Cytochrome c (m.w. 12,500) is an electron transport protein from mitochondria. It is released from mitochondria to cytoplasm during the early stages of apoptosis，prior to caspase activation，DNA fragmentation，and loss of membrane potential. The cytoplasmic cytochrome c is associated with Apaf-1 and caspase-9 to activate caspase-3 and other caspases.
