AviTag fusion proteins are engineered recombinant proteins that incorporate a short, 15-amino-acid peptide sequence (Gly-Leu-Asn-Asp-Ile-Phe-Glu-Ala-Gln-Lys-Ile-Glu-Trp-His-Glu) at the N- or C-terminus of a target protein. This tag is specifically recognized by the biotin ligase enzyme (BirA) and is enzymatically biotinylated at a designated lysine, ensuring a single, well-defined biotin attachment. The result is a uniform, functionally active protein that maintains its native structure while gaining the capacity to bind avidin/streptavidin with very high affinity.