Cytochrome c is a 12-15 kDa electron transporting protein located in the inner mitochondrial membrane. Upon apoptotic stimulation,cytochrome c can be released from mitochondria into cytoplasm,resulting in caspase-3 activation and apoptosis. Measurement of cytochrome c release from the mitochondria is useful for detection of the onset of apoptosis in cells. In addition,cytochrome c can also leave cells and be detectable in extra-cellular medium of apoptotic cells and serum of cancer patients. The level of serum cytochrome c may serve as a prognostic maker during cancer therapy.