This gene encodes an inhibitor of apoptosis protein (IAP)-binding protein. The encoded mitochondrial protein enters the cytosol when cells undergo apoptosis, and it moderates the caspase inhibition of IAPs. Multiple polyadenylation sites have been found for this gene. Several alternatively spliced transcript variants that encode distinct isoforms have been described for this gene but the validity of some transcripts, and their predicted ORFs, has not been determined conclusively. The inhibitor of apoptosis (IAP) proteins regulate programmed cell death by inhibiting members of the caspase family of enzymes. A novel mammalian protein that binds to IAPs and neutralizes their inhibitory effect on caspases has been designated Smac/DIABLO. This is a mitochondrial protein that is released along with cytochrome c during apoptosis and activates the cytochrome c/Apaf-1/caspase-9 pathway. Analysis of the structural basis of Smac/DIABLO reveals that the N-terminal amino acids are required for binding of Smac/DIABLO to IAPs and activation of caspases. Smac/DIABLO is expressed in a variety of human and mouse tissues.