JC-1 is a mitochondrial dye that stains mitochondria in Living cells in a membrane potential-dependent fashion. JC-1 monomer is in equilibrium with so-called J-aggregates, which are favored at higher mitochondrial membrane potential. The monomer JC-1 has green fluorescence (lem = 527 nm), while the J-aggregates have red fluorescence (lem = 590 nm). Therefore, it has been possible to use a fluorescence rationing technique to study mitochondrial membrane potential. 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.