KMG-301AM is the acetoxy methyl esterified form of KMG-301. KMG-301AM successfully accumulates in mitochondria and then it is hydrolyzed to KMG-301. KMG-301 is an Mg 2+ -selective fluorescent probe functional in mitochondria in intact cells. Since the mitochondrial membrane is impermeable to KMG-301, it is not released upon depolarization of the mitochondrial membrane potential. KMG-301 can indicate changes in mitochondrial Mg2+ concentration and shows Mg 2+ transport across the mitochondrial membrane in the early phases of a cellular model.