FAST pertains to a chemical-genetic strategy for specific labeling of proteins.  A peptide domain, called "tag", is genetically encoded to be bound to a protein of interest (by combination of their respective genes by means of transfection or infection).  This tag is the anchor for a synthetic fluorescent probe to be further added.  Such chemical-genetic approach was already implemented besides natural fluorescent proteins such as GFP or their derivatives such as mCherry in several systems already widely used:
* since 2003, SNAP-tag, a bi-component reporting system consisting of a 19 kDa peptide derived from a human enzyme, O-methylguanine-ADN methyltransferase, evolved to form covalent bonds with fluorescent O-benzylguanine derivatives; SNAP-tag was later evolved into an orthogonal tag, CLIP-tag;
* since 2008, HaloTag, a bi-component reporting system consisting of a 33 kDa peptide derived from a bacterial enzyme, a haloalkane deshalogenase, which can specifically bind functional halogenated synthetic ligands, most often fluorescent for cell imaging (e.g., Coumarine, Oregon Green, Alexa Fluor 488, diAcFAM, TMR).
Several versions of FAST have been described differing by a small number of mutations, e.g., FAST1 (a.k.a. Y-FAST), FAST2 (a.k.a. iFAST), or a dimer, td-FAST.  Also, a complementation split version for monitoring protein-protein interactions was developed, splitFAST.  A number of plasmids displaying FAST or splitFAST genes are available at Addgene.