Butterfly1.2 is a genetically encoded fluorescent voltage indicator that is designed to report subthreshold voltage fluctuations in neurons. It consists of the voltage-sensing domain from the Ciona intestinalis VSP gene (GB:AB183035) sandwiched between the mKate2 (FRET acceptor) and the Citrine (FRET donor) fluorescent proteins. In addition, a Golgi export motif from Kir2.1 has been added to the N-terminal end of the coding sequence, to improve targeting to the plasma membrane. Butterfly1.2 protein shows a voltage-dependent FRET with a decrease in Citrine emission and an increase in mKate2 emission upon membrane depolarization (PMID:22815406).