The most thoroughly characterized mammalian IAP is XIAP/BIRC4, which can inhibit caspases 9, 3 and 7, but may alsoregulate apoptosis through interactions with other proteins such as Smac/DIABLO, HtrA2/Omi, XAF1, TAK1, cIAP1, andcIAP2. High throughput sequencing of the mouse genome revealed the existence of a gene resembling Xiap/Birc4 onmouse chromosome 7. To confirm the existence of this gene, and to determine its functional significance, we performedSouthern and Northern blot analysis. This showed the presence of the Xiap-like gene in both wild-type and Xiap geneknock-out mice, but the corresponding mRNA was not detected in any tissues examined by Northern blot. Analysis of thegene sequence in all three possible reading frames predicts that expression of this gene would not give rise to a full-lengthprotein, but only non-functional truncated polypeptides. Because its nucleotide sequence is 92% identical to Xiap, but it hasno introns corresponding to those of Xiap, we conclude that Xiap-ps1 is a pseudogene generated by retro-transposition of aspliced Xiap message to chromosome 7.