QuasAr2-mOrange2-eFRET is a genetically encoded voltage indicator that acts as a fluorescent reporter for membrane voltage. It is derived from the QuasAr2-mOrange2 sensor, which consists of the 'QuasAr2' voltage-sensitive sequence (a mutated form of the Halorubrum sodomense Archaerhodopsin-3 (aop3) gene, UniProtKB:P96787, which does not generate a photocurrent) fused to the mOrange2 fluorescent protein. Compared to QuasAr2-mOrange2, QuasAr2-mOrange2-eFRET contains a deletion of the 14 C-terminal residues of QuasAr2 (APEPSAGADVSAAD) and a deletion of the N-terminal 7 residues of mOrange2 (VSKGEEN) and the linker sequence between the two sequences has been changed from RPVVA to LR. QuasAr2-mOrange2-eFRET reports changes in voltage via electrochromic FRET (eFRET); voltage-induced changes in the absorption spectrum of the QuasAr2 protein alter the degree of nonradiative quenching of the closely fused mOrange2 protein (PMID:25118186).