One of the most exciting opportunities offered by genetic engineering is the development of gene therapy. A surprising number of human diseases are brought about by the presence of inherited faulty genes. Genetic engineering offers us the hope that in the not-too-distant future it will be possible to replace the faulty genes with healthy copies.

Most current research is looking at modifying the genetic material in the affected cells by adding healthy genes or switching off damaged genes. This is known as somatic (body) cell gene therapy. Such therapy has the potential to make a vast difference to the lives of affected people by removing the symptoms of their inherited condition. The problem is that if we simply change the genes which do not work properly in an individual – for example adding healthy CFTR genes to the lungs of people affected by cystic fibrosis – then we are only treating the disease, not curing it. The affected individual may pass on the faulty genes to their children, who in turn will need to be treated.

Gene therapy actually offers a way of curing genetic diseases. It would involve changing the genes of a fertilised egg or very early embryo so that the baby subsequently born had healthy genes in all its cells. This is known as germ line gene therapy. The individuals treated would not only have a healthy body themselves but would pass healthy genes on to any children they might have. But intervening at this level raises some major ethical issues and opinion across the world is strongly divided about it. Most countries, including the UK, have so far completely banned germ line gene therapy.