Summary: Thymidylate synthase
Pfam includes annotations and additional family information from a range of different sources. These sources can be accessed via the tabs below.
This is the Wikipedia entry entitled "Thymidylate synthase". More...
The Wikipedia text that you see displayed here is a download from Wikipedia. This means that the information we display is a copy of the information from the Wikipedia database. The button next to the article title ("Edit Wikipedia article") takes you to the edit page for the article directly within Wikipedia. You should be aware you are not editing our local copy of this information. Any changes that you make to the Wikipedia article will not be displayed here until we next download the article from Wikipedia. We currently download new content on a nightly basis.
Does Pfam agree with the content of the Wikipedia entry ?
Pfam has chosen to link families to Wikipedia articles. In some case we have created or edited these articles but in many other cases we have not made any direct contribution to the content of the article. The Wikipedia community does monitor edits to try to ensure that (a) the quality of article annotation increases, and (b) vandalism is very quickly dealt with. However, we would like to emphasise that Pfam does not curate the Wikipedia entries and we cannot guarantee the accuracy of the information on the Wikipedia page.
Editing Wikipedia articles
Before you edit for the first time
Wikipedia is a free, online encyclopedia. Although anyone can edit or contribute to an article, Wikipedia has some strong editing guidelines and policies, which promote the Wikipedia standard of style and etiquette. Your edits and contributions are more likely to be accepted (and remain) if they are in accordance with this policy.
You should take a few minutes to view the following pages:
How your contribution will be recorded
Anyone can edit a Wikipedia entry. You can do this either as a new user or you can register with Wikipedia and log on. When you click on the "Edit Wikipedia article" button, your browser will direct you to the edit page for this entry in Wikipedia. If you are a registered user and currently logged in, your changes will be recorded under your Wikipedia user name. However, if you are not a registered user or are not logged on, your changes will be logged under your computer's IP address. This has two main implications. Firstly, as a registered Wikipedia user your edits are more likely seen as valuable contribution (although all edits are open to community scrutiny regardless). Secondly, if you edit under an IP address you may be sharing this IP address with other users. If your IP address has previously been blocked (due to being flagged as a source of 'vandalism') your edits will also be blocked. You can find more information on this and creating a user account at Wikipedia.
If you have problems editing a particular page, contact us at email@example.com and we will try to help.
The community annotation is a new facility of the Pfam web site. If you have problems editing or experience problems with these pages please contact us.
Thymidylate synthase Edit Wikipedia article
|Symbols||; HST422; TMS; TS|
|External IDs||ChEMBL: GeneCards:|
|RNA expression pattern|
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
Thymidylate synthetase (EC 126.96.36.199) is an enzyme that catalyzes the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP). dTMP is one of the three nucleotides (dTMP, dTTP, and dTDP) that form thymine. Thymine is a nucleic acid in DNA. With inhibition of TS, an imbalance of deoxynucleotides and increased levels of dUTP arise. Both cause DNA damage.
The following reaction catalyzed by thymidylate synthetase:
- 5,10-methylenetetrahydrofolate + dUMP dihydrofolate + dTMP
This provides the sole de novo pathway for production of dTMP and is the only enzyme in folate metabolism in which the 5,10-methylenetetrahydrofolate is oxidised during one-carbon transfer. The enzyme is essential for regulating the balanced supply of the 4 DNA precursors in normal DNA replication: defects in the enzyme activity affecting the regulation process cause various biological and genetic abnormalities, such as thymineless death. The enzyme is an important target for certain chemotherapeutic drugs. Thymidylate synthase is an enzyme of about 30 to 35 Kd in most species except in protozoan and plants where it exists as a bifunctional enzyme that includes a dihydrofolate reductase domain. A cysteine residue is involved in the catalytic mechanism (it covalently binds the 5,6-dihydro-dUMP intermediate). The sequence around the active site of this enzyme is conserved from phages to vertebrates.
Thymidylate synthase is induced by a transcription factor LSF/TFCP2 and LSF is an oncogene in hepatocellular carcinoma. LSF and Thymidylate synthase plays significant role in Liver Cancer proliferation and progression and Drug resistance.
