A genetic disease that gradually robs sufferers of their sight may finally have met its match.
Scientists have used gene therapy to restore sight in laboratory animals with the most common genetic form of blindness - retinitis pigmentosa.
Their pioneering treatment raises the prospect of a cure for the human condition that is at present completely untreatable.
Bill Hauswirth and Alfred Lewin at the University of Florida have carried out a series of successful trials on rats and mice.
Experiments involving pigs, whose eyes are of a comparable size to humans, have begun to show indications of a similar effect.
"If we can rescue the vision of a large animal, then we could begin human trials within five years," Dr Hauswirth explained.
The root cause of the condition is a single faulty gene.
This mutant gene produces a protein that hinders the formation of a key light-detecting molecule in the retina.
It can take years for this effect to become apparent.
The first the sufferer knows of it is when they start to lose their night and peripheral vision.
By this stage, half their eyes' photoreceptors have been destroyed and there is nothing medicine can do to prevent the slow walk into complete darkness.
Dr Hauswirth and his colleagues have developed a way to block the effects of the faulty gene that does not simply halt the progress of the disease but actually restores vision to the carrier.
At the heart of their technique is ribozyme, a chemical that targets and cuts up specific sequences of RNA, the molecule that translates genes into proteins inside a living cell.
By creating a ribozyme that only homes in on the faulty gene's RNA, the scientists have in effect made a guided missile that stops the manufacture of the protein that disrupts the retina's function.
A specially modified virus was designed to deliver the ribozymes into the photoreceptor cells in the form of an injection.
Tests on rats with the equivalent of retinitis pigmentosa have so far been positive.
The scientists have demonstrated that their technique works even when the disease has already shut down half of the photoreceptors. The technique can restore vision for eight months - more than half the lifetime of the animal.
While the untreated eye had lost 90 per cent of its vision, the treated eye retained 50 percent of its vision - more than sufficient for normal function.
Besides its promise as a gene therapy for retinitis pigmentosa, Dr Hauswirth said the method could be adapted to treat other genetic disorders unrelated to the eye.
It could give scientists a relatively problem-free way to create "knock-out" animals that lack particular genes.
These animals could then be used to create models of human diseases.