X-linked inhibitor of apoptosis protein (XIAP), also known as inhibitor of apoptosis protein 3 (IAP3) and baculoviral IAP repeat-containing protein 4 (BIRC4), is a protein that stops apoptotic cell death. In humans, this protein (XIAP) is produced by a gene named XIAP gene located on the X chromosome. XIAP is a member of the inhibitor of apoptosis family of proteins (IAP). IAPs were initially identified in baculoviruses, but XIAP is one of the homologous proteins found in mammals. It is so called because it was first discovered by a 273 base pair site on the X chromosome. The protein is also called human IAP-like Protein (hILP), because it is not as well conserved as the human IAPS: hIAP-1 and hIAP-2. XIAP is the most potent human IAP protein currently identified.