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). Inhibits the activity of BIRC6/bruce by inhibiting its binding to caspases. Isoform 3 attenuates the stability and apoptosis-inhibiting activity of XIAP/BIRC4 by promoting XIAP/BIRC4 ubiquitination and degradation through the ubiquitin-proteasome pathway. Isoform 3 also disrupts XIAP/BIRC4 interacting with processed caspase-9 and promotes caspase-3 activation. Isoform 1 is defective in the capacity to down-regulate the XIAP/BIRC4 abundance. Homodimer (PubMed:10972280). Interacts with BEX3 (By similarity). Interacts with BIRC2/c-IAP1 (via BIR3 domain) (PubMed:19153467). Interacts with BIRC6/bruce (PubMed:15200957). Interacts with BIRC7/livin (PubMed:16729033). Interacts with XIAP/BIRC4 (via BIR3 domain) (PubMed:21695558, PubMed:11140637). Interacts with the monomeric and dimeric form of BIRC5/survivin (PubMed:21536684). Interacts with AREL1 (via HECT domain); in the cytoplasm following induction of apoptosis (PubMed:23479728). Released into the cytosol when cells undergo apoptosis. Ubiquitously expressed with highest expression in testis. Expression is also high in heart, liver, kidney, spleen, prostate and ovary. Low in brain, lung, thymus and peripheral blood leukocytes. Isoform 3 is ubiquitously expressed. The mature N-terminus mediates interaction with XIAP/BIRC4. Ubiquitinated by BIRC7/livin. The disease is caused by variants affecting the gene represented in this entry.