Forster Resonance Energy Transfer (FRET) offers a tool to examine the physical distance of < 10 nanometers between two molecules, which is in a range for inter-molecular interactions. This technique overcomes the traditional optical resolution limit (approximately 200 nanometers) determined by the Rayleigh criterion, through utilizing the mechanism of fluorescence resonance energy transfer from a donor fluorophore in an excited state to a nearby acceptor fluorophore. Various fluorescent protein fusion vectors for FRET pairs, including the traditional ECFP-EYFP and more advanced variants Cerulean-Venus, are available through Plasmid Distribution Service.