X-chromosome-linked Inhibitor of Apoptosis (XIAP) is one of the most potent and versatile regulators of apoptosis. It belongs to the family of Inhibitors of Apoptosis Proteins (IAPs), which are intrinsic cellular regulators of cell death by binding and inhibiting distinct caspases. XIAP inhibits the initiator caspase-9 as well as the effector caspases-3 and -7. XIAP displays a modular structure consisting of three BIR domains and a C-terminal RING finger. The second BIR domain (BIR2) and its preceding linker region are required for inhibiting caspase-3 and -7 , whereas BIR3 is sufficient to inhibit caspase-9. More