About 1 in every 400 people carry a disease-causing mutation of mitochondrial DNA, inherited from the CYTOPLASM of their mother's ovum which was fertilised by their father's sperm. Spermatozoa have little or no cytoplasm, so paternal inheritance of these diseases does not occur. Many of the resulting illnesses are devastating and not amenable to curative treatment. This has led to proposals to prevent such transmission by using IN VITRO FERTILISATION (IVF) where the nucleus of the affected ovum is isolated and inserted into a healthy donor ovum from which the nucleus has been removed. In this way, the resulting fetus receives nuclear DNA from his mother but not mitochondrial DNA. In the UK, any such procedure would require individual permission from the Human Fertilisation and Embryology Authority (see ASSISTED CONCEPTION).