Reduced expression or mutation of TRAIL death receptor DR4 (6,7), dysfunction of caspase-8 (4), lack of Smac/DIABLO in the cytoplasm or a reduction in the release of Smac/DIABLO from mitochondria (8), overexpression of Bcl-2, loss of Bax or Bak function (3), or overexpression of the inhibitor of apoptosis (IAP) family of proteins, including X-linked inhibitor of apoptosis protein (XIAP), may all lead to the development of resistance of tumor cells to TRAIL. This evidence concerns the gene XIAP and neoplasm.