The pro-domain of initiator caspases contains amino acid sequences that are caspase recruitment domains (CARD) or death effector domains (DED) that enable the caspases to interact with other molecules that regulate their activation. Activation of caspases involves proteolytic processing between domains, and then association of large and small subunits to form a heterodimer with both subunits contributing to the catalytic site. Two heterodimers associate to form a tetramer that has 2 catalytic sites that function independently. Some isoforms of caspases (e.g., caspase-9, isoform 2) are inactive proteolytically and function as dominant negative inhibitors of active forms.