US scientists have claimed success using gene therapy to try to reverse a severe inherited sight disorder.
They injected material containing a corrective gene into the eyes of three patients with Leber congenital amaurosis (LCA).
The journal Proceedings of the National Academy of Sciences reports all three showed signs of "significant" improvement in their vision.
UK researchers carried out a similar procedure on three patients last year.
They believe the method could be ready for use within two years to treat people suffering from some inherited diseases of the retina, which affect 20,000 people in Britain.
Within three years, they believe it could be ready for testing on people who suffer age related macular degeneration, a condition that affects 500,000 Britons.
Gene therapy works on a simple principle - to replace a malfunctioning gene, and restore function to a part of the body affected by a genetic disorder.
In practice, however, it has proved very difficult to find ways to introduce the new gene copies in the correct tissues, and experiments in animals have had mixed results.
In the eye, however, gene therapy has shown more promise.
LCA affects approximately one in 80,000 people, causing progressively worsening vision, often starting in the first few years of life.
It is responsible for one in 10 severe sight disorders in children.
A fault in the RPE65 gene is to blame, and the gene therapy injects working copies of the gene into the back of the eye.
Just 30 days after the treatment was delivered into one eye of each of the three young adults involved in the US study, the improvements could be measured.
The researchers, from Pennsylvania University, the University of Florida and Cornell University, suggested that the function of "cones" in the retina, which are used in daytime and colour vision, could be boosted up to 50-fold - a "dramatic" improvement in function.
However, vision in the treated eyes was not perfect - with the patients showing an abnormally slow adaptation to low light levels.
In the UK, scientists and doctors at the Institute of Ophthalmology and Moorfields Eye Hospital in London have been working for some years on a similar approach.
In 2007, they were the first to perform a gene therapy operation on three people with LCA, and earlier this year reported significant improvement in one of these.
Dr James Bainbridge, one of the Institute of Ophthalmology researchers involved, said the US findings were a step forward, helping to fine tune the technique and maximising the chances of success.
He said: "This paper is important because it provides further evidence that gene replacement therapy can improve vision in people with this form of LCA.
"It also confirms the prediction that the extent of improvement depends on the number of surviving target cells at the time of intervention, and shows that both rod and cone photoreceptor cells can benefit.
"The paper adds to the body of evidence supporting the value of gene therapy for people with inherited disease."
Professor Robin Ali, who also worked on the UK study, said the latest paper gave scientists a much better idea of what dose of gene therapy would give the best results.
He said: "We can now begin to find out how much useful vision is restored by improving retinal sensitivity and whether the results are better in young children when we treat the whole retina using a high dose."