Thymidylate synthase (TS) plays a crucial role in the early stages of DNA biosynthesis. DNA damage or deletion occur on a daily basis as a result of both endogenous and environmental factors. Such environmental factors include ultraviolet damage and cigarette smoke that contain a variety of carcinogens. Therefore, synthesis and insertion of healthy DNA is vital for normal body functions and avoidance of cancerous activity. In addition, inhibition in synthesis of important nucleotides necessary for cell growth is important. For this reason, TS has become an important target for cancer treatment by means of chemotherapy. The sensitivity of TS to succumb to TS inhibitors is a key part to its success as treatment for colorectal, pancreatic, ovarian, gastric, and breast cancers.
Using TS as a drug target
The use of TS inhibitors has become a main focus of using TS as a drug target. The most widely used inhibitor is 5-Fluorouracil (5-FU), which acts as an antimetabolite that irreversibly inhibits TS by competitive binding. However, due to a low level of 5-FU found in many patients, it has been discovered that in combination with leucovorin (LV), 5-FU has greater success in down regulating tumor progression mechanisms and increasing immune system activity.
Experimentally, it has been shown that low levels of TS expression leads to a better response to 5-FU and higher success rates and survival of colon and liver cancer patients. However, additional experiments have merely stated that levels of TS may be associated with stage of disease, cell proliferation and tumor differentiation for those with lung adenocarcinoma but low levels are not necessarily indicators of high success. Expression levels of TS mRNA may be helpful in predicting the malignant potential of certain cancerous cells, thus improving cancer treatment targets and yielding higher survival rates among cancer patients [Hashimoto].
TS’s relation to the cell cycle also contributes to its use in cancer treatment. Several cell-cycle dependent kinases and transcription factors influence TS levels in the cell cycle that increase its activity during the S phase but decrease its activity while cells are no longer proliferating. In an auto-regulatory manner, TS not only controls its own translation but that of other proteins such as p53 that through mutation is the root of much tumor growth. Through its translation, TS has a varying expression in cancer cells and tumors, which leads to early cell death.
Interactive pathway map
Click on genes, proteins and metabolites below to link to respective articles. [§ 1]
- The interactive pathway map can be edited at WikiPathways: "FluoropyrimidineActivity_WP1601".
In the proposed mechanism, TS forms a covalent bond to the substrate dUMP through a 1,4-addition involving a cysteine nucleophile. The coenzyme tetrahydrofolate donates a methyl group to the alpha carbon while reducing the new methyl on dUMP to form dTMP.
It has been proven that the imine formed through reaction with THF and dUMP is an intermediate in the reaction with dUMP through mutations in the structure of TS that inhibit the completion of the mechanism. V316Am TS, a mutant with deletion of C terminal valines from both subunits, allows the catalysis of dehalogenation of BrdUMP preceding the mechanism described above and the covalent bond to THF and dUMP. The mutant TS is unable to accomplish the C-terminal conformational change needed to break covalent bonds to form dTMP, thus showing the proposed mechanism to be true. The structure was deduced through x-ray crystallography of V316Am TS to illustrate full homodimer TS structure (Figure 1). In addition, it showed possible interactions of the 175Arg and 174Arg between dimers. These arginines are thought to stabilize the UMP structures within the active sites by creating hydrogen bonds to the phosphate group (Figure 2). [Stroud and Finer-Moore] 5-FU is an inhibitor of TS. Upon entering the cell, 5-fluorouracil (5-FU) is converted to a variety of active metabolites, intracellularly. One such metabolite is FdUMP which differs from dUMP by a fluorine in place of a hydrogen on the alpha carbon. FdUMP is able to inhibit TS by binding to the nucleotide-binding site of dUMP. This competitive binding inhibits the normal function of dTMP synthesis from dUMP [Longley]. Thus the dUMP is unable to have an elimination reaction and complete the methyl donation from THF.
Figure 1. This figure depicts the homodimer that is TS. As you can see the orange and teal backbones never connect or intertwine, but there are side chains interactions between the dimers. On the orange protein, you can visibly detect two long side chains that enter the teal protein (this is located within the yellow circle). The other beige parts are side chains that interact within the active site. Just below the yellow circle, you are able to see the same pattern of side chains and configuration.
