The BH3 domain-only protein, BID, a death agonist member of the Bcl-2/Bcl-xL family (1), is localized in the cytosolic fraction of cells as an inactive precursor (2,3). Its active form is generated upon proteolytic cleavage by caspase-8 in the Fas signaling pathway. Cleaved BID translocates to mitochondria and induces cytochrome c release and mitochondrial damage (2-5). Thus, BID relays an apoptotic signal from the cell surface to mitochondria. However, the precise molecular mechanism for the translocation of the cleaved BID, and for the subsequent release of cytochrome c during apoptosis, is still unclear.