XAF-1 binds to XIAP, an inhibitor of caspases-3, -7, -9 and triggers its relocation from the cytosol to the nucleus. Overexpression of XAF-1 results in the neutralization of XIAP’s ability to inhibit cell death. XAF-1is normally expressed in all adult and fetal tissues but was found to be present in very low levels in a variety of cancer cell lines. In contrast, XIAP levels have been shown to be high in a majority of cell lines.