The iATPSnFR(Unk) entry in FlyBase represents a genetically encoded single-wavelength fluorescent sensor for ATP, where it is known that the sensor is based on the design in PMID:30755613, but the particular iATPSnFR sensor variant used is unknown/unspecified. The iATPSnFR design in PMID:30755613 is based on the epsilon subunit of Bacillus sp. PS3 F[[0]]F[[1]]-ATP synthase (UniProtKB:P07678), into which a circularly permuted form of sfGFP has been inserted, flanked by linkers. Upon binding to ATP, conformational changes in the F[[0]]F[[1]]-ATP synthase epsilon subunit result in an increase in fluorescence.