text: FRET in lanthanide probes is a widely used technique to measure the distance between two points separated by approximately 15–100 Angstrom. Measurements can be done under physiological conditions in vitro with genetically encoded dyes, and often in vivo as well. The technique relies on a distant- dependent transfer of energy from a donor fluorophore to an acceptor dye. Lanthanide probes has been used to study DNA-protein interactions (using a terbium chelate complex) to measure distances in DNA complexes bent by the CAP protein.