Two decades of genetically encoded biosensors based on Förster resonance energy transfer

: Two decades have passed since the development of the first calcium indicator based on the green fluorescent protein (GFP) and the principle of Forster resonance energy transfer (FRET). During this period, researchers have advanced many novel ideas for the improvement of such genetically encoded FRET biosensors, which have allowed them to expand their targets from small molecules to signaling proteins and physicochemical properties. Although the merits of "genetically encoded" FRET biosensors became clear once various cell lines were established and several transgenic organisms were generated, the road to these developments was not necessarily a smooth one. Moreover, even today the development of new FRET biosensors remains a very labor-intensive, trial-and-error process. Therefore, at this junction, it may be worthwhile to summarize the progress of the FRET biosensor and discuss the future direction of its development and application.Key words: FRET, biosensor, fluorescent protein.