The BH3 domain-only protein BID, a death agonist member of the Bcl-2/Bcl-xL family, is localized in the cytosolic fraction of cells as an inactive precursor. 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. 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 unclear.