Bid is a pro-apoptotic “BH3 domain-only” member of the Bcl-2 family originally discovered to interact with both the anti-apoptotic family member Bcl-2 and the pro-apoptotic protein Bax. Bid is normally localized in the cytosolic fraction of cells as an inactive precursor and is cleaved at Asp60 by caspase-8 during Fas signaling, leading to translocation of the carboxyl terminal p15 fragment (tBid) to the mitochondrial outer membrane. Translocation of Bid is associated with release of cytochrome c from the mitochondria, leading to complex formation with Apaf-1 and caspase-9 and resulting in caspase-9 activation. Thus, Bid relays an apoptotic signal from the cell surface to the mitochondria triggering caspase activation.