Fluorescent Proteins: FRET
Förster (or Fluorescence) Resonance Energy Transfer (FRET) is a process by which energy is non-radiatively transferred from an excited donor fluorophore to an acceptor. Because this process is extremely sensitive to the distance between the fluorophores at the single-nanometer scale, scientists can use FRET to report on protein binding or conformational changes. Organic dyes, fluorescent proteins, and other molecules have been demonstrated for use in FRET experiments.
FRET efficiency can be measured using fluorescence microscopy or spectroscopy by exciting the donor and detecting the emission of the acceptor. Alternatively, the fluorescence lifetime (the time between excitation of the fluorophore and emission of a photon) of the donor can be measured using fluorescence lifetime imaging microscopy (FLIM) or FLIM-FRET, where the increased energy transfer through FRET reduces the donor lifetime. FRET is often used to study (1) protein-protein interactions where each protein is separately fused to a donor or acceptor molecule (also referred to as intermolecular or bimolecular FRET) or (2) conformational changes within a protein where the donor and acceptor are both fused to the same protein (also referred to as intramolecular or unimolecular FRET).
Learn more about FRET on our blog and explore the tables below for plasmids and information to help plan your next FRET experiment.
Looking for FRET-based biosensors? Check out our Biosensors Collection to find fluorescent biosensors targeting metal ions, small molecules, and more.
Popular FRET Pairs
This table lists some popular fluorescent proteins used as FRET pairs along with their properties. Where available, we have listed cloning backbones and control plasmids.
| Donor | Acceptor | λDex | QYD | λAem | εA | QYA | R0 | J(λ) | Plasmids |
|---|---|---|---|---|---|---|---|---|---|
| mTagBFP | sfGFP | 399 | 0.64 | 510 | 83,000 | 0.65 | 4.6 | 2.6 | mTagBFP2-pBAD, sfGFP-pBAD |
| ECFP | EYFP | 434 | 0.41 | 527 | 67,000 | 0.67 | 4.8 | 1.5 | pmECFP-1, pmEYFP-1, pECFP18aaEYFP |
| mCerulean | mVenus | 433 | 0.49 | 527 | 104,000 | 0.64 | 5.3 | 2.3 | mCerulean N1, mVenus N1 |
| mTurquoise2 | mVenus | 434 | 0.93 | 527 | 104,000 | 0.64 | 5.9 | 2.3 | pmTurquoise2-N1, mVenus N1, pmVenus(L68V)-mTurquoise2 |
| mTurquoise2 | mNeonGreen | 434 | 0.93 | 517 | 116,000 | 0.8 | 6.2 | 3.1 | pmTurquoise2-N1, pCS2+mNeonGreen-C Cloning Vector, mNeonGreen-mTurquoise2 |
| CyPet | YPet | 435 | 0.51 | 530 | 104,000 | 0.77 | 5.3 | 2.3 | CYPet-N1, YPet-N1 |
| mTFP1 | mVenus | 462 | 0.85 | 527 | 104,000 | 0.64 | 6.0 | 2.9 | mTFP1-N1, mVenus N1 |
| mTFP1 | mCitrine | 462 | 0.85 | 529 | 94,000 | 0.74 | 5.9 | 2.6 | mTFP1-N1, mCitrine-pBAD |
| EGFP | mCherry | 488 | 0.60 | 610 | 72,000 | 0.22 | 5.3 | 1.9 | mEGFP-N1, mCherry2-N1, pcDNA3.1 CMV mCherry-eGFP BGH |
| Clover | mRuby2 | 505 | 0.76 | 600 | 113,000 | 0.38 | 6.3 | 4.5 | pcDNA3-Clover, pcDNA3-mRuby2, pcDNA3.1-Clover-mRuby2 |
| mClover3 | mRuby3 | 506 | 0.78 | 592 | 128,000 | 0.45 | 6.4 | 4.9 | pNCS-mClover3, pNCS-mRuby3, pKanCMV-mClover3-mRuby3 |
| mNeonGreen | mRuby3 | 506 | 0.80 | 592 | 128,000 | 0.45 | 6.4 | 4.7 | pCS2+mNeonGreen-C Cloning Vector, pcDNA3-mRuby2, mNeonGreen-mRuby2-FRET-10 |
| SYFP2 | mScarlet3 | 515 | 0.68 | 592 | 104,000 | 0.75 | 6.3 | 4.8 | pSYFP2-C1, pmScarlet3_C1, pDx_mScarlet3-SYFP2 |
| mVenus | mKO2 | 515 | 0.64 | 565 | 63,800 | 0.62 | 5.7 | 2.9 | mVenus N1, mKO2-N1 |
| mOrange2 | mCherry | 549 | 0.60 | 610 | 72,000 | 0.22 | 6.1 | 4.4 | mOrange2-N1, mCherry2-N1 |
| miRFP670 | miRFP720 | 642 | 0.14 | 720 | 87,400 | 0.061 | 5.7 | 13 | pmiRFP670-N1, pmiRFP720-N1 |
| LSSmOrange* | mKate2 | 437 | 0.45 | 633 | 62,500 | 0.40 | 5.6 | 3.7 | pLSSmOrange-N1, mKate1.31-pBAD, pLSSmOrange-mKate2 |
