U.S. researchers have helped nearly blind children and adults see again through an experimental gene therapy.

Five children and seven adults who suffered a special form of congenital blindness were able to see again enough to navigate a low-light obstacle course, the researchers from the University of Pennsylvania School of Medicine and the Center for Cellular and Molecular Therapeutics at the Children’s Hospital of Philadelphia said. Results were published in the journal The Lancet.

The patients suffered from Leber’s congenital amaurosis (LCA), a condition which sets on in childhood and can lead to total blindness at age 30 to 40. The 12 patients were treated with genetic material in their worst eye. The material was carried piggyback on a virus into the defective eye cells.

All 12 patients showed improvements, but the biggest progress was made by children age 8, 9, 10 and 11. The success of the therapy depended on how far the retina had degenerated.

The patients did not attain normal eyesight, but half of them improved enough that they can no longer be classified as legally blind.

The clinical benefits have persisted for nearly two years since the first subjects were treated with injections of therapeutic genes into their retinas.

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