a, On the basis of the latest structural information2, we speculate that the binding of BIM SAHB to the ‘back’ of BAX, and the ensuing conformational changes, constrain the ‘front’ of the molecule, including the hydrophobic BH pocket, causing the carboxy-terminal region (not shown) and α-helix 4 containing the BH3 domain (green) to move out of the BH pocket and create a groove. Two such BAX molecules could then dimerize ‘nose-to-nose’ through interactions between the BH3 domain and the hydrophobic BH groove. b, Interaction at an additional, undefined interface might result in the formation of BAX oligomers and so permeabilization of the outer mitochondrial membrane. On BAX activation, the inducer of activation — in this case BIM SAHB, but presumably any activator — might dissociate to allow higher-order oligomer formation.