Tectonic, a novel regulator
of the Hedgehog pathway
required for both activation
Jeremy F. Reiter1,4and William C. Skarnes2,3
1Developmental and Stem Cell Biology Program, and Diabetes
Center, University of California, San Francisco, California
94143-0525, USA;2Department of Molecular and Cell
Biology, University of California,
Berkeley, California 94720, USA
We report the identification of a novel protein that par-
ticipates in Hedgehog-mediated patterning of the neural
tube. This protein, named Tectonic, is the founding
member of a previously undescribed family of evolution-
arily conserved secreted and transmembrane proteins.
During neural tube development, mouse Tectonic is re-
quired for formation of the most ventral cell types and
for full Hedgehog (Hh) pathway activation. Epistasis
analyses reveal that Tectonic modulates Hh signal trans-
duction downstream of Smoothened (Smo) and Rab23.
Interestingly, characterization of Tectonic Shh and Tec-
tonic Smo double mutants indicates that Tectonic plays
an additional role in repressing Hh pathway activity.
Supplemental material is available at http://www.genesdev.org.
Received August 9, 2005; revised version accepted November
Hh signals are secreted proteins essential for normal de-
velopment and tissue homeostasis. Misregulation of Hh
signaling in humans can lead to congenital defects and
cancers (McMahon et al. 2003). The most extensively
studied function of the Hedgehog (Hh) family member
Sonic hedgehog (Shh) is its role in the developing neural
tube. There, Shh acts as a morphogen to direct the pro-
duction of particular neuronal subtypes at defined dor-
soventral positions (Jacob and Briscoe 2003). Shh medi-
ates its effects by binding to its receptor, Patched (Ptch).
Ptch, in the absence of Shh, represses the downstream
signaling pathway by inhibiting the activity of Smooth-
ened (Smo), a seven-transmembrane protein. Binding of
Shh to Ptch relieves the repression of Smo, triggering
events that culminate in the activation of transcription
factors of the Gli family. How Hh signals are transduced
is incompletely understood.
Results and Discussion
Through a screen for genes encoding secreted and trans-
membrane proteins (Skarnes et al. 1995; Mitchell et al.
2001), we identified a novel gene which, because it is
involved in a diverse range of developmental processes,
we named Tectonic after the Greek word for builder.
Conceptual translation of Tectonic indicates that it en-
codes a 63-kDa protein with no recognized domains
other than an N-terminal signal peptide. Genomic data-
base searches identify two other mammalian Tectonic
family members, Tect2 and Tect3, which are 49% and
58% similar to Tectonic, respectively (Supplementary
Fig. 1). The Drosophila genome contains a single Tec-
tonic homolog. Thus, Tectonic is the founding member
of an evolutionarily conserved family of proteins of un-
To assess whether Tectonic is secreted as predicted,
we created a fusion between the putative Tectonic signal
peptide and alkaline phosphatase. This fusion is robustly
secreted by Cos7 cells, indicating that the signal peptide
is functional (Supplementary Fig. 2). Interestingly, full-
length Tectonic is not secreted by Cos7 cells, suggesting
that its secretion may be regulated.
Insertion of the gene trap vector in Tectonic occurs in
the first of 12 introns (Fig. 1A). The resultant mutant
allele encodes a fusion between the first 57 amino acids
of Tectonic and a membrane-spanning ?geo reporter
(Mitchell et al. 2001). Given that no wild-type transcript
is detectable in Tectonic mutants by RT–PCR and
Northern blot analyses (Fig. 1B,C), and that transmem-
brane ?geo fusion proteins are retained in intracellular
compartments (Skarnes et al. 1995), the Tectonic gene
trap is likely to be a null allele.
During embryonic development, Tectonic is expressed
in regions that participate in Hh signaling. Tectonic is
first expressed during gastrulation stages in the ventral
node (Fig. 1D,E). At embryonic day 9.5 (E9.5), Tectonic is
expressed in the gut endoderm, limb buds, notochord,
somites, neural tube and floorplate (Fig. 1F). Unlike regu-
lators of Hh signaling such as Ptch and Hhip (Goodrich
et al. 1996; Marigo and Tabin 1996; Chuang and McMa-
hon 1999), Tectonic is not a transcriptional target of Hh
signaling (Supplementary Fig. 3B,C).
