XLP-2 instead arises from congenital mutations of the X-linked inhibitor of apoptosis (XIAP, also termed BIRC4; MIM no. 300635), a 497-amino acid member of the inhibitor of apoptosis protein (IAP) family that serves as a central regulator of apoptotic cell death by inhibiting caspases 3 and 7 (25–29). This evidence concerns the gene XIAP and X-linked lymphoproliferative disease.