The SunTag(Unk) entry in FlyBase represents an epitope that is known to be derived from the the Saccharomyces cerevisiae GCN4 gene ( SGDID:S000000735 ) and which is bound robustly and specifically by the scFv(GCN4) single-chain variable fragment antibody, but where the precise engineered variant is unknown/unspecified. If a protein of interest is tagged with a repeating array of the SunTag(Unk) epitope, multiple copies of scFv(GCN4) can be recruited to the SunTag(Unk) 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(Unk) array can be visualized with very high sensitivity, even at the single molecule level (PMID:25307933).