Cytochrome c is a 15kD protein found in the mitochondrial intermembrane space with a heme-binding domain. Cytochrome c is a component of the electron transport chain; the heme group transfers electrons from cytochrome b-c1 complex to cytochrome oxidase complex. Cytochrome c initiates apoptosis by release to cytoplasm and binding Apaf-1 which activates procaspase 9. Cytochrome c interacts with the cytochrome b-c1 complex, cytochrome oxidase complex, heme, Apaf-1, and Caspase 9 proteins.