The field got off to a bad start, with the widely reported death of an American liver patient in 1999. In 2003 some French children who were being treated with it for an immune-system problem called SCID developed leukaemia. Since then, though, things have improved. Indeed one procedure, for lipoprotein lipase deficiency (which causes high levels of blood fats, with all the problems those can bring), has been approved, in Europe, for clinical use.

The most recent success, announced last month in theLancet, was of an experimental treatment for choroideremia, a type of blindness. This is caused by mutation of the gene for a protein called REP1. Without REP1, the eye’s light receptors degenerate. Robert MacLaren of Oxford University used a virus to deliver working versions of the REP1 gene to the most light-sensitive part of the retina. Five of the six participants in the trial duly experienced an improvement in their sensitivity to light. Two were so improved that they could read more letters than previously on a standard eye chart.

Dr MacLaren’s work complements that of Albert Maguire and Jean Bennett at the University of Pennsylvania, who use gene therapy to treat another eye disease, Leber’s congenital amaurosis. A defective version of a gene called RPE65 means that, in this condition, retinal cells are starved of vitamin A, which also causes blindness. Putting normal copies of RPE65 into the retina leads, as with REP1, to greater light sensitivity and—sometimes—clearer vision.