The inhibitor of apoptosis domain -- also known as IAP repeat, ** B aculovirus I nhibitor of apoptosis protein R epeat**, or BIR -- is a structural motif found in proteins with roles in apoptosis, cytokine production, and chromosome segregation. Proteins containing BIR are known as inhibitor of apoptosis proteins (IAPs), or BIR-containing proteins (BIRPs or BIRCs), and include BIRC1 (NAIP), BIRC2 (cIAP1), BIRC3 (cIAP2), BIRC4 (xIAP), BIRC5 (survivin) and BIRC6. BIR domains belong to the zinc-finger domain family and characteristically have a number of invariant amino acid residues, including 3 conserved cysteines and one conserved histidine, which coordinate a zinc ion. They are typically composed of 4-5 alpha helices and a three-stranded beta sheet.