JC-1 (5,5',6,6'-Tetrachloro-1,1',3,3'-tetraethylbenzimidazolylcarbocyanine, iodide) is a mitochondrial dye that stains mitochondria in living cells in a membrane potential-dependent fashion. JC-1monomer is in equilibrium with so-called J-aggregates, which are favored at higher dye concentration or higher mitochondrial membrane potential. The monomer JC-1 has green fluorescence ( Em = 527 nm), while the J-aggregates have red fluorescence ( Em = 590 nm). Therefore, it is possible to use fluorescence ratioing technique to study mitochondrial membrane potentials. JC-1 is particularly useful for apoptosis studies. In apoptotic cells, the dye stays in the cytoplasm and fluoresces green. It has also been used in high throughput drug screening applications.; ;