Mitochondria, which play an essential role in apoptosis, are specialized organelles, which contain an outer membrane separated from an inner membrane by an intermembrane space that contains many proapoptotic proteins, including cytochrome c. Decrease of MMP causes disruption of the outer mitochondrial membrane, which in turn contributes to release of cytochrome c. Because generation and acumination of ROS in cancer cells might be related to mitochondrial dysfunction and cell apoptosis, we attempted to characterize the relationship between ROS production and changes in the MMP. The gene discussed is CYCS; the disease is cancer.