BID, a BH3 domain containing proapoptotic Bcl2 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 Bcl2. Mutation in the SH3 domain can diminish the cytochrome c releasing activity. In animal model studies, Bid deficient mice are found resistant to the lethal effects of death factor signals relayed through Fas.