Procaspases are the inactive precursors to caspases. The procaspases are cleaved into two subunits that form a dimer. Two of these dimers combine to form the activated caspase.

Initiator procaspases have a long prodomain that contains a caspase recruitment domain (CARD). This domain enables the initiator pro-caspases to assemble with the adaptor proteins to form activation complexes. This happens when the cell receives a signal to undergo apoptosis. 

Next, when incorporated in such a complex, the initiator procaspases are brought in close proximity so that they can be activated and thus, can cleave each other to make the process irreversible. These in turn cleave and activate executioner procaspases and hence, a proteolytic caspase cascade is initiated.