A human cell is made up of many components, including mitochondria which provide the main source of energy for the cell. Mitochondria contain their own genetic material (DNA) mutations in which can give rise to variable effects ranging from neurodegenerative disease to diabetes. Some women are known to be carriers of defective mitochondrial DNA, which means that they may pass devastating conditions on to any children they have. These researchers have developed a new application of a technique which is routine in their in vitro fertilisation (IVF) unit. By testing a woman's egg cells, they can estimate how likely it is that any children she might have would be severely affected, so that she can be properly advised before becoming pregnant.