A team at London's Moorfields Eye Hospital has made the world's first attempt to treat a sight disorder using gene therapy.
The condition causes the retina to deteriorate
They operated on Robert Johnson, a UK man born with a sight disorder which means he can see very little at night, and which deteriorates with age.
It will be several months before the researchers know whether their work has been a success.
What is the condition?
Mr Johnson has Leber's congenital amaurosis (LCA), a type of inherited degeneration of the retina.
The retina is made up of a layer of cells called photoreceptors which detect light, convert it into electrical impulses and then transfer these impulses to the brain via the optic nerve.
LCA is caused by mutations in the RPE65 gene that controls production of an enzyme responsible for the recycling of retinol, a chemical necessary for capturing light.
This recycling takes place in the retinal pigment epithelium (RPE), a supportive tissue underlying the layer of photoreceptor cells in the retina.
If used retinol cannot be recycled, the photoreceptor cells run out of supplies and can no longer function.
This results in severely impaired vision from a very young age.
What do the researchers hope to achieve?
In this form of retinal degeneration the photoreceptor cells are present, but not functioning.
The researchers aim to restore the activity in these cells and therefore restore vision by implanting healthy copies of the key gene into the RPE at the back of the eye.
The genes are contained in a harmless virus, which should, in theory, not be neutralised by the body's defences, as the immune system is not strong in the retina.
Some indications of the result may be available within several months. However, the subjects - Mr Johnson and 11 others - will need to be followed up to assess the long term effect of the treatment.
It is anticipated that the best results will be achieved in younger patients, as they will be treated when the disease is in its earlier stages of development.
However, as this trial is the first to treat an eye disease using gene therapy, the first thing the researchers will need to do is to establish that it is safe.
Has the experiment worked in animal experiments?
The researchers have been able to successfully treat dogs with the same defect by administering an engineered virus that contains the missing gene.
They were able to restore vision in the dogs to the extent that they could walk through a maze without difficulty - something they could not do before the treatment.
There are many similarities between the eye of a dog and human, and the disease in dogs is very similar to the disease in human patients, so the researchers are hopeful that humans will benefit from a similar treatment.
Are there plans for future trials for other forms of retinal degeneration?
For each form of retinal degeneration, the use of gene therapy will need to be developed independently and the treatment will need to be tested in a separate clinical trial for that disease.
Nevertheless, the results from this first trial are likely to provide an important basis for many more gene therapy protocols in the future.
The researchers hope that eventually their work could lead to ways to treat more common sight problems, such as age-related macular degeneration.