It has long been believed that correcting genes in the womb is one of the best ways to treat congenital diseases. Studies on animals have included injection of modified viruses into amniotic fluid, maternal blood or the foetus. However, this has failed to deliver enough genes, sometimes even injuring the foetus. Now, Marie-Claude Senut at the Salk Institute for Biological Studies in La Jolla, California, USA, has managed to deliver hormones and enzymes to the foetal bloodstream without permanently altering the tissues through genetically modified rat placenta. The researchers took tissues from pregnant rats and inserted a gene for human growth hormone (HGH) into them using a retrovirus. These cells were then injected into the placentas of pregnant rats midway through gestation. Foetuses removed several days later revealed significant levels of HGH in the blood. This technique uses the placenta, which transports material from the maternal to the foetal bloodstream. Paul McCray of the College of Medicine, University of Iowa, says that this can be used to deliver any soluble factor, such as clotting proteins for foetuses with haemophilia ( New Scientist , Vol 158, No 2132).