The pro-apoptotic molecules Bak and Bax are similar in their amino acid sequence and three-dimensional structure to the anti-apoptotic proteins Bcl-2, Bcl-xL and others. We use X-ray crystallography to understand the signature chemical differences between the pro-and anti-apoptotic family members and to interrogate the events that lead to Bak and Bax transiting from inert monomeric molecules to pore-forming oligomers.