JC-1 is a dual-emission potential-sensitive probe that can be used to measure mitochondrial membrane potential. JC-1 is a green-fluorescent (λex 520 nm) monomer at low membrane potential. At higher potentials, JC-1 forms red-fluorescent (λem 596 nm ) "J-aggregates," which exhibit broad excitation and very narrow emission spectra. The ratio of red to green fluorescence of JC-1 is dependent only on membrane potential, and not influenced by mitochondrial size, shape, or density. JC-1 is particularly useful for apoptosis studies. In apoptotic cells, the dye stays in the cytoplasm and fluoresces green, while in healthy cells, the dye aggregates in the mitochondria and fluoresces red.