Tectonic mutants die between E13.5 and E16.5 and
display holoprosencephaly (Fig. 1G), a defect associated
with reduced Hh signaling (Chiang et al. 1996). Shh me-
diates induction of the floorplate, a histologically dis-
tinct cell population at the ventral midline of the neural
tube. Like Shh mutants and Gli2 mutants (Chiang et al.
1996; Ding et al. 1998; Matise et al. 1998), Tectonic mu-
tants fail to form floorplates and, instead, cells of neural
morphology are present at the midline (Fig. 2A). Molecu-
lar analysis with the markers Shh and FoxA2 (Hnf3?)
confirms that Tectonic is required for floorplate forma-
tion (Figs. 2B, 3B). However, the notochord forms nor-
mally in Tectonic mutants as judged by Shh and Brachy-
ury expression (Fig. 2B; Supplementary Fig. 3D). Thus,
axial defects in Tectonic mutants are confined to the
In addition to the floorplate, high levels of Hh signal-
ing are required for the induction of the adjoining V3
interneurons (Litingtung and Chiang 2000; Wijgerde et
[Keywords: Shh; signal transduction; mouse development; neural pat-
terning; open brain]
3Present address: Wellcome Trust Sanger Institute, Wellcome Trust Ge-
nome Campus, Hinxton, Cambridge, CB10 1SA, United Kingdom.
E-MAIL firstname.lastname@example.org; FAX (415) 514-2346.
Article published online ahead of print. Article and publication date are
22GENES & DEVELOPMENT 20:22–27 © 2006 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/06; www.genesdev.org
al. 2002). Analysis of neural tube patterning reveals that,
like Shh, Tectonic is required for formation of the Sim1-
expressing V3 interneurons (Fig. 2C). Nkx2.2, a marker
of the progenitors of the V3 interneurons (Briscoe et al.
1999), is also lost in Tectonic mutants (Fig. 3C), suggest-
ing that these defects are not due to defects in neuronal
maturation, but in their specification. Moreover, the
Tectonic-dependent defects in ventral neural develop-
ment are not limited to the V3 interneurons. Tectonic
mutants also display a variable reduction in the number
of Islet1/2-positive motor neurons (Fig. 3D). However,
Tectonic is not required for the expression of Dbx1 or
Dbx2 (Fig. 2D; data not shown), indicating that Tectonic
function is not essential for the development of more
dorsal cell fates within the neural tube.
The loss of ventral neural markers in Tectonic mu-
tants is accompanied by a ventral expansion of genes
normally restricted to more dorsal domains. High levels
of Hh signaling exclude expression of Irx3 from the V3
and motor neuron progenitor (p3 and pMN) domains
(Briscoe et al. 2000). In Tectonic mutants, Irx3 expres-
sion expands to include all but a small number of ventral
cells (Fig. 2E). Similarly, expression of Pax6, another fac-
tor repressed by high Hh signaling (Ericson et al. 1997), is
dramatically expanded in Tectonic mutants (Fig. 3B).
Taken together, these changes in marker expression in-
dicate that Tectonic is essential for the induction of the
ventral-most cell types of the neural tube. These pattern-
ing defects are qualitatively similar to those caused by
mutations in Shh or Gli2 (Chiang et al. 1996; Ding et al.
1998; Matise et al. 1998; Litingtung and Chiang 2000),
suggesting that Tectonic participates in Hh signaling.
To test this hypothesis, we examined the expression of
Gli1 and Ptch, two general Hh transcriptional targets.
Significantly, Gli1 expression is reduced throughout
Tectonic mutant embryos at E9.5 (Fig. 2F). In the devel-
oping neural tube of Tectonic mutants, expression of
Gli1 and Ptch is similarly dramatically reduced (Fig.
2G,H). Hh signaling in the neural tube is antagonized by
Bmp activity (Barth et al. 1999; Kawakami et al. 2005).
