IAP (Inhibitors of Apoptosis) is a family of functionally and structurally related proteins, which serve as endogenous inhibitors of programmed cell death (apoptosis). A common feature of all IAPs is the presence of a BIR in one to three copies. The human IAP family consists of 8 members, and IAP homologs have been identified in numerous organisms. The members of the IAPs included IAPs, Cp-IAP, Op-IAP, XIAP, c-IAPl, C-IAP2, NAIP, Livin and Survivin. The best characterized IAP is XIAP, which binds caspase-9, caspase-3 and caspase 7, thereby inhibiting their activation and preventing apoptosis. Also cIAP1 and cIAP2 have been shown to bind caspases, although how the IAPs inhibit apoptosis mechanistically at the molecular level is not completely understood.