family members, ZFP36L1 and ZFP36L2, were identified, we did not find sequences corresponding to TTP in any bird species. Sequences corresponding to TTP were identified in both lizards and alligators, close evolutionary relatives of birds. The induction kinetics of Zfp36l1 and Zfp36l2 mRNAs in LPS-stimulated chicken macrophages or serum-stimulated chick embryo fibroblasts did not resemble the normal mammalian TTP response to these stimuli, suggesting that the other two family members might not compensate for the TTP deficiency in regulating rapidly induced mRNA targets. Several mammalian TTP target transcripts have chicken counterparts that contain one or more potential TTP binding sites, raising the possibility that birds express other proteins that subsume TTP's function as a rapidly inducible regulator of AU-rich element (ARE)-dependent mRNA turnover.

Defense genes missing from the flight division.

Birds have a smaller repertoire of immune genes than mammals. In our efforts to study antiviral responses to influenza in avian hosts, we have noted key genes that appear to be missing. As a result, we speculate that birds have impaired detection of viruses and intracellular pathogens. Birds are missing TLR8, a detector for single-stranded RNA. Chickens also lack RIG-I, the intracellular detector for single-stranded viral RNA. Riplet, an activator for RIG-I, is also missing in chickens. IRF3, the nuclear activator of interferon-beta in the RIG-I pathway is missing in birds. Downstream of interferon (IFN) signaling, some of the antiviral effectors are missing, including ISG15, and ISG54 and ISG56 (IFITs). Birds have only three antibody isotypes and IgD is missing. Ducks, but not chickens, make an unusual truncated IgY antibody that is missing the Fc fragment. Chickens have an expanded family of LILR leukocyte receptor genes, called CHIR genes, with hundreds of members, including several that encode IgY Fc receptors. Intriguingly, LILR homologues appear to be missing in ducks, including these IgY Fc receptors. The truncated IgY in ducks, and the duplicated IgY receptor genes in chickens may both have resulted from selective pressure by a pathogen on IgY FcR interactions. Birds have a minimal MHC, and the TAP transport and presentation of peptides on MHC class I is constrained, limiting function. Perhaps removing some constraint, ducks appear to lack tapasin, a chaperone involved in loading peptides on MHC class I. Finally, the absence of lymphotoxin-alpha and beta may account for the observed lack of lymph nodes in birds. As illustrated by these examples, the picture that emerges is some impairment of immune response to viruses in birds, either a cause or consequence of the host-pathogen arms race and long evolutionary relationship of birds and RNA viruses.

Shh signalling restricts the expression of Gcm2 and controls the position of the developing parathyroids.

The parathyroid glands originate from the endoderm of the caudal pharyngeal pouches. How these parathyroids are restricted to developing in the caudal pouches is unclear. In this paper we investigate the role of Shh signalling in patterning the vertebrate pharyngeal pouches, and show that Hh signalling may be involved in restricting the expression of the parathyroid marker Gcm2 in the pharyngeal epithelium. In the chick and mouse, Shh signalling is excluded or highly reduced in the posterior/caudal pouches, where the parathyroid marker Gcm2 is expressed, while remaining at high levels in the more anterior pouches. Moreover, though the block of Shh signalling at early developmental stages results in the loss of chick Gcm2 expression, at later stages, it induces ectopic Gcm2 expression domains in the second and first pharyngeal epithelium, suggesting that HH signalling prevents Gcm2 in those tissues. These ectopic domains go on to express other parathyroid markers but do not migrate and develop into ectopic parathyroids. Differences in the expression of Gcm2 in the chick, mouse and zebrafish, correlate with changing patterns of Shh signalling, indicating a conserved regulatory mechanism that acts to define pouch derivatives.

GCN5 regulates the superoxide-generating system in leukocytes via controlling gp91-phox gene expression.

