Mitochondria are the powerhouse of the cell. They are thought to have evolved more than a billion years ago from primitive bacterium which was engulfed by an early eukaryotic cell resulting in endosymbiotic relationships between the host cell and the newly formed organelle. During evolution the vast majority of the mitochondrial genetic material left the organelle and got integrated into the nucleus of the host cell. Hence, most of the mitochondrial proteins are synthesized outside of the organelle and have to be imported into the various internal mitochondrial compartments.