The Lily Foundation website states mitochondrial disease is caused by the mitochondria in the cells not producing enough energy, sometimes being not very efficient, or by not working at all. "Every person with Mitochondrial Disease is affected differently. Each individual affected will have a different combination of mitochondria that are working and not working within each cell." There is also a very useful Question & Answer section on the website. Not sure if this info is useful, although I found it helpful in getting an insight into the importance of ATP energy production in cells.