X linked inhibitor of apoptosis protein (XIAP) associates with the active caspase 9 (CASP9) within the APAF1 apoptosome complex. XIAP consists of three baculoviral IAP repeat (BIR) domains and a COOH terminal RING domain (Duckett CS et al. 1996). The BIR3 region of XIAP binds to the amino terminus of the linker peptide on the small subunit of CASP9, which becomes exposed after proteolytic processing of procaspase 9 at Asp315 (Srinivasula SM et al. 2001). SMAC (DIABLO) competes with CASP9 for binding to BIR3 domain of XIAP promoting the release of XIAP from the CASP9:apoptosome complex (Du et al. 2000; Liu Z et al. 2000; Srinivasula SM et al. 2001).