Apoptosis, the process of active cell death, is a phenomenon important in the etiology and treatment of various diseases, including cancer. Existing cancer therapies kill tumor cells often by inducing apoptosis, and tumor cells that develop resistance to apoptosis can therefore be refractory to therapy. An understanding of the mechanisms of apoptosis is thus essential groundwork for the development of novel approaches to cancer therapy. Previous studies using cell-free systems and cultured cells have shown that the cytochrome c protein can play an important role in the activation of caspases (apoptotic proteases). However, evidence also shows that under some circumstances, caspases can be activated in a manner independent of cytochrome c. To test the hypothesis that cytochrome c is required for apoptosis in multiple tissues, the investigators will employ a genetic approach in mice. Homozygous null mutations of the somatic cytochrome c gene would be lethal, because this protein is required for mitochondrial respiration. However, they have identified mutant or variant forms of cytochrome c that exhibit no, or drastically reduced, pro-apoptotic activity in vitro, while retaining electron transport function. The investigators propose to generate mice in which the wild-type cytochrome c gene is replaced by one or more of these apoptosis-defective cytochrome c genes. If apoptosis in these mutant animals is disrupted in certain tissues, then this will confirm the hypothesis that cytochrome c is required for apoptosis in those tissues. However, if apoptosis proceeds normally, then it will be concluded that apoptosis pathways independent of cytochrome c predominate. These experiments will decide how important cytochrome c-mediated apoptotic pathways are in vivo and will help in the future design of pro-apoptotic therapy for cancer.