A second-generation synthetic antisense oligonucleotide with potential antineoplastic activity. AEG35156 selectively blocks the cellular expression of X-linked inhibitor of apoptosis protein (XIAP), a pivotal inhibitor of apoptosis that is overexpressed in many tumors. This agent reduces total levels of XIAP in tumor cells, working synergistically with cytotoxic drugs to overcome tumor cell resistance to apoptosis. XIAP interferes with both the intrinsic and extrinsic program-death signaling pathways, which may render tumor cells resistant to apoptosis. OR A substance being studied in the treatment of cancer. AEG35156 may kill cancer cells by blocking the production of a protein called XIAP that helps cells live longer. It also makes cancer cells more sensitive to anticancer drugs. It is a type of antisense oligonucleotide, and a type of chemosensitizing agent. Also called GEM640.