KeyTec® TR-FRET Streptavidin-Solar Eu is designed for developing the TR-FRET Assay. In the Protein-Protein Interaction assay, one Biotinylated protein binds to the donor (KeyTec® TR-FRET Streptavidin-Solar Eu*1), and the other protein is labeled (directly or indirectly) with the acceptor (KeyTec® TR-FRET LA/HX*2). 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.