OGsor is a genetically-encoded FRET nanosensor for 2-oxoglutarate. It contains the 2-oxoglutarate-binding GAF domain of the nifA gene from Azotobacter vinelandii, flanked by the fluorescent proteins CFP and YFP, which act as FRET donor and acceptor respectively. Binding of the GAF domain by 2-oxoglutarate results in a conformational change which decreases FRET efficiency (PMID:23893310).