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BID is a member of the BH3-only subgroup of Bcl-2 family proteins that displays pro-apoptotic activity. The NH₂-terminal region of BID contains a caspase-8 (Casp-8) cleavage site and the cleaved form of BID translocates to mitochondrial membranes where it is a potent inducer of cytochromec release. Secondary structure and fold predictions suggest that BID has a high degree of α-helical content and structural similarity to Bcl-X_L, which itself is highly similar to bacterial pore-forming toxins. Moreover, circular dichroism analysis confirmed a high α-helical content of BID. Amino-terminal truncated BIDΔ1–55, mimicking the Casp-8-cleaved molecule, formed channels in planar bilayers at neutral pH and in liposomes at acidic pH. In contrast, full-length BID displayed channel activity only at nonphysiological pH 4.0 (but not at neutral pH) in planar bilayers and failed to form channels in liposomes even under acidic …