The superoxide anion (O(2)(-))-generating system is an important mechanism of innate immune response against microbial infection in phagocytes and is involved in signal transduction mediated by various physiological and pathological signals in phagocytes and other cells, including B lymphocytes. The O(2)(-)-generating system is composed of five specific proteins: p22-phox, gp91-phox, p40-phox, p47-phox, p67-phox, and a small G protein, Rac. Little is known regarding epigenetic regulation of the genes constituting the O(2)(-)-generating system. In this study, by analyzing the GCN5 (one of most important histone acetyltransferases)-deficient DT40 cell line, we show that GCN5 deficiency causes loss of the O(2)(-)-generating activity. Interestingly, transcription of the gp91-phox gene was drastically downregulated (to ∼4%) in GCN5-deficient cells. To further study the involvement of GCN5 in transcriptional regulation of gp91-phox, we used in vitro differentiation system of U937 cells. When human monoblastic U937 cells were cultured in the presence of IFN-γ, transcription of gp91-phox was remarkably upregulated, and the cells were differentiated to macrophage-like cells that can produce O(2)(-). Chromatin immunoprecipitation assay using the U937 cells during cultivation with IFN-γ revealed not only that association of GCN5 with the gp91-phox gene promoter was significantly accelerated, but also that GCN5 preferentially elevated acetylation levels of H2BK16 and H3K9 surrounding the promoter. These results suggested that GCN5 regulates the O(2)(-)-generating system in leukocytes via controlling the gp91-phox gene expression as a supervisor. Our findings obtained in this study should be useful in understanding the molecular mechanisms involved in epigenetic regulation of the O(2)(-)-generating system in leukocytes.

Chicken TLR21 is an innate CpG DNA receptor distinct from mammalian TLR9.

TLRs comprise a family of evolutionary conserved sensory receptors that respond to distinct classes of ligands. For one major evolutionary branch of TLRs, the ligands are still largely unknown. Here we report the cloning and function of one member of this group, chicken TLR21 (chTLR21). This TLR is absent in the human species but has homologs in fish and frog and displays similarity with mouse TLR13. Expression of chTLR21 in HEK293 cells resulted in activation of NF-kappaB in response to unmethylated CpG DNA, typically recognized by mammalian TLR9. Silencing of chTLR21 (but not chTLR4) in chicken macrophages inhibited the response to CpG-DNA (but not to LPS), indicating similar functionality of the endogenous receptor. ChTLR21 responded to human- and murine-specific TLR9 ligands, as well as to bacterial genomic DNA isolated from Salmonella enterica serovar Enteritidis. Confocal microscopy located chTLR21 in the same intracellular compartments as human TLR9. Inhibition of the chTLR21 response by the endosomal maturation inhibitor chloroquine suggested that the receptor is functional in endolysosomes, as known for TLR9. The analogous localization and function of the phylogenetically only distantly related chTLR21 and mammalian TLR9 suggest that during evolution different classes of TLRs have emerged that recognize the same type of ligands.

ATR activation and replication fork restart are defective in FANCM-deficient cells.

Fanconi anaemia is a chromosomal instability disorder associated with cancer predisposition and bone marrow failure. Among the 13 identified FA gene products only one, the DNA translocase FANCM, has homologues in lower organisms, suggesting a conserved function in DNA metabolism. However, a precise role for FANCM in DNA repair remains elusive. Here, we show a novel function for FANCM that is distinct from its role in the FA pathway: promoting replication fork restart and simultaneously limiting the accumulation of RPA-ssDNA. We show that in DT40 cells this process is controlled by ATR and PLK1, and that in the absence of FANCM, stalled replication forks are unable to resume DNA synthesis and genome duplication is ensured by excess origin firing. Unexpectedly, we also uncover an early role for FANCM in ATR-mediated checkpoint signalling by promoting chromatin retention of TopBP1. Failure to retain TopBP1 on chromatin impacts on the ability of ATR to phosphorylate downstream molecular targets, including Chk1 and SMC1. Our data therefore indicate a fundamental role for FANCM in the maintenance of genome integrity during S phase.

MAGUK-controlled ubiquitination of CARMA1 modulates lymphocyte NF-kappaB activity.

