Epitope tags
Explore widely used epitope tags, such as c-Myc and HA, including their recommended applications and key considerations for use in antibody-based assays. Epitope tags support protein detection, purification, and analysis, but experimental design should account for tag properties, antibody specificity, and potential effects on protein behavior and assay performance.
Table of contents
Human influenza hemagglutinin (HA)
What are epitope tags?
An epitope is part of an antigen recognized by antibodies, and tags used in antibody-based assays are therefore known as epitope tags. These tags are typically shorter than affinity tags and are less likely to disrupt protein function. Although usable for purification, antibody-based columns are often more costly and less efficient than affinity systems1.
Despite these limitations, epitope tags are highly specific to their primary antibodies, making them valuable tools for detecting fusion proteins. They are widely applied in cell culture experiments, immunoprecipitation (IP), and protein complex immunoprecipitation (Co-IP)2 (Figure 1), where precise antibody recognition is essential for reliable experimental outcomes.
Figure 1: This diagram shows the steps for co-immunoprecipitation. First lyse your sample to release the proteins. When this lysate is added to the tube, along with antibodies against the fusion tag, the antibody will recognize the fusion tag. The antibodies are then bound to Protein A or G-coupled beads, which will pull out your protein of interest, along with any proteins that are complexed with it.
c-Myc
Molecular Weight: 1.2 kDa
Size: 10 amino acids (EQKLISEEDL)
Tag location: C- or N- terminals
Applications: Western blot, immunoprecipitation, flow cytometry. Can be used in affinity purification.
Affinity Resin: divinyl sulphone-activated agarose
Strengths: Can be placed at either N- or C-terminal. c-Myc-tagged proteins can be crystallized successfully.
Limitations: Elution for affinity chromatography is at low pH, which could affect fusion protein functionality.
Overview
Human c-Myc is expressed at low levels in proliferating cells and plays a central role in oncogenesis. The c-Myc tag is derived from the protein’s C-terminal region and is efficiently recognized by antibodies. It is widely used for protein detection in western blotting, immunoprecipitation, and flow cytometry, with successful crystallization of tagged proteins3.
What to watch out for
Fusing the c-Myc tag to a secretory signal is not recommended, as it can disrupt translocation through the secretory pathway. Although suitable for purification, its use is limited because elution requires a low pH, which may adversely affect protein structure and function in downstream experimental applications.
Top tip
It is not recommended to fuse c-Myc to a secretory signal, as this can interfere with translocation into the secretory pathway and potentially compromise protein localization and function in downstream analyses.
Human influenza hemagglutinin (HA)
Molecular Weight: 1.1 kDa
Size: 9 amino acids (YPYDVPDYA)
Tag Location: C- or N- terminals
Affinity Resin: Anti-HA antibody immobilized onto agarose beads
Applications: ELISA, western blot, immunoprecipitation and immunofluorescence, protein purification
Strengths: Unlikely to affect protein functionality
Limitations: Not recommended for use in apoptotic cells
Overview
The HA tag is derived from the influenza virus hemagglutinin glycoprotein, which mediates viral infectivity. As a small peptide tag, it rarely affects protein function, supporting its use in detection assays such as ELISA, western blotting, immunoprecipitation, and immunofluorescence. Anti-HA antibodies can be immobilized for affinity-based protein purification workflows.
What to watch out for
HA-tagged proteins should not be used in samples derived from apoptotic cells. The tag can be cleaved by caspases 3 and 7, leading to a loss of immunoreactivity and reduced detectability, which can compromise experimental reliability in apoptosis-related studies.
DDDK (FLAG®, Sigma)
Molecular Weight: 1.01 kDa
Size: 8 amino acids (DYKDDDDK).
Tag Location: C- or N- terminals, or internal.
Affinity Resin: Immobilized DDDK antibodies.
Applications: ELISA, western blot, protein purification, protein crystallization.
Strengths: Not likely to affect the functionality of fusion protein. Contains an internal cleavage site.
Limitations: Affinity resin is not as stable as others and can be expensive.
Overview
The DDDDK (FLAG®) tag is the only patented epitope tag and is more hydrophilic than many alternatives, reducing its impact on protein function. It contains an enterokinase cleavage site, enabling removal from the fusion protein when required, which supports flexibility in experimental design and downstream structural or functional studies3.
What to watch out for
Although purification using immobilized anti-DDDK antibodies can be effective, such affinity resins are often more expensive than other purification systems. Cost and resin stability should therefore be considered when selecting this tag for large-scale or repeated purification workflows.
Top tip
The DDDK tag includes an enterokinase cleavage sequence, enabling efficient removal of the protein of interest when necessary, which is advantageous for downstream applications that require native protein structure or activity.
V5
Molecular Weight: 0.95–1.4 kDa
Size: 9–14 amino acids (IPNPLLGLD or GKPIPNPLLGLDST)
Tag Location: C- or N- terminals
Applications: Western blot, ELISA, flow cytometry, protein visualization, ChIP, immunoprecipitation
Strengths: Available in two peptide lengths.
Limitations: Potential cross-reactivity in mammalian systems.
Overview
The V5 tag is derived from the P and V proteins of simian virus 5. Two versions are available, with the longer variant being the more commonly used. Combining the V5 tag with a His-tag is sometimes recommended to enhance detection or purification performance across different experimental systems.
What to watch out for
When using V5 tags in mammalian expression systems, cross-reactivity may occur. This can affect specificity and lead to background signal, so appropriate controls and antibody validation steps should be included to ensure reliable interpretation of experimental results.