Expression of Msx1, a readout of Bmp pathway activity
in the dorsal neural tube (Liu et al. 2004), is normal in
ventral neural tube. (A) Hematoxylin-and-eosin-stained transverse
sections of E9.5 embryos. Tectonic mutants lack a histologically
distinct floorplate (arrow). (B–H) In situ hybridization of E9.5 whole-
mount embryos (F), or transverse sections of E9.5 (B) or E10.5 (C–
E,G,H) embryos. (B) Shh, a marker of the floorplate, is not expressed
in the Tectonic mutant neural tube. However, Tectonic mutants
express Shh normally in the notochord and gut epithelium. (C) Simi-
larly, Sim1, a marker of V3 interneurons, is not expressed in the
Tectonic mutant neural tube. (D) Expression of Dbx1, a marker of
V0 interneuron precursors, is expressed in Tectonic mutants. (E)
Expression of Irx3, a gene normally expressed dorsal to the pMN
domain, is expanded almost to the ventral midline of Tectonic mu-
tants. (F) Gli1, a general transcriptional target of Hh signaling, is
broadly diminished in Tectonic mutants. (G) Similarly, Gli1 expres-
sion is reduced in the neural tubes of Tectonic mutants. (H) Ptch,
another general Hh transcriptional target, is also down-regulated in
the Tectonic mutant neural tube.
Tectonic is required for Hh-mediated patterning of the
essential for embryonic development. (A) The mouse Tectonic gene
is comprised of 13 exons on chromosome 5. The gene trap consists
of a strong splice acceptor (SA) followed by an ORF encoding a
transmembrane domain (TM) and ?GEO. The gene trap also in-
cludes an IRES and PLAP coding sequence followed by a polyade-
nylation sequence (pA). (B) RT–PCR analysis of Tectonic gene ex-
pression in E11.5 wild-type, heterozygous, and homozygous mutant
embryos. Primers are specific for the Tectonic coding sequence 3? to
the gene trap (Tect), the ?GEO transcript, and G3PD. Included is a
−RT control using G3PD-specific primers. (C) Northern blot analy-
sis of Tectonic and ?GEO expression in wild-type, heterozygous,
and mutant embryos. (D–F) ?-Galactosidase staining of Tectonic
heterozygotes. (D) Lateral and distal views of late headfold stage
embryos, demonstrating restricted Tectonic expression in the node
(arrow). (E) Tectonic is expressed in the ventral epithelium of the
node, as revealed in a transverse section through the node of a six-
somite stage embryo. (F) At E9.5, Tectonic is expressed in the neural
tube, gut epithelium (arrow), notochord, and somites (arrowhead), as
seen both in whole-mount and transverse section. (G) E10.5 Tec-
tonic mutants exhibit reduced telencephalon size and holoprosen-
Tectonic is expressed in domains of Hh signaling, and is
Tectonic modulates Hedgehog signaling
GENES & DEVELOPMENT23
Tectonic mutants (Supplementary Fig. 4), suggesting
that Tectonic does not influence Hh signaling indirectly
by altering Bmp activity. Together, these results argue
that Tectonic acts in neural patterning by positively
regulating the Hh pathway.
Conceptually, Tectonic could contribute to Hh signal-
ing by participating in the creation of the Hh protein
gradient or in the interpretation of that gradient. To dis-
tinguish between these two possibilities, we carried out
epistasis experiments with Ptch mutants. If Tectonic
acts in Hh processing, release or distribution, Ptch
should be epistatic to Tectonic. However, if Tectonic
acts in Hh signal transduction, Tectonic should be epi-
static to Ptch. Ptch-dependent defects in embryonic
turning and dorsal neural tube closure are ameliorated in
Tectonic Ptch double mutants (Fig. 3A). Embryos lacking
Ptch function show marked expansion of the ventral do-
mains of the neural tube (Fig. 3B–D; Goodrich et al.
1997). Examination of the dorsoventral patterning of the
neural tube of Tectonic Ptch double mutants reveals a
loss of ventral neural fates indistinguishable from those
of Tectonic single mutants (Fig. 3B–D).
Like Ptch, Rab23 is a negative regulator of the Hh
pathway (Eggenschwiler et al. 2001; Huangfu et al. 2003).
