Mitochondria store a group of cysteine proteases called caspases in the space between the inner and outer mitochondrial membranes. These caspases can cleave various cellular substrates to disintegrate cellular contents. Activation of caspases lies at the heart of apoptosis regulation. Caspases exist as inactive proenzymes or zymogens and are regulated by Bcl-2 family members. During apoptosis, Bcl-2 family members regulate the release of caspases, which triggers a downstream event in apoptosis pathways.