Figure 2. This figure shows the possible H-bond interactions between the arginines and the UMP in the active site of thymidylate synthase. This can be seen by the faint lines between the blue tips and the red tips. These arginines are used to hold the position of the UMP molecule so that the interaction may occur correctly. The two arginines in the top right corner that are located next to each other on the back bone are actually from the other protein of this dimer enzyme. This interaction is one of the many intermolecular forces that holds these two tertiary structures together. The yellow stand in the top-middle region shows a sulfur bond that forms between a cysteine side chain and UMP. This covalently holds the UMP within the active site until it is reacted to yield TMP.
- "Entrez Gene: TYMS thymidylate synthetase".
- "DNA: Form and Function" (PDF).
- "DNA Synthesis".
- Stroud RM, Santi DV, Hardy LW, Montfort WR, Jones MO, Finer-Moore JS (1987). "Atomic structure of thymidylate synthase: target for rational drug design". Science 235 (4787): 448–455. doi:10.1126/science.3099389. PMID 3099389.
- Gotoh O, Shimizu K, Kaneda S, Nalbantoglu J, Takeishi K, Seno T, Ayusawa D (1990). "Structural and functional analysis of the human thymidylate synthase gene". J. Biol. Chem. 265 (33): 20277–20284. PMID 2243092.
- Santhekadur PK, Rajasekaran D, Siddiq A, Gredler R, Chen D, Schaus SE, Hansen U, Fisher PB, Sarkar D (2012). "The transcription factor LSF: a novel oncogene for hepatocellular carcinoma" (PDF). Am J Cancer Res 2 (3): 269–85. PMC 3365805. PMID 22679558.
- Peters GJ, Backus HH, Freemantle S, van Triest B, Codacci-Pisanelli G, van der Wilt CL, Smid K, Lunec J, Calvert AH, Marsh S, McLeod HL, Bloemena E, Meijer S, Jansen G, van Groeningen CJ, Pinedo HM (2002). "Induction of thymidylate synthase as a 5-fluorouracil resistance mechanism". Biochim. Biophys. Acta 1587 (2-3): 194–205. doi:10.1016/S0925-4439(02)00082-0. PMID 12084461.
- "Leucovorin". MedlinePlus Drug Information. U.S. National Library of Medicine.
- Papamichael D (2000). "The Use of Thymidylate Synthase Inhibitors in the Treatment of Advanced Colorectal Cancer: Current Status". NCBI 4 (6): 478–87. PMID 10631692.
- Papamichael D (1999). "The use of thymidylate synthase inhibitors in the treatment of advanced colorectal cancer: current status". Oncologist 4 (6): 478–87. PMID 10631692.
- Nicolini A, Conte M, Rossi G, Ferrari P, Duffy M, Barak V, Carpi A, Miccoli P (2011). "Additional 5-FU-LV significantly increases survival in gastrointestinal cancer". Front Biosci (Elite Ed) 3: 1475–82. PMID 21622151.
- Carreras CW, Santi DV (1995). "The catalytic mechanism and structure of thymidylate synthase". Annu. Rev. Biochem. 64: 721–62. doi:10.1146/annurev.bi.64.070195.003445. PMID 7574499.
- Carreras CW, and Santi DV (1995). "The Catalytic Mechanism and Structure of Thymidylate Synthase". Annual Review of Biochemistry 64 (1): 721–762. doi:10.1146/annurev.bi.64.070195.003445. PMID 7574499.
- Banerjee D; Mayer-Kuckuk P; Capiaux G; et al. (2002). "Novel aspects of resistance to drugs targeted to dihydrofolate reductase and thymidylate synthase". Biochim. Biophys. Acta 1587 (2–3): 164–73. doi:10.1016/S0925-4439(02)00079-0. PMID 12084458. Unknown parameter
- Liu J; Schmitz JC; Lin X; et al. (2002). "Thymidylate synthase as a translational regulator of cellular gene expression". Biochim. Biophys. Acta 1587 (2–3): 174–82. doi:10.1016/s0925-4439(02)00080-7. PMID 12084459. Unknown parameter
- Chu J, Dolnick BJ (2002). "Natural antisense (rTSalpha) RNA induces site-specific cleavage of thymidylate synthase mRNA". Biochim. Biophys. Acta 1587 (2–3): 183–93. doi:10.1016/s0925-4439(02)00081-9. PMID 12084460.
