Bid, a BH3 domain-containing proapoptotic Bcl-2 family member, 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 releases its potent proapoptotic activity, which in turn induces cytochrome c release and mitochondrial damage. The cytochrome c releasing activity of Bid was antagonized by Bcl-2. Mutation in the SH3 domain can diminish the cytochrome c releasing activity. In the animal model studies, Bid-deficient mice are found resistant to the lethal effects of death factor signals relayed throμgh Fas.