Caspase family of cysteine proteases plays a key role in apoptosis. Caspase-9 is one of the most important caspases among the caspase family members. Upon induction of apoptosis, cytochrome c released from mitochondria associates with procaspase-9/Apaf-1. The complex processes procaspase-9 into a large subunit (35 kDa or 17-25 kDa) and small (10 kDa) by self-cleavage at D315. Activated caspase-9 further cleaves other caspase members including caspase-3, one of the proteases responsible for the proteolytic cleavage of many key proteins in apoptosis. In addition to self-cleavage, procaspase-9 can also be cleaved in vivo by caspase-3 at D330. The process served as a positive feedback to amplify the apoptotic signal in caspase activation pathway.