SunTag(v4) is a 19 residue peptide (EELLSKNYHLENEVARLKK) that is bound robustly and specifically by the scFv(GCN4) single-chain variable fragment antibody (PMID:25307933). SunTag(v4) is an optimized version of a scFv(GCN4) compatible epitope originally derived (PMID:10644744) from the Saccharomyces cerevisiae GCN4 gene ( SGDID:S000000735 ). If a protein of interest is tagged with a repeating array of the SunTag(v4) epitope, multiple copies of scFv(GCN4) can be recruited to the SunTag(v4) 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(v4) array can be visualized with very high sensitivity, even at the single molecule level (PMID:25307933).