BID is a pro-apoptotic member of the BH3-only (BOPS) subset of the BCL-2 family of proteins that constitute a critical control point in apoptosis. Bid is the first of the BOPs reported to bind and activate Bcl-2, Bax, and Bak. Bid serves as a death-inducing ligand that moves from the cytosol to the mitochondrial membrane to inactivate Bcl-2 or to activate Bax.