DEATH ENZYMES. “Caspases are constitutively present in cells as inactive zymogens with variable-length amino-terminal prodomains. Caspases themselves have caspase cleavage sites separating each domain, and activation of the pro-caspases requires proteolysis at these sites.”
http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1097-0177(200005)218:1%3C68::AID-DVDY6%3E3.0.CO;2-9/full