Embryos homozygous for the opb2mutation in Rab23
display a ventralized neural tube (Fig. 3E,F; Eggen-
schwiler and Anderson 2000). As with Ptch, embryos
mutant for both Rab23 and Tectonic display neural tube
patterning defects identical to those of Tectonic single
mutants (Fig. 3E,F). Together, these results indicate that
Tectonic is epistatic to both Ptch and Rab23. As Rab23
has been reported to act downstream of Smo (Huangfu et
al. 2003), these data suggest that Tectonic modulates Hh
transduction at a point downstream of Ptch, Smo, and
To investigate whether the Tectonic-mediated effects
on neural tube patterning reflect changes in Hh pathway
activity, we assayed the expression of Gli1 in Tectonic
Ptch double mutants (Fig. 3G). Loss of Ptch function
causes ectopic expression of high levels of Gli1 in the
dorsal neural tube. In contrast, Tectonic Ptch double mu-
tants display uniform low levels of Gli1 expression (Fig.
3G). These data confirm that Tectonic is essential for
maximal activation of the Hh pathway. Furthermore,
our results strongly suggest that Tectonic functions
downstream of both Ptch and Rab23 in the Hh signal
transduction pathway, and not in Hh production or re-
lease. Consistent with this conclusion, Shh protein is
distributed in a dorsoventral gradient in Tectonic mu-
tant neural tubes similar to that of wild-type neural
tubes (Supplementary Fig. 5).
One of the most prominent defects displayed by Shh
mutants is the severe reduction in forebrain develop-
ment (Chiang et al. 1996). Strikingly, Tectonic Shh and
of E9.5 littermates. Ptch mutants display a characteristic open neu-
ral tube and defective turning whereas normal turning is largely
restored in Tectonic Ptch double mutants. (B–D) Transverse sec-
tions of E9.5 embryos stained for expression of Pax6 in red and, in
green, FoxA2 (B), Nkx2.2 (C), or Islet1/2 (D). Nuclei are visualized
with DAPI staining (blue). (B) Tectonic mutants lack floorplate ex-
pression of FoxA2 and show expanded Pax6 expression. Conversely,
Ptch mutants display expanded FoxA2 expression and reduced Pax6
expression. Tectonic Ptch double mutants closely resemble Tec-
tonic single mutants. (C) Tectonic mutants lack Nkx2.2 expression,
a marker of the p3 domain, whereas Ptch mutants display expanded
Nkx2.2 expression. Tectonic Ptch double mutants exhibit a loss of
Nkx2.2 expression identical to that of Tectonic single mutants. (D)
Motor neuron expression of Islet1/2 is reduced in most (n = 4/5)
Tectonic mutants, expanded in Ptch mutants, and reduced in Tec-
tonic Ptch double mutants. (E,F) Transverse sections of E10.5 em-
bryos. (E) Similar to Ptch mutants, Rab23 mutants exhibit an ex-
pansion of FoxA2 (green) and a dorsal shift in expression of Olig2, a
marker of motor neuron precursors (red). In contrast, Tectonic
Rab23 double mutants resemble Tectonic mutants. (F) Expression
of the dorsal markers Pax3 and Pax6 is shifted dorsally in Rab23
mutants, but not in Tectonic Rab23 double mutants. (G) Gli1 in
situ hybridization of transverse sections of E9.5 neural tubes.
Whereas Gli1 is normally expressed in a dorsoventral gradient, in
Ptch mutants, Gli1 is widely up-regulated and expressed ectopically
in the dorsal neural tube. In Tectonic Ptch double mutants, Gli1 is
expressed at a uniform low level throughout the dorsoventral extent
of the neural tube.
Tectonic is epistatic to Ptch and Rab23. (A) Lateral views
Reiter and Skarnes
24GENES & DEVELOPMENT
Tectonic Smo double mutants have considerably larger
forebrains than do either Shh or Smo mutants (Fig. 4A;
Supplementary Fig. 6). Although these results appear
paradoxical given the reduced forebrains of Tectonic mu-
tants, they suggest that there is a higher level of Hh
activity in double mutants than in single mutants, im-
plying that in addition to its role in pathway activation,
Tectonic exerts a repressive effect on the pathway. To
test whether this is the case, we examined neural tube
expression of Dbx1 and Dbx2, markers of the p0 and p1
precursors that are induced by low Hh levels (Wijgerde et
al. 2002). If p0 and p1 formation in Tectonic mutants
requires Shh activity, Tectonic Shh double mutants
should show a reduction in Dbx1 and Dbx2 expression
similar to that displayed by Shh mutants. However, our
analysis reveals a dramatic increase in Dbx1- and Dbx2-
expressing cells in Tectonic Shh double mutants as com-
pared to Shh single mutants (Fig. 4B,C). These surprising
results demonstrate that Dbx1 and Dbx2 expression in
Tectonic mutants is independent of Shh, and suggest
that Hh pathway activity is higher in Tectonic Shh
double mutants than in Shh mutants.