- Peters GJ; Backus HH; Freemantle S; et al. (2002). "Induction of thymidylate synthase as a 5-fluorouracil resistance mechanism". Biochim. Biophys. Acta 1587 (2–3): 194–205. doi:10.1016/S0925-4439(02)00082-0. PMID 12084461. Unknown parameter
- Costi MP; Tondi D; Rinaldi M; et al. (2002). "Structure-based studies on species-specific inhibition of thymidylate synthase". Biochim. Biophys. Acta 1587 (2–3): 206–14. doi:10.1016/s0925-4439(02)00083-2. PMID 12084462. Unknown parameter
- Lin D; Li H; Tan W; et al. (2007). "Genetic polymorphisms in folate- metabolizing enzymes and risk of gastroesophageal cancers: a potential nutrient-gene interaction in cancer development". Forum of nutrition. Forum of Nutrition 60: 140–5. doi:10.1159/000107090. ISBN 3-8055-8216-1. PMID 17684410. Unknown parameter
This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.
Thymidylate synthase Provide feedback
This is a family of proteins that are flavin-dependent thymidylate synthases.
Leduc D, Graziani S, Meslet-Cladiere L, Sodolescu A, Liebl U, Myllykallio H;, Biochem Soc Trans. 2004;32:231-235.: Two distinct pathways for thymidylate (dTMP) synthesis in (hyper)thermophilic Bacteria and Archaea. PUBMED:15046578 EPMC:15046578
Leduc D, Escartin F, Nijhout HF, Reed MC, Liebl U, Skouloubris S, Myllykallio H;, J Bacteriol. 2007;189:8537-8545.: Flavin-dependent thymidylate synthase ThyX activity: implications for the folate cycle in bacteria. PUBMED:17890305 EPMC:17890305
Internal database links
|SCOOP:||Nif11 ASD1 CotH Tap-RNA_bind RRN9 LIAS_N|
External database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR000398Thymidylate synthase (EC) [PUBMED:6996564, PUBMED:2117882] catalyzes the reductive methylation of dUMP to dTMP with concomitant conversion of 5,10-methylenetetrahydrofolate to dihydrofolate:
The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.
|Molecular function||thymidylate synthase activity (GO:0004799)|
|Biological process||dTMP biosynthetic process (GO:0006231)|
Below is a listing of the unique domain organisations or architectures in which this domain is found. More...
The graphic that is shown by default represents the longest sequence with a given architecture. Each row contains the following information:
- the number of sequences which exhibit this architecture
a textual description of the architecture, e.g. Gla, EGF x 2, Trypsin.
This example describes an architecture with one
Gladomain, followed by two consecutive
EGFdomains, and finally a single
- a link to the page in the Pfam site showing information about the sequence that the graphic describes
- the UniProt description of the protein sequence
- the number of residues in the sequence
- the Pfam graphic itself.
Note that you can see the family page for a particular domain by clicking on the graphic. You can also choose to see all sequences which have a given architecture by clicking on the Show link in each row.
Finally, because some families can be found in a very large number of architectures, we load only the first fifty architectures by default. If you want to see more architectures, click the button at the bottom of the page to load the next set.
Loading domain graphics...
We store a range of different sequence alignments for families. As well as the seed alignment from which the family is built, we provide the full alignment, generated by searching the sequence database using the family HMM. We also generate alignments using four representative proteomes (RP) sets, the NCBI sequence database, and our metagenomics sequence database. More...
There are various ways to view or download the sequence alignments that we store. We provide several sequence viewers and a plain-text Stockholm-format file for download.
