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{{short description|Fragment}}
[[File:ScFv-rotation.gif|300px|thumb|Rotating scFv fragment with highlighted complementarity determining regions (CDRs)]]
[[File:Single chain variable fragment.svg|thumb|The two possible structures of a single-chain variable fragment, with the antigen binding sites including the [[N-terminus|N-termini]] on the left and the [[C-terminus|C-termini]] on the right. The linker peptides are shown as arrows.]]
A '''single-chain variable fragment''' ('''scFv''') is not actually a [[Antibody fragment|fragment]] of an antibody, but instead is a [[fusion protein]] of the variable regions of the [[Immunoglobulin heavy chain|heavy]] (V<sub>H</sub>) and [[Immunoglobulin light chain|light chains]] (V<sub>L</sub>) of [[immunoglobulins]], connected with a short linker [[peptide]] of ten to about 25 [[amino acid]]s.<ref>{{cite journal | last1 = Huston | first1 = J. S. | last2 = Levinson | first2 = D. | last3 = Mudgett-Hunter | first3 = M. | last4 = Tai | first4 = M. S. | last5 = Novotný | first5 = J. | last6 = Margolies | first6 = M. N. | last7 = Crea | first7 = R. | year = 1988 | title = Protein engineering of antibody binding sites: recovery of specific activity in an anti-digoxin single-chain Fv analogue produced in Escherichia coli | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 85 | issue = 16| pages = 5879–5883 | doi=10.1073/pnas.85.16.5879| pmid = 3045807 | pmc = 281868 | bibcode = 1988PNAS...85.5879H | doi-access = free }}</ref> The linker is usually rich in [[glycine]] for flexibility, as well as [[serine]] or [[threonine]] for solubility, and can either connect the [[N-terminus]] of the V<sub>H</sub> with the [[C-terminus]] of the V<sub>L</sub>, or ''vice versa''.<ref name="Schirrmann">{{cite
This protein retains the specificity of the original immunoglobulin, despite removal of the constant regions and the introduction of the linker.<ref name="Peterson" /> The image to the right shows how this modification usually leaves the specificity unaltered.
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==Purification==
Single-chain variable fragments lack the constant [[Fc region]] found in complete antibody molecules, and, thus, the common binding sites (e.g., [[protein G]]) cannot be used to purify antibodies. These fragments can often be purified or immobilized using [[protein L]], since protein L interacts with the variable region of kappa light chains. More commonly, scientists incorporate a six histidine tag on the c-terminus of the scFv molecule and purify them using immobilized metal affinity chromatography (IMAC). Some scFv can also be captured by [[protein A]] if they contain a human VH3 ___domain.<ref name="Vostakolaei 14711–14724">{{Cite journal |last1=Vostakolaei |first1=Mehdi Asghari |last2=Molavi |first2=Ommoleila |last3=Hejazi |first3=Mohammad Saeid |last4=Kordi |first4=Shirafkan |last5=Rahmati |first5=Saman |last6=Barzegari |first6=Abolfazl |last7=Abdolalizadeh |first7=Jalal |date=September 2019 |title=Isolation and characterization of a novel scFv antibody fragments specific for Hsp70 as a tumor biomarker |url=https://onlinelibrary.wiley.com/doi/10.1002/jcb.28732 |journal=Journal of Cellular Biochemistry |language=en |volume=120 |issue=9 |pages=14711–14724 |doi=10.1002/jcb.28732 |pmid=30998271 |s2cid=121351794 |issn=0730-2312|url-access=subscription }}</ref><ref>{{Cite journal|url=https://pubmed.ncbi.nlm.nih.gov/9186782-staphylococcal-protein-a-binding-to-vh3-encoded-immunoglobulins/|pmid = 9186782|year = 1997|last1 = Potter|first1 = K. N.|last2 = Li|first2 = Y.|last3 = Pascual|first3 = V.|last4 = Capra|first4 = J. D.|title = Staphylococcal protein a binding to VH3 encoded immunoglobulins|journal = International Reviews of Immunology|volume = 14|issue = 4|pages = 291–308|doi = 10.3109/08830189709116521}}</ref><ref>{{Cite journal |last1=Kordi |first1=Shirafkan |last2=Rahmati-Yamchi |first2=Mohammad |last3=Asghari Vostakolaei |first3=Mehdi |last4=Barzegari |first4=Abolfazl |last5=Abdolalizadeh |first5=Jalal |date=2019-02-21 |title=Purification of a Novel Anti-VEGFR2 Single Chain Antibody Fragmentand Evaluation of Binding Affinity by Surface Plasmon Resonance |url=https://apb.tbzmed.ac.ir/Abstract/apb-22975 |journal=Advanced Pharmaceutical Bulletin |language=en |volume=9 |issue=1 |pages=64–69 |doi=10.15171/apb.2019.008 |issn=2228-5881 |pmc=6468230 |pmid=31011559}}</ref>
==Bivalent and trivalent scFvs==
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* C6.5, a diabody targeting [[HER2/neu]]<ref name="Adams" /> found in some [[breast cancer]]s
* [[Brolucizumab]], a scFV binding to VEGF-A and used to treat wet age-related macular degeneration
* '''G6A''', a scFV binding to human Hsp70.1 and can be used to target this protein as a potential marker in vast cancers.<ref name="Vostakolaei 14711–14724"/>
==References==
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