The apoptosome is a large enzyme complex composed of Apaf-1 and cytochrome c. The proposed holoenzyme structure comprises of seven pairs of Apaf-1-cytochrome c subunits arranged circularly like spokes forming a wheel. In the intrinsic pathway of apoptosis, the apoptosome is assembled upon the release of cytochrome c from the mitochondria into the cytoplasm. In the presence of ATP it then recruits and activates procaspase-9 to activate downstream caspases which carry out apoptosis. For the crucial role of the apoptosome in cell death and proliferation, antibodies that target components Apaf-1 and cytochrome c may serve as highly useful tools in areas such as cancer research and therapeutics.