X-linked inhibitor of apoptosis protein (XIAP)-associated factor 1 (XAF1) is azinc finger protein that blocks the anti-apoptotic activity of XIAP. XIAP is amember of the family of intrinsic inhibitors of apoptosis proteins (IAPs), whichsuppress apoptosis through the inhibition of caspases. In the presence ofXAF1, XIAP protein redistributes from the cytosol to the nucleus. XAF1 transcripts(3.9-, 4.5-, 6.0- and 7.0-kb) are present at high levels in heart and ovary.Low expression of XAF1 mRNA is an indicator for certain cancers (WM164melanoma, WM35 melanoma, U937 pro-monocytic leukemia and HT1080fibrosarcoma), suggesting that low levels of XAF1 transcript may enhancecancer cell-survival through the relative increase in XIAP anti-apoptotic function.IFN-α and IFN-β activate the human XAF1 gene, which maps to chromosome17p13.1.