X-linked inhibitor of apoptosis protein (XIAP) is a protein that stops apoptotic cell death. XIAP is a member of the inhibitor of apoptosis family of proteins (IAP). XIAP stops apoptotic cell death that is induced either by viral infection or by overproduction of caspases. Caspases are the enzymes primarily responsible for cell death. XIAP binds to and inhibits caspase 3, 7 and 9. The BIR2 domain of XIAP inhibits caspase 3 and 7, while BIR3 binds to and inhibits caspase 9. The RING domain utilizes E3 ubiquitin ligase activity and enables IAPs to catalyze ubiquination of self, caspase-3, or caspase-7 by degradation via proteasome activity. However, mutations affecting the RING Finger do not significantly affect apoptosis, indicating that the BIR domain is sufficient for the protein’s function. When inhibiting caspase-3 and caspase-7 activity, the BIR2 domain of XIAP binds to the active-site substrate groove, blocking access of the normal protein substrate that would result in apoptosis.