JC-1 is a novel cationic carbocyanine dye that accumulates in mitochondria. This dye exists as a monomer at low concentrations and fluoresces green, similar to fluorescein. At higher concentrations, this dye forms J-aggregates with a broad excitation spectrum with an emission wavelength upper limit of ∼590 nm. These properties make JC-1 a sensitive marker of mitochondrial membrane potential. Another dye with similar properties is JC-9 (D-22421).