Promotes apoptosis by activating caspases in the cytochrome c/Apaf-1/caspase-9 pathway. Acts by opposing the inhibitory activity of inhibitor of apoptosis proteins (IAP) (By similarity). inhibitory activity of inhibitor of apoptosis proteins (IAP). Inhibits the activity of BIRC6/bruce by inhibiting its binding to caspases (By similarity). Homodimer (By similarity). Interacts with BEX3 (By similarity). Interacts with BIRC2/c-IAP1 (via BIR3 domain) (By similarity). Interacts with BIRC6/bruce (By similarity). Interacts with BIRC7/livin (By similarity). Interacts with XIAP/BIRC4 (via BIR3 domain) (By similarity). Interacts with the monomeric and dimeric form of BIRC5/survivin (By similarity). Interacts with AREL1 (via HECT domain); in the cytoplasm following induction of apoptosis (By similarity). Released into the cytosol when cells undergo apoptosis. The mature N-terminus mediates interaction with XIAP/BIRC4. Ubiquitinated by BIRC7/livin.