In the first apparent success for gene therapy in adults, two German men have been cured of an genetic immune deficiency which made them vulnerable to life-threatening infections. A paper in Nature Medicine says that a 16-year-old and a 25-year-old had chronic granulomatous disease resulting from a defective gene on the maternal X chromosome. They were treated at a Frankfurt hospital with transfusions of genetically altered stem cells which carried the effective gene. Within 50 days, the men were free of chronic liver and lung infections.

The researchers warn that the results are still preliminary. "Each gene therapy with stem cells carries a certain risk of leukaemia," says Christof von Kalle, of the National Centre for Tumour Disease in Heidelberg. Gene therapy, though highly promising, has had a mixed history. In 1999 an American teenager died during a clinical trial. Three years later three French children developed leukaemia after treatment. Since 2002, all gene therapy trials in Germany were banned unless the patients were severely ill, had no other treatment option and were fully informed of the risks.