All cells need energy which is ultimately derived from what we eat. By a series of enzyme reactions principally in the intestinal tract and the liver, food is turned into glucose which then circulates in the blood. Cells take up this glucose and the mitochondria take over, using oxygen to produce energy. Mitochondria themselves are small particles, known as organelles, that reside in the cell cytoplasm, outside the nucleus but inside the cell membrane. They have a fascinating evolution, being at one time free-living organisms related to bacteria. More than 1 billion years ago mitochondria were somehow "captured" by primitive cells with which they developed a symbiotic relationship. The cells provided mitochondria with nutrients and a safe environment while the mitochondria enabled cells to use oxygen for aerobic metabolism, something they were unable to do on their own. Aerobic metabolism is much more efficient than its anaerobic equivalent, which is all that primitive cells could manage. Mitochondria have retained their original "bacterial" DNA which is arranged in a circle and contains only 27 genes. A small segment of the mitochondrial DNA, known as the 'control' region, contains most of the genetic variation and it is this region, 400 DNA bases in length, that we sequence. The details of the control region sequence allow us to place any individual within one of the mitochondrial clan clusters. The matrilineal inheritance of mitochondria and hence mitochondrial DNA follow from the fact that mammalian eggs are full of mitochondria whereas sperm have very little, none of which survives in the egg after fertilisation. Thus we all inherit our mitochondrial DNA from our mothers alone. This is true for both men and women but only women pass their mitochondrial DNA on to the next generation .