The key role of cytochrome c in apoptosis is widely accepted. However, the requirement of cytochrome c for apoptosome formation has been defined mostly by in vitro assays. In vivo, cytochrome c translocation from the mitochondria to the cytosol is an indicator of apoptosis, but this association does not address the actual mechanism, requirements and the sequence of events leading to apoptosis. Proof of cytochrome c requirement for in vivo apoptosis has been hampered by the lack of mutants and its dual role. In addition, we and others have also shown that defects in oxidative phosphorylation can protect cells from an apoptotic death Therefore, it is important to differentiate the oxidative phosphorylation effect from the apoptosome assembly catalysis role of cytochrome c. To better define the mechanisms associated with the intrinsic apoptotic mechanism (the mitochondrial pathway), we propose to develop and analyze cytochrome c deficient cells and mice These studies have far reaching implications to different areas of medical and biological research, including cancer, neurodegeneration and development.