The adaptor protein CARMA1 is required for antigen receptor-triggered activation of IKK and JNK in lymphocytes. Once activated, the events that subsequently turn off the CARMA1 signalosome are unknown. In this study, we found that antigen receptor-activated CARMA1 underwent lysine 48 (K48) polyubiquitination and proteasome-dependent degradation. The MAGUK region of CARMA1 was an essential player in this event; the SH3 and GUK domains contained the main ubiquitin acceptor sites, and deletion of a Hook domain (an important structure for maintaining inactive MAGUK proteins) between SH3 and GUK was sufficient to induce constitutive ubiquitination of CARMA1. A similar deletion promoted the ubiquitination of PSD-95 and Dlgh1, suggesting that a conserved mechanism may control the turnover of other MAGUK family protein complexes. Functionally, we demonstrated that elimination of MAGUK ubiquitination sites in CARMA1 resulted in elevated basal and inducible NF-kappaB and JNK activation as a result of defective K48 ubiquitination and increased persistence of this ubiquitination-deficient CARMA1 protein in activated lymphocytes. The coordination of degradation with the full activation of the CARMA1 molecule likely provides an intrinsic feedback control mechanism to balance lymphocyte activation upon antigenic stimulation.

E2A expression stimulates Ig hypermutation.

Ig hypermutation is limited to a region of approximately 2 kb downstream of the transcription start sites of the Ig loci. The process requires transcription and the presence of Ig enhancer sequences, and is initiated by the activation-induced cytidine deaminase (AID)-mediated deamination of cytidine bases. It remains unknown why AID causes mutations selectively in the Ig genes and not in most other transcribed loci of B cells. In this study, we report that the inactivation of the E2A gene strongly reduces the rate of Ig L chain mutations in the chicken B cell line DT40 without affecting the levels of surface Ig or AID expression. The defect is complemented by the expression of cDNAs corresponding to either of the two E2A splice variants E12 or E47. The results suggest that E2A-encoded proteins enhance Ig hypermutation by recruitment of AID to the Ig loci.

Bves modulates epithelial integrity through an interaction at the tight junction.

We first identified Bves (blood vessel/epicardial substance) as a transmembrane protein that localized to the lateral compartment of the epithelial epicardium. Bves traffics to sites of cell-cell contact in cultured epicardial cells and promotes adhesion following transfection into non-adherent fibroblastic L-cells, reminiscent of a cell adhesion molecule. Currently, no function for Bves in relation to epithelial cell adhesion has been identified. We hypothesize that Bves plays a role at cell junctions to establish and/or modulate cell adhesion or cell-cell interactions in epithelial cell types. In this study, we demonstrate that Bves regulates epithelial integrity and that this function may be associated with a role at the tight junction (TJ). We report that Bves localizes with ZO-1 and occludin, markers of the TJ, in polarized epithelial cell lines and in vivo. We find that the behavior of Bves following low Ca2+ challenge or TPA treatment mimics that observed for ZO-1 and is distinct from adherens junction proteins such as E-cadherin. Furthermore, GST pull-down experiments show an interaction between ZO-1 and the intracellular C-terminal tail of Bves. Finally, we demonstrate that Bves modulates tight junction integrity, as indicated by the loss of transepithelial resistance and junction protein localization at the membrane following Bves knock-down in cultured cells. This study is the first to identify a function for Bves in epithelia and supports the hypothesis that Bves contributes to establishment and/or maintenance of epithelial cell integrity.

The vertebrate Hef ortholog is a component of the Fanconi anemia tumor-suppressor pathway.

The helicase-associated endonuclease for fork-structured DNA (Hef) is an archaeabacterial protein that processes blocked replication forks. Here we have isolated the vertebrate Hef ortholog and investigated its molecular function. Disruption of this gene in chicken DT40 cells results in genomic instability and sensitivity to DNA cross-links. The similarity of this phenotype to that of cells lacking the Fanconi anemia-related (FA) tumor-suppressor genes led us to investigate whether Hef functions in this pathway. Indeed, we found a genetic interaction between the FANCC and Hef genes. In addition, Hef is a component of the FA nuclear protein complex that facilitates its DNA damage-inducible chromatin localization and the monoubiquitination of the FA protein FANCD2. Notably, Hef interacts directly with DNA structures that are intermediates in DNA replication. This discovery sheds light on the origins, regulation and molecular function of the FA tumor-suppressor pathway in the maintenance of genome stability.