Loss of mitochondrial membrane potential is indicative of apoptosis. ICT’s MitoPT TMRE Assay utilizes the potentiometric fluorescent dye TMRE to detect mitochondrial membrane permeability and membrane depolarization. The TMRE dye has a delocalized positive charge dispersed throughout its molecular structure. In addition, its lipophilic solubility enables it to be readily membrane permeant and penetrate living cells. The TMRE dye enters the negatively charge mitochondria where it accumulates and fluoresces orange upon excitation. When the mitochondrial membrane potential collapses in apoptotic cells, MitoPT TMRE becomes distributed throughout the cytosol. Detection of the loss of orange-red fluorescence in TMRE stained cells is a reliable method for assessing apoptosis induction or oxidative stress-induced mitochondrial depolarization in experimental cell populations.