The iATPSnFR(2) entry in FlyBase represents a genetically encoded single-wavelength fluorescent sensor for ATP, as described in PMID:38748581. It 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. iATPSnFR(2) is a variant of iATPSnFR(1.0) in which the linkers joining the two domains have been optimized to produce a five to sixfold improvement in the dynamic range and affinity variants varying from 4 μM to 500 μM. Upon binding to ATP, conformational changes in the F[[0]]F[[1]]-ATP synthase epsilon subunit result in an increase in fluorescence.