E3 ubiquitin-protein ligase XIAP / BIRC4, also known as inhibitor of apoptosis protein 3, X-linked inhibitor of apoptosis protein, and IAP-like protein, is a protein that belongs to a family of apoptotic suppressor proteins. Members of this family share a conserved motif termed, baculovirus IAP repeat, which is necessary for their anti-apoptotic function. XIAP / BIRC4 functions through binding to tumor necrosis factor receptor-associated factors TRAF1 and TRAF2 and inhibits apoptosis induced by menadione, a potent inducer of free radicals, and interleukin 1-beta converting enzyme. XIAP / BIRC4 also inhibits at least two members of the caspase family of cell-death proteases, caspase-3 and caspase-7. Mutations in this encoding gene are the cause of X-linked lymphoproliferative syndrome. Alternate splicing results in multiple transcript variants. Thought to be the most potent apoptosis suppressor, XIAP / BIRC4, directly binds and inhibits caspases -3, -7 and -9. Survivin, which also binds to several caspases, is up-regulated in a many tumour cell types. Defects in XIAP / BIRC4 are the cause of lymphoproliferative syndrome X-linked type 2 (XLP2). XLP is a rare immunodeficiency characterized by extreme susceptibility to infection with Epstein-Barr virus (EBV). Symptoms include severe or fatal mononucleosis, acquired hypogammaglobulinemia, pancytopenia and malignant lymphoma.