The discovery of a novel proteolytic network in the cytosol of metazoan cells comes as a surprise to a field that previously considered intracellular proteolysis to mainly be a mechanism to process antigen or remove effete proteins. The network, supported by caspases, a family of cysteine proteases that specifically cleave proteins after Asp residues, involves limited proteolysis of a growing number of cellular substrates. It is not surprising that a signaling pathway utilizes proteolysis, for this is the essence of the blood coagulation cascade. What is surprising is that the caspase pathway takes place in the interior of a cell. Caspase precursors are usually activated at internal conserved Asp residues, with the result that most activated caspases can process their own and other caspase zymogens given sufficient time and a high enough concentration in vitro (see for example,