FAST (Fluorescence-Activating and absorption-Shifting Tag) is a small, genetically-encoded, protein tag which allows for fluorescence reporting of proteins of interest.  Unlike natural fluorescent proteins and derivates such as GFP or mCherry, FAST is not fluorescent by itself.  It can bind selectively a fluorogenic chromophore derived from 4-hydroxybenzylidene rhodanine (HBR), which is itself non fluorescent unless bound.  Once bound, the pair of molecules goes through a unique fluorogen activation mechanism based on two spectroscopic changes, increase of fluorescence quantum yield and absorption red shift, hence providing high labeling selectivity.  The FAST-fluorogen reporting system can be used in fluorescence microscopy, flow cytometry and any other fluorometric method to explore the living world: biosensors, protein trafficking.
FAST, a small 14 kDa protein, was engineered from the photoactive yellow protein (PYP) by directed evolution.  It was reported for the first time in 2016 by researchers from Ecole normale supérieure de Paris.
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