Recent experiments from our own laboratory have established that culturing human cancer cells in the presence of metformin significantly enhances the expression of cytochrome c oxidase I (COX-1) and mitochondrial succinate dehydrogenase (SDH-A), which are encoded by mitochondrial and nuclear genomes, respectively (Oliveras-Ferraros C, Cufí S, Vazquez-Martin A, Menendez OJ, Martin-Castillo B, Joven J, Menendez JA. Here, MT-CO1 is linked to cancer.