We make a range of alignments for each Pfam-A family:
- the curated alignment from which the HMM for the family is built
- the alignment generated by searching the sequence database using the HMM
- Representative Proteomes (RPs) at 15%, 35%, 55% and 75% co-membership thresholds
- alignment generated by searching the NCBI sequence database using the family HMM
- alignment generated by searching the metagenomics sequence database using the family HMM
You can see the alignments as HTML or in three different sequence viewers:
- a Java applet developed at the University of Dundee. You will need Java installed before running jalview
- an HTML page showing the whole alignment.Please note: full Pfam alignments can be very large. These HTML views are extremely large and often cause problems for browsers. Please use either jalview or the Pfam viewer if you have trouble viewing the HTML version
- an HTML-based representation of the alignment, coloured according to the posterior-probability (PP) values from the HMM. As for the standard HTML view, heatmap alignments can also be very large and slow to render.
- Pfam viewer
- an HTML-based viewer that uses DAS to retrieve alignment fragments on request
You can download (or view in your browser) a text representation of a Pfam alignment in various formats:
You can also change the order in which sequences are listed in the alignment, change how insertions are represented, alter the characters that are used to represent gaps in sequences and, finally, choose whether to download the alignment or to view it in your browser directly.
You may find that large alignments cause problems for the viewers and the reformatting tool, so we also provide all alignments in Stockholm format. You can download either the plain text alignment, or a gzipped version of it.
We make a range of alignments for each Pfam-A family. You can see a description of each above. You can view these alignments in various ways but please note that some types of alignment are never generated while others may not be available for all families, most commonly because the alignments are too large to handle.
1Cannot generate PP/Heatmap alignments for seeds; no PP data available
Key: available, not generated, — not available.
Format an alignment
We make all of our alignments available in Stockholm format. You can download them here as raw, plain text files or as gzip-compressed files.
You can also download a FASTA format file containing the full-length sequences for all sequences in the full alignment.
MyHits provides a collection of tools to handle multiple sequence alignments. For example, one can refine a seed alignment (sequence addition or removal, re-alignment or manual edition) and then search databases for remote homologs using HMMER3.
HMM logos is one way of visualising profile HMMs. Logos provide a quick overview of the properties of an HMM in a graphical form. You can see a more detailed description of HMM logos and find out how you can interpret them here. More...
If you find these logos useful in your own work, please consider citing the following article:
This page displays the phylogenetic tree for this family's seed alignment. We use FastTree to calculate neighbour join trees with a local bootstrap based on 100 resamples (shown next to the tree nodes). FastTree calculates approximately-maximum-likelihood phylogenetic trees from our seed alignment.
Note: You can also download the data file for the tree.
Curation and family details
This section shows the detailed information about the Pfam family. You can see the definitions of many of the terms in this section in the glossary and a fuller explanation of the scoring system that we use in the scores section of the help pages.
|Number in seed:||561|
|Number in full:||19061|
|Average length of the domain:||277.20 aa|
|Average identity of full alignment:||53 %|
|Average coverage of the sequence by the domain:||95.98 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 80369284 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||15|
|Download:||download the raw HMM for this family|
Weight segments by...
Change the size of the sunburst
selected sequences to HMM
a FASTA-format file
- 0 sequences
- 0 species
This visualisation provides a simple graphical representation of the distribution of this family across species. You can find the original interactive tree in the More....
This chart is a modified "sunburst" visualisation of the species tree for this family. It shows each node in the tree as a separate arc, arranged radially with the superkingdoms at the centre and the species arrayed around the outermost ring.
How the sunburst is generated
The tree is built by considering the taxonomic lineage of each sequence that has a match to this family. For each node in the resulting tree, we draw an arc in the sunburst. The radius of the arc, its distance from the root node at the centre of the sunburst, shows the taxonomic level ("superkingdom", "kingdom", etc). The length of the arc represents either the number of sequences represented at a given level, or the number of species that are found beneath the node in the tree. The weighting scheme can be changed using the sunburst controls.
In order to reduce the complexity of the representation, we reduce the number of taxonomic levels that we show. We consider only the following eight major taxonomic levels:
Colouring and labels
Segments of the tree are coloured approximately according to their superkingdom. For example, archeal branches are coloured with shades of orange, eukaryotes in shades of purple, etc. The colour assignments are shown under the sunburst controls. Where space allows, the name of the taxonomic level will be written on the arc itself.
