KeyTec® TR-FRET Streptavidin-HX is designed for developing the TR-FRET Assay. In the Protein-Protein Interaction assay, one Biotinylated protein binds to the acceptor (KeyTec® TR-FRET Streptavidin-HX), and the other protein is labeled (directly or indirectly) with the donor (KeyTec® TR-FRET Eu/Tb). When the two proteins interact, the donor molecule is brought into proximity with the acceptor molecule. Excitation of the donor will result in the generation of the TR-FRET signal at 665 nm, proportional to the extent of protein interaction.