To assess whether increased Dbx1 and Dbx2 expres-
sion reflects increased Hh pathway activation, we exam-
ined Ptch expression in Tectonic Shh double mutants.
We found that the abrogation of Ptch expression exhib-
ited by Shh mutants is indeed partially alleviated by loss
of Tectonic function (Fig. 4D), indicating that levels of
Hh pathway activation are in fact higher in Tectonic Shh
double mutants than in Shh mutants. The genetic inter-
action between Shh and Tectonic is similar to that ob-
served between Shh and Gli3 (Litingtung and Chiang
2000) and suggest that Tectonic acts in a Shh-indepen-
dent fashion to repress the Hh pathway. Taken with the
forebrain data, these results reveal that Tectonic plays
dual essential roles in both activating and inhibiting the
Hh pathway in the anterior and posterior neural tube.
The loss of the activator function can be depicted as a
rightward shift in the Hh pathway activity gradient of
Tectonic mutants (Fig. 4E). Our additional finding that
Tectonic inhibits Hh pathway activation in the absence
of Shh can be represented graphically as a leftward shift
in the Hh pathway activity gradient of Tectonic Shh
double mutants relative to Shh mutants (Fig. 4E). This
evidence that Tectonic functions in Hh signal transduc-
tion to fully activate the pathway in the presence of high
Hh levels and to repress the pathway in the absence of
Hh signals may reflect a combination of decreased func-
tion of both Gli activators and Gli repressors. In this
regard, Tectonic joins a number of recently described
regulators of Hh signal transduction including mouse
IFT proteins (Huangfu et al. 2003; Liu et al. 2005) and
signaling, Tectonic functions to repress low levels of Hh pathway
activation. (A) Lateral view of E10.5 embryos. Shh mutants are one-
third the size of littermates and show severely diminished fore-
brains. Tectonic Shh double mutants are larger than Shh single mu-
tants and develop markedly larger telencephalons (arrow). (B–D) In
situ hybridization of transverse sections of E10.5 embryos. (B) In
Shh mutants, Dbx1 expression, a marker of the p0 domain, is re-
duced to a very few cells at the ventral midline. Tectonic Shh
double mutants exhibit increased Dbx1 expression relative to Shh
single mutants. (C) Dbx2, a marker of the p0 and p1 domains, is
markedly reduced or not expressed in Shh mutants. In contrast,
Tectonic Shh double mutants express Dbx2 robustly at the ventral
midline. (D) Tectonic Shh double mutants display higher levels of
Ptch expression than do Shh single mutants. (E) Levels of Hh path-
way activity within the developing ventral neural tube are trans-
lated into distinct fates, including floorplate (FP) and five neural
precursor domains (p3–p0) at defined dorsoventral positions. In Tec-
tonic mutants, neural fates that require the highest levels of Hh
signaling are lost, represented as a rightward shift in the Hh path-
way activity curve. Disruption of Shh function causes severe reduc-
tion of the p0 and p1 domains and loss of more ventral fates. In
contrast, loss of both Shh and Tectonic function results in increased
Hh pathway activity and restored p0 and p1 development, revealing
an inhibitory role for Tectonic in the Hh pathway.
In addition to its role in mediating high levels of Hh
Tectonic modulates Hedgehog signaling
GENES & DEVELOPMENT 25
zebrafish Iguana (Sekimizu et al. 2004; Wolff et al. 2004).
Additionally, a Drosophila protein complex that in-
cludes Cos2 is similarly required for full pathway acti-
vation (Robbins et al. 1997; Sisson et al. 1997; Wang and
Holmgren 2000; Wang et al. 2000; Lefers et al. 2001) and
inhibition (Methot and Basler 2000; Stegman et al. 2000;
Wang et al. 2000; Lefers et al. 2001). Tectonic is the first
extracytosolic factor shown to act in this dual capacity.
