A mitochondrial dye; incorporates into the cardiolipin-rich inner mitochondrial membrane; selectively binds to cardiolipin-containing liposomes over phosphatidylserine- and phosphatidylinositol-containing liposomes in vitro (Kas = 0.002, 7,000, and 7,000 mM, respectively); binding dependent on mitochondrial membrane potential; ex/em = 490/510 nm, respectively; has been used to quantify cardiolipin content and mitochondrial mass in isolated mitochondria and cellular systems,