The FLAG-tag (DYKDDDDK) is a short peptide added to proteins using recombinant DNA technology. It is commonly used for protein purification via affinity chromatography and for studying proteins in living cells. The tag helps distinguish recombinant proteins from native ones and allows isolation of multi-subunit complexes without disrupting them. It enables purification of proteins suitable for structural studies like x-ray crystallography. The FLAG-tag is recognized by specific antibodies, making it useful in assays such as immunofluorescence, SDS-PAGE, and Western blotting, especially when no antibody is available for the target protein.