Cellular inhibitor of apoptosis protein1 (cIAP-1, also known as BIRC2, MIHB, and HIAP2) and 2 (cIAP-2, also known as BIRC3, MIHC, and HIAP1), are members of the inhibitor of apoptosis (IAP) family of proteins that inhibit the proteolytic activity of mature caspases. Structurally, cIAP-1 and cIAP-2 are each comprised of 3 BIR (baculovirus inhibitor of apoptosis) domains, a RING finger domain, and a caspase recruitment domain (CARD). Human cIAP-1 and cIAP-2 share 70% sequence identity. Functionally, cIAPs inhibit caspases through the direct interaction of its BIR domain with the active caspase. The ring finger domain of cIAP-1 and cIAP-2 also functions as an E3 ubiquitin ligase to ubiqutinylate specific target proteins. Caspase activity may be restored by mitochondrial proteins, such as SMAC/Diablo or HtrA2/Omi, through interactions with the Reaper-like motif and the BIR domain. cIAPs are reported to be cleaved by HtrA2/Omi.