The present invention is directed to novel peptides and compositions capable of modulating apoptosis in cells, and to methods of modulating apoptosis employing the novel peptides and compositions of the invention. In one aspect, the invention is directed to a novel peptide designated the "GD domain," which is essential both to Bak's interaction with Bcl-x.sub.L, and to Bak's cell killing function. Methods of identifying agonists or antagonists of GD domain function are provided. The GD domain is responsible for mediating key protein/protein interactions of significance to the actions of multiple cell death regulatory molecules.