BBC science correspondent
See how the treatment has changed Steven Howarth's vision
A 18-year-old whose sight was failing has had his vision improved in a pioneering operation carried out by doctors at Moorfields Eye Hospital.
The London researchers used gene therapy to regenerate the dying cells in Steven Howarth's right eye.
As a result he can now confidently walk alone in darkened rooms and streets for the first time.
Steven, from Bolton, is the third person to have the operation - doctors expect better results in future cases.
Before the procedure, he could hardly see at all at night and in time he would have lost his sight completely.
Confidence
His condition - Leber's congenital amaurosis - was due to a faulty gene that meant that the light-detecting cells at the back of his eye were damaged and slowly degenerating further.
RETINAL GENE THERAPY -
STEP BY STEP
But, in a delicate operation, surgeons at Moorfields injected working copies of the gene into the back of Steven's eye.
After a few months, doctors detected some improvements.
But Steven did not notice these changes until he confidently strode through a dimly-lit maze designed to test his vision.
Until then he had kept walking into walls - and it would take him nearly a minute to walk a few feet.
His doctors were shocked at the improvement.
Professor Robin Ali, of the Institute for Ophthalmology, who led the trial, said: "To get this indication after only three patients is hugely exciting.
"I find it difficult to remember being as excited as I am today about our science and what it might achieve."
'Cracks in the pavement'
The operation gave Steven the confidence to try out his improved night-time vision on the streets near his home in Bolton.
Before he had only been able to see the bright lights of passing cars, street lamps and brightly-lit buildings but, to his amazement, he found he could see beyond the bright lights. For the first time he could see the cracks on the pavement, the edge of the kerb and markings on the street.
He recently began walking home late at night from the railway station.
James Bainbridge, the consultant surgeon who carried out the operation, said: "It's hugely rewarding and exciting to see that this new treatment can have this impact on a person's quality of life."
'To not have to worry about losing my sight is great'
Steven also says that it has really helped his confidence.
He is now able to socialise more late at night with his friends. And, as an aspiring musician, he says he can see the frets on his guitar better - and can move around more on a darkened stage.
There may well be further improvements. But without the operation it was likely that Steven would have lost his sight altogether.
The prospect made him depressed. Now he says he can get on with his life.
"When I used to think about it, it would get me really down and depressed. But now I don't have to think about it. It's a big burden lifted."
Child sight hope
The gene therapy has not improved the vision of the other two patients who have received it so far - but it may well stop their vision from declining further.
Robert Johnson was the first person to undergo the operation, as reported by BBC News in May 2007.
He welcomed the results so far: "For the team, I am thrilled that their hard work has come off.
 |  This is only the beginning  James Bainbridge Surgeon |
"For me - I am simply pleased that I left what I entered with - a level of sight that gives me my freedom. What more could I ask for?"
Professor Ali said that the team now hoped to treat children: "The next stage is to increase the dose of the gene which we anticipate will improve the outcome - and it's also to treat younger patients, who have better residual vision and in whom we expect to see a much greater benefit."
Although the genetic condition that is being treated is rare, the researchers believe that their technique could be used to treat a wide variety of sight disorders, possibly even age-related sight loss.
Mr Bainbridge added: "This is only the beginning.
"What we've demonstrated so far is proof of principle that gene therapy can be used to treat a particular gene disorder."
The research, which has been funded by the Department of Health, has been published online in the New England Journal of Medicine.
Health Minister Dawn Primarolo said: "This is absolutely brilliant.
"It's been done here in the UK with the expertise of the NHS and the science and research of the Department of Health all coming together to offer such hope for gene therapy for the correction of sight - but also for gene therapy generally."
David Head, of the British Retinitis Pigmentosa Society, thanked Professor Ali and his team for their "outstanding" work.
He said: "Of course, we must temper our excitement and enthusiasm with an acknowledgement that these are very early days, and the trial is working on one flawed gene."
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