The loss of mitochondrial membrane potential is a hallmark for apoptosis. It is an early event preceding phosphatidylserine externalization and coinciding with caspase activation. JC-1 Mitochondrial Membrane Potential Detection Kit is used to measure mitochondrial membrane potential changes in cells. In non-apoptotic cells, JC-1 accumulates as aggregates in the mitochondrial membranes, resulting in red fluorescence (590 nm) . The brightness of red fluorescence is proportional to the potential and varies among different cell types. However, in apoptotic and necrotic cells, which have diminished mitochondrial membrane potential, JC-1 exists in the green fluorescent (529 nm) monomeric form. This kit provides a step-by-step protocol and ready-to-use reagents for performing assays for flow cytometry, fluorescence microscopy, or fluorometric microplate reader.