FAP-tags are genetically encoded polypeptides (~25 kD) that bind small fluorogen molecules at nanomolar concentrations to yield fluorescent signal.  Neither the FAP nor the fluorogen is fluorescent by itself.



FAP technology is particularly useful for measuring receptor internalization and trafficking.

Different FAP/fluorogen pairings enable a variety of dynamic studies and quantitative assays. For example, when cells expressing a GPCR with a FAP-tag at its extracellular N-terminus are exposed to a membrane impermeant fluorogen, only receptor molecules at the cell surface, or molecules that have visited the surface while the cells were exposed to the fluorogen, will exhibit fluorescence.  This is in contrast to GFP tags, where the entire population of receptors is fluorescent.

We offer two types of FAPs – α and β – and a variety of cognate α and b fluorogens.

  • Three fluorogens – membrane-impermeant, membrane-permeant, and pH sensitive/membrane impermeant – are available for the alpha FAPs. They enable quantitative assays for receptors at the surface, total protein, receptor internalization/desensitization, and receptor resensitization
  • Two fluorogens, both membrane impermeant, are available for the beta FAPs. They enable quantitative assays for receptors at the surface, receptor internalization/desensitization, receptor resensitization, and 2-color pulse-chase experiments.


A bibliography of over 70 papers describing FAP-based assays and other FAP applications can be viewed here.

Please contact us at for more information about these assays, or to discuss your specific assay needs.

Properties of our standard FAPs and fluorogens are summarized below

A chart of our standard FAPs and fluorogens is shown below.

FAP tags were developed at Carnegie Mellon University over the past decade and a half and are the subject of multiple patents including US8664364, US9023998, US9249306, US9688743, US9995679, and foreign equivalents. SpectraGenetics is the exclusive world-wide commercial licensee of these patents in the field of GPCR assays.