Multi-functional protein which regulates not only caspases and apoptosis, but also acts as an E3 ubiquitin-protein ligase mediating ubiquitination and subsequent proteasomal degradation of its target proteins. Acts as a direct caspase inhibitor. A key apoptotic suppressor in eggs. Acts as a positive regulator of Wnt signaling. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Monomer, and homodimer. The BIR and RING-type domains are dispensable for anti-apoptotic function. Degraded in a 2-step mechanism; a caspase-independent first step and a caspase-dependent second step. Stabilized indirectly by MAPK, which acts to delay caspase activation, rather than directly phosphorylating xiap. Belongs to the IAP family.