| LSSmOrange* | mScarlet3 | 437 | 0.45 | 592 | 104,000 | 0.75 | 5.8 | 4.3 | pLSSmOrange-N1, pmScarlet3_C1 |
| mT-Sapphire* | tdTomato | 399 | 0.6 | 581 | 138,000 | 0.69 | 6.4 | 5.9 | mT-Sapphire-N1, tdTomato-N1 |
| CyOFP1* | mCardinal | 497 | 0.76 | 659 | 87,000 | 0.19 | 6.7 | 6.5 | pNCS-CyOFP, mCardinal-N1 |
| mAmetrine* | tdTomato | 406 | 0.58 | 581 | 138,000 | 0.69 | 6.5 | 7.1 | mAmetrine-N1, tdTomato-N1 |
| mTurquoise2 | sREACh** (super-REACh, aka Nữ) | 434 | 0.93 | 531** | 115,000 | 0.07 | 6.1 | 3.0 | pmTurquoise2-N1, sREACh-C1, sReach-mTurquoise2 |
| EGFP | sREACh** (super-REACh, aka Nữ) | 488 | 0.60 | 531** | 115,000 | 0.07 | 6.0 | 4.1 | mEGFP-N1, sREACh-C1, mGFP-10-sREACh-N3 |
| EGFP | ShadowY** | 488 | 0.6 | 531** | 136,000 | 0.01 | 6.1 | 4.5 | mEGFP-N1, CMV-ShadowY, EGFP-ShadowY |
λDex: Donor maximum excitation wavelength (nm), QYD: Donor quantum yield, λAem: Acceptor maximum emission wavelength (nm), εA: Acceptor exctinction coefficient (M-1 cm-1), QYA: Acceptor quantum yield, R0: Förster Radius (nm, distance at which 50% FRET efficiency occurs), J(λ): Overlap integral of the donor emission and acceptor excitation spectra (×1015 M-1 cm-1 nm4). Values obtained from FPbase.org FRET calculator (Link opens in a new window)
*: Long Stokes shift donor
**: Non-fluorescent (dark) acceptor, used for FLIM measurements
Note that FRET measurements can be influenced by many factors, such as the organism or cell type, relative position and orientation of the fluorophores through linkers, fluorescent protein expression level and maturation time, and other environmental factors. When designing your experiment, be sure to refer to published reports and include appropriate controls to help interpret your results. See Algar et al. (2019) (Link opens in a new window) for more detailed discussion.
FRET Calibration Standards
The articles or collections listed below feature plasmids with positive and negative FRET controls or calibration standards consisting of donor and acceptor proteins fused with linkers of varying lengths.
Additional Resources at Addgene
- Learn more on the Addgene Blog: Introduction to FRET and Tips for Using FRET in Your Experiments.
- Find many FRET-based biosensors in Addgene's Biosensors Collection.
- Generate custom FRET-based biosensors with the cpFRET kit from the Pertz Lab or the 2in1 Plasmid Toolkit from the Grefen Lab.
- FRET pairs and biosensors can also incorporate organic fluorophores via HaloTag or SNAP-Tag. Read more about self-labeling tags on our blog.
- Luciferase can be paired with a fluorescent protein acceptor for Bioluminescence Resonance Energy Transfer (BRET). Browse Addgene's Luciferase Plasmid Collection or the Promega Plasmid Collection to find related tools for BRET assays.
References
- Bajar, B. T., Wang, E. S., Zhang, S., Lin, M. Z., & Chu, J. (2016). A Guide to Fluorescent Protein FRET Pairs. Sensors, 16(9), 1488. doi: https://doi.org/10.3390/s16091488 (Link opens in a new window) PMID: 27649177 (Link opens in a new window)
- Algar, W. R., Hildebrandt, N., Vogel, S. S., & Medintz, I. L. (2019). FRET as a biomolecular research tool — understanding its potential while avoiding pitfalls. Nature Methods, 16(9), 815–829. doi: https://doi.org/10.1038/s41592-019-0530-8 (Link opens in a new window) PMID: 31471616 (Link opens in a new window)
- Kappel, C. Kuschel, L. DeRose, J. (2022). What is FRET with FLIM (FLIM-FRET)? Leica Microsystems Blog. https://www.leica-microsystems.com/science-lab/life-science/what-is-fret-with-flim/ (Link opens in a new window)
External Resources
- FPbase.org FRET calculator (Link opens in a new window)
- Nikon MicroscopyU: Basics of FRET Microscopy (Link opens in a new window)
- Fluorescent Biosensor Database (Link opens in a new window) designed by Eric Greenwald and the Jin Zhang Lab at UCSD
Content last reviewed: 25 September 2025