Apoptosis is a type of cell death carried out by an intrinsic cellular machinery. Apoptosis is an attractive target for cancer therapy because even drug-resistant cancer cells contain the apoptotic machinery, which can destroy the cell within an hour. Our current knowledge is sufficient to design and implement tools that kill cells by apoptosis quickly and efficiently. However, because the apoptotic machinery is present in most if not all mammalian cells, the major problem is how to induce apoptosis in cancer cells selectively. One approach is based on a paradoxical observation that some oncogenes, including Myc and adenoviral E1A, either induce apoptosis or sensitize cells to cytotoxic agents, including those used for chemotherapy. This implies that some oncogenes have pro-apoptotic activities that are specific for transformed cells. We think that understanding how apoptosis is activated in response to expression of oncogenes will help to understand how to kill cancer cells selectively. During the previous funding period we found that E1A sensitizes primary fibroblasts to chemotherapeutic agents by facilitating activation of caspase-9, a protease involved in apoptosis. We learned that expression of E1A affects at least two steps in caspase-9 activation: the release of cytochrome c from mitochondria, and the subsequent formation of the caspase-9 holoenzyme. Our results indicate that E1A affects these steps using a dual mechanism, by activating pro-apoptotic proteins and by repressing inhibitors of apoptosis. Here we propose to determine how precisely E1A accomplishes these effects. This will be accomplished (i) by learning how E1A regulates activation of Bax, a protein involved in cytochrome c release; (ii) by characterizing and identifying IODA, an activity that inhibits this release; and (iii) by investigating how oncogenes regulate formation of the caspase-9 holoenzyme. These studies will advance our understanding of the mechanisms a cell uses to kill itself and will help determine how these mechanisms can be manipulated to selectively induce cell death in cancer cells.