By FIONA MACRAE
Scientists have cured colour blindness in monkeys in a world first.
As well as allowing colour-blind humans to tell red from green, the innovative technique could restore sight to the blind.
Sufferers of age-related macular degeneration - the most common cause of blindness in the elderly - are among the millions who could eventually benefit.
Researcher Jay Neitz said: 'If we could find a way to do this with complete safety in human eyes, as we did with monkeys, there would be a lot of people who would want it.
'We hope the technology will be useful in correcting a lot of different vision disorders.'
Professor Neitz used gene therapy - injections of genes - to allow two male squirrel monkeys called Sam and Dalton to see in full colour for the first time.
Like some humans with red-green colour blindness, the monkeys lacked a pigment that the cones - the colour-detector cells at the back of the eye - need to see red and green. As a result, they saw both red and green as shades of grey. Other colours, such as orange, blue and brown appeared washed-out.
To fix their vision, the U.S. scientists injected their eyes with millions of copies of a gene needed to make the missing pigment, the journal Nature reports.
mportantly, the monkeys were injected with a human gene, suggesting the same technique would work on people. Four months later, their vision suddenly improved.
Professor Neitz, of the University of Washington in Seattle, said: 'It was as if they woke up and saw these new colours. They unquestionably responded to colours that had been invisible to them.'
A version of the colour blindness test that is used in schools around the world showed just how much their vision has improved.
Dr Katherine Mancuso, the study's lead author, said: 'Their performance on red and green was similar, but not quite as good, as a female monkey who had normal colour vision since birth.'
But the need to prove that adding genes to the body does not cause harmful side-effects means it will be some time before the method is routinely used to correct colour blindness.
Despite this, the researchers are optimistic and point out that gene therapy is already being tested on Britons and Americans with a rare, hereditary form of blindness called Leber's congenital amaurosis.
The technique could also be adapted to treat other conditions that involve problems with genes in the colour and detail-detecting cone cells at the back of the eye.
These include age-related macular degeneration, in which deterioration of central vision makes it increasingly difficult to carry out everyday tasks such as reading and driving. Blindness linked to diabetes might also be treated.
Cathy Yelf, of the Macular Disease Society, said the research was 'very interesting' but cautioned: 'A practical gene therapy for macular degeneration is still some way off.'