BID is a novel BH3 domain-only death agonist that interacts with both death agonists (BAX) and antagonists (BCL-2) of the BCL-2 family. It contains only the BH3 domain and lacks a carboxy-terminal signal-anchor segment, and is found in both cytosolic and membrane locations. BID counteracts the protective effect of BCL-2 and can induce apoptosis without additional death signals. Mutagenesis studies showed that the BH3 domain of BID is required for binding to the BH1 domain of BCL-2 or BAX. A BH3 mutant of BID that still heterodimerizes with BCL-2 failed to promote apoptosis, while a mutant that interacts with BAX but not BCL-2 retained death-promoting activity. These findings suggest that BID functions as a death ligand for the membrane-bound receptor BAX. BID's BH3 domain is essential for its death agonist activity and its interaction with BCL-2 or BAX. The study highlights the role of BID in the BCL-2 family's regulation of apoptosis, providing insights into the molecular mechanisms underlying cell death.BID is a novel BH3 domain-only death agonist that interacts with both death agonists (BAX) and antagonists (BCL-2) of the BCL-2 family. It contains only the BH3 domain and lacks a carboxy-terminal signal-anchor segment, and is found in both cytosolic and membrane locations. BID counteracts the protective effect of BCL-2 and can induce apoptosis without additional death signals. Mutagenesis studies showed that the BH3 domain of BID is required for binding to the BH1 domain of BCL-2 or BAX. A BH3 mutant of BID that still heterodimerizes with BCL-2 failed to promote apoptosis, while a mutant that interacts with BAX but not BCL-2 retained death-promoting activity. These findings suggest that BID functions as a death ligand for the membrane-bound receptor BAX. BID's BH3 domain is essential for its death agonist activity and its interaction with BCL-2 or BAX. The study highlights the role of BID in the BCL-2 family's regulation of apoptosis, providing insights into the molecular mechanisms underlying cell death.