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mEos2 & mEos4

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mEos2 & mEos4

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A New Photo-Convertible Fluorescent Protein Probe for Fluorescence Microscopy

A new photo-convertible fluorescent protein for use in fluorescence microscopy, mEos4, has been developed by researchers at the Howard Hughes Medical Institute’s Janelia Farm Research Campus. mEos4 offers several advantages over the previous protein probe EosFP.

EosFP, or Eos Fluorescent Protein, is a photo-convertible probe that can switch from green fluorescence to red fluorescence, with a single excitation wavelength typically around 400nm. Derived from a “wild” or living biological organism, EosFP is a unique photo-convertible FP probe in that it has a large separation in the emission spectra, making the peak colors very easy to differentiate.

The new florescence probe, mEos4, is highly monomeric in nature (having no more than one protein per bundle) and offers many new advantages while avoiding many of the disadvantages of EosFP.


  • High photo-stability for longer term imaging  compared to EosFP
  • Functions well in a broad range of protein fusions for dynamic investigations in live cell imaging
  • Can be used in fusions that are problematic with tdEos or dEos
  • Provides localization precision of its parent in the 10-nm range for super resolution requirements
  • Very resistant to chemical fixatives used to preserve samples, such as PFA, gluteraldehyde, and OsO4
  • Effective at higher temperatures than EosFP
  • Higher emission amplitude in both the green     and red states
  • Photo-converts at a faster rate than the previous proteins
  • Functions well in a GCaMP-like calcium indicator in the green channel for neural imaging and drug screening
  • Distinct FP not covered by GFP patents
  • Can be used as the foundation for other formulations or variants of application-specific, photo-convertible protein-based probes


  • Photo-activation localization microscopy (PALM)
  • Correlative light/electron microscopy (CLEM)
  • Activity indicators (i.e. real-time imaging in live cells)
  • Activity integration (i.e. post-hoc imaging of samples indicating activity over time)


Free for Non-Profit Research and available for Commercial License

Vectors available from Addgene: 

For inquiries, please reference:

Janelia 2013-024

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Photo-Stability, broad applications, facilitative.
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Michael Perham
Director, Innovations and External Relations
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