Although the molecular mechanism by which Tec-
tonic functions is not clear, our double mutant analyses
suggest that it modulates Hh signal transduction at a
point fairly downstream in the pathway. As Rab23 acts
in the same region of the pathway and is thought to
control vesicle transport, it will be interesting to assess
whether it regulates the trafficking of Tectonic.
Materials and methods
The mouse embryonic stem cell line KST296 carrying an insertion of the
pGT1pfs secretory trap vector in the Tectonic gene was isolated as de-
scribed in Mitchell et al. (2001). Tectonic F1 heterozygotes were back-
crossed to C57Bl/6 mice for six generations prior to intercrossing. Geno-
typing of Tectonic was performed using genomic PCR with a pair of
wild-type-specific primers (5?-CGCCTCTTTAGCCCTCTGTT-3? and
5?-AGAACCTCCACGAGAGCAGA-3?) and a mutant-allele-specific
primer (5?-TCTAGGACAAGAGGGCGAGA-3?). Ptch, Rab23, Shh, and
Smo embryos were genotyped as described (Chiang et al. 1996; Goodrich
et al. 1997; Eggenschwiler et al. 2001; Zhang et al. 2001).
Cos7 cells were transfected using Fugene6 (Roche) with APTag5 (Gen-
Hunter) or APTag5-TectSignal, a vector in which the SEAP signal se-
quence has been replaced with that of Tectonic. Alkaline phosphatase
activity in the supernatant was chemiluminescently measured using the
Phospha-Light Assay (Applied Biosystems) and a 20/20nluminometer
RT–PCR and Northern blot analyses
RT–PCR was performed using exon-spanning primers complementary to
Tectonic cDNA 3? to the gene trap insertion (5?-AATCCGCTGTTCC
TTCCAC-3? and 5?-TGCGTCAGTGTGTGATTCAG-3?), to the ?GEO
transcript (5?-CTTGGGTGGAGAGGCTATTC-3? and 5?-AGGTGAG
ATGACAGGAGATC-3?), and to G3PD (5?-GTGTTCCTACCCCCAAT
GTG-3? and 5?-TGTGAGGGAGATGCTCAGTG-3?). Northern blots
were hybridized to a Tectonic cDNA probe spanning exons 2–12 and a
probe to ?geo.
Immunohistochemistry and in situ hybridization
X-gal staining, in situ hybridization, and immunohistochemical staining
were carried out using antibodies and protocols as previously described
(Ericson et al. 1997; Briscoe et al. 1999, 2000; Takebayashi et al. 2000;
Gritli-Linde et al. 2001) with the exception of rabbit ?-Pax6 antibody
(Covance Research Products), which was used at 1:300. The ?-FoxA2,
?-Nkx2.2, ?-Islet1/2, ?-Msx1/2, and ?-Pax3 antibodies were obtained
from the Developmental Studies Hybridoma Bank maintained by the
University of Iowa under contract NO1-HD-7-3263 from the NICHD.
Gene analysis and accession numbers
Sequences of Tectonic family members were aligned using ClustalW and
Boxshade 3.21. Domain analysis was performed with SignalP 3.0 and
HMMTOP 2.0. Mouse Tectonic cDNA sequence, GenBank accession
number DQ278867; human Tectonic cDNA sequence, GenBank acces-
sion number DQ278868; mouse Tect2 cDNA sequence, GenBank acces-
sion number DQ278869; human Tect2 cDNA sequence, GenBank acces-
sion number DQ278870; mouse Tect3 cDNA sequence, GenBank acces-
sion number DQ278871; human Tect3 cDNA sequence, GenBank
accession number DQ278872; Drosophila dTectonic cDNA sequence,
GenBank accession number DQ278873.
We are grateful to Debbie Pangilinan for expert mouse husbandry, to
Andrew Norman for technical assistance, and to Pao-Tien Chuang and
Andrew McMahon for the anti-Shh antibody Ab80. We thank Kevin
Mitchell, Chulho Kang, and Marc Tessier-Lavigne for help generating the
KST296 ES cell line and the Tectonic mice. This work was assisted by
valuable conversations with Gail Martin, Pao-Tien Chuang, Didier
Stainier, and Tom Kornberg. This research was supported by funding
from the Chicago Community Trust and grants from the NICHD and
NIMH. J.R. is a UCSF Fellow generously supported by the Sandler Foun-
dation and a March of Dimes Basil O’Connor Award.
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