The apoptosome is a large 1-megadalton complex comprised of proteins Apaf-1 and cytochrome c. The holoenzyme is understood to be composed of seven pairs of Apaf-1 and cytochrome c circularly ordered together like a wheel. When the intrinsic pathway of apoptosis is triggered, the release of mitochondrial cytochrome c into the cytoplasm induces binding with Apaf-1, forming the apoptosome. This in turn and activates procaspase-9 to activate the other apoptotic caspases. The apoptosome is critical player in cell death and proliferation- ELISA kits that measure its components Apaf-1 and cytochrome c may serve as valuable tools in therapeutic development and cancer research.