The Bcl-2 family is composed of over 20 members that regulate processes leading to apoptosis. Most members such as Bcl-2, Bcl-Xl, and Bcl-W inhibit apoptosis by preventing the release of cytochrome c and downstream caspase activation. This, in turn, is opposed by the function of family members Bax and Bak which interact directly with mitochondrial membrane proteins. Other pro-apoptotic members such as Bid, Bad, Puma, Bim and Bik are not significantly similar in protein structure to Bcl-2, but are considered family members based on their ability to bind and inhibit the antiapoptotic Bcl-2 proteins. ELISA kits targeting Bcl-2 family proteins are useful tools in investigating apoptosis and cell proliferation.