As you move your mouse across the sunburst, the current node will be highlighted. In the top section of the controls panel we show a summary of the lineage of the currently highlighed node. If you pause over an arc, a tooltip will be shown, giving the name of the taxonomic level in the title and a summary of the number of sequences and species below that node in the tree.
Anomalies in the taxonomy tree
There are some situations that the sunburst tree cannot easily handle and for which we have work-arounds in place.
Missing taxonomic levels
Some species in the taxonomic tree may not have one or more of the main eight levels that we display. For example, Bos taurus is not assigned an order in the NCBI taxonomic tree. In such cases we mark the omitted level with, for example, "No order", in both the tooltip and the lineage summary.
Unmapped species names
The tree is built by looking at each sequence in the full alignment for the family. We take the name of the species given by UniProt and try to map that to the full taxonomic tree from NCBI. In some cases, the name chosen by UniProt does not map to any node in the NCBI tree, perhaps because the chosen name is listed as a synonym or a misspelling in the NCBI taxonomy.
So that these nodes are not simply omitted from the sunburst tree, we group them together in a separate branch (or segment of the sunburst tree). Since we cannot determine the lineage for these unmapped species, we show all levels between the superkingdom and the species as "uncategorised".
Since we reduce the species tree to only the eight main taxonomic levels, sequences that are mapped to the sub-species level in the tree would not normally be shown. Rather than leave out these species, we map them instead to their parent species. So, for example, for sequences belonging to one of the Vibrio cholerae sub-species in the NCBI taxonomy, we show them instead as belonging to the species Vibrio cholerae.
Too many species/sequences
For large species trees, you may see blank regions in the outer layers of the sunburst. These occur when there are large numbers of arcs to be drawn in a small space. If an arc is less than approximately one pixel wide, it will not be drawn and the space will be left blank. You may still be able to get some information about the species in that region by moving your mouse across the area, but since each arc will be very small, it will be difficult to accurately locate a particular species.
The tree shows the occurrence of this domain across different species. More...
We show the species tree in one of two ways. For smaller trees we try to show an interactive representation, which allows you to select specific nodes in the tree and view them as an alignment or as a set of Pfam domain graphics.
Unfortunately we have found that there are problems viewing the interactive tree when the it becomes larger than a certain limit. Furthermore, we have found that Internet Explorer can become unresponsive when viewing some trees, regardless of their size. We therefore show a text representation of the species tree when the size is above a certain limit or if you are using Internet Explorer to view the site.
If you are using IE you can still load the interactive tree by clicking the "Generate interactive tree" button, but please be aware of the potential problems that the interactive species tree can cause.
For all of the domain matches in a full alignment, we count the number that are found on all sequences in the alignment. This total is shown in the purple box.
We also count the number of unique sequences on which each domain is found, which is shown in green. Note that a domain may appear multiple times on the same sequence, leading to the difference between these two numbers.
Finally, we group sequences from the same organism according to the NCBI code that is assigned by UniProt, allowing us to count the number of distinct sequences on which the domain is found. This value is shown in the pink boxes.
We use the NCBI species tree to group organisms according to their taxonomy and this forms the structure of the displayed tree. Note that in some cases the trees are too large (have too many nodes) to allow us to build an interactive tree, but in most cases you can still view the tree in a plain text, non-interactive representation. Those species which are represented in the seed alignment for this domain are highlighted.
You can use the tree controls to manipulate how the interactive tree is displayed:
- show/hide the summary boxes
- highlight species that are represented in the seed alignment
- expand/collapse the tree or expand it to a given depth
- select a sub-tree or a set of species within the tree and view them graphically or as an alignment
- save a plain text representation of the tree
Please note: for large trees this can take some time. While the tree is loading, you can safely switch away from this tab but if you browse away from the family page entirely, the tree will not be loaded.
There are 3 interactions for this family. More...
We determine these interactions using iPfam, which considers the interactions between residues in three-dimensional protein structures and maps those interactions back to Pfam families. You can find more information about the iPfam algorithm in the journal article that accompanies the website.
For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the Thymidylat_synt domain has been found. There are 495 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.
Loading structure mapping...