This mix-and-match of domains in Bcl-2 family proteins is important, because these domains are protein-protein interaction domains that govern the ability of Bcl-2 proteins to associate with other proteins, including other Bcl-2 proteins. Under normal conditions, it's the guys with the white hats, like Bcl-2 and Bcl-X_L, who run the town and keep the bad guys  in their place. They do this by binding to and "sequestering" the black hats. In short, under normal, "survival" conditions, the white hats keep the black hats locked up in the county jail.

But what happens after a pro-apoptotic insult?