Mitochondria are tiny structures found in most human cells. Their job is to make energy available for the cell’s operation, which is why they are often likened to batteries. A cell can contain hundreds of mitochondria. Each one holds a small amount of genetic material: a mere 37 genes.

The majority of our DNA – the 20,000 or so genes that affect how we look and behave – are held separately in the cell nucleus. People inherit all of their mitochondrial DNA from their mothers. None is passed down the male line.

Mitochondrial diseases are caused by faults in the DNA carried by mitochondria. They typically affect tissues and organs that need a lot of energy, such as the muscles, brain and heart. The conditions are often diagnosed in childhood, and steadily worsen as the child grows. Medical problems include epilepsy, strokes, early dementia, depression, cataracts, blindness, abnormal heartbeats, kidney failure, inability to absorb nutrients properly, muscle pain and weakness, breathing difficulties and damage to the nervous system. Most children diagnosed with a mitochondrial disease die prematurely.

Mitochondrial diseases are far from rare. Roughly one in every 250 babies carries abnormal mitochondrial DNA. While many of them will not suffer serious problems, at least one in 10,000 adults in the UK is severely affected by mitochondrial diseases.