Voltron is a chemigenetic voltage sensor. It consists of a rhodopsin from Acetabularia acetabulum, mutated to carry a D81N amino acid substitution which inactivates the proton pump, fused to a Halo-tag which can covalently bind a fluorophore dye ligand such as JF[[525]]. Changes in membrane potential result in a change in the level of fluorescence resonance energy transfer (FRET) quenching of the fluorophore dye by the rhodopsin; depolarization results in decreased fluorescence (FBrf0246298).