Caspases are a family of cysteine proteases that centrally controls apoptotic machinery. Caspases can be grouped according to their substrate specificities that are largely determined by the amino acids preceding the cleavage site. One group of caspases that include -6, -8, (MACH/FLICE), and –9(V/LEXD) is specific for the substrate V/LEXD. This substrate is a site similar to those found in caspase proenzymes. This group of caspases may function as initiators of a proteolytic cascade by activating pro-caspases to amplify a death signal. A second group of caspases (-2, -3 and -7) is specific for the substrate DEXD that is related to sites found on target proteins cleaved during apoptosis. This suggests that this second group of caspases function during the effector phase of cell death. Protolytic proteases activate caspases that are expressed as inactive precursors. However, the caspase cascade is not yet well defined, and some caspases may function both upstream and downstream. The Bcl-2 protein inhibits apoptosis and is cleaved at ASP-34 by caspases during apoptosis and by recombinant caspase-3 in vitro. The cleavage of Bcl-2 and caspase-7 is abolished during the immunodepletion of caspase-3, however, the immunodepletion of caspase-7 has no effect on Bcl-2 cleavage. Caspase-3-dependent cleavage of Bcl-2 may promote further caspase activation as part of a positive feedback loop for apoptosis. (1)