scFv(GCN4) is a single-chain variable fragment antibody that robustly and specifically binds 'SunTag' epitopes; these epitopes are originally derived (PMID:10644744) from the Saccharomyces cerevisiae GCN4 gene ( SGDID:S000000735 ) and may have been further optimized for binding to scFv(GCN4) (PMID:25307933). If a protein of interest is tagged with a repeating array of a SunTag epitope, multiple copies of scFv(GCN4) can be recruited to the SunTag epitope array. This property of 'signal amplification via protein multimerization' has a number of applications in vivo. For example, if the scFv(GCN4) antibody sequence is fused to a fluorescent protein, expression of a protein of interest tagged with a SunTag epitope array can be visualized with very high sensitivity, even at the single molecule level (PMID:25307933).