One of the most extensively studied types of cellular stress is pro-apoptotic stress. In apoptosis, extrinsic, or intrinsic pathways lead to programmed disassembly of the cell. Cysteine proteases called caspases are activated and cleave a select set of cellular proteins during programmed cell death. Different types of stress activate specific initiator caspases, which then activate effector caspases (Boatright and Salvesen, 2003). Not all caspases are involved in cell death however.