British doctors say they have made the world's first surgical attempt to treat a human sight disorder using gene therapy.

The patient, Robert Johnson, 23, has an inherited disorder called Leber's congenital amaurosis. The disorder, linked to a mutation in a gene called RPE65, is thought to lead to degeneration in the retina — the layer of cells at the back of the eye that detects light.

Since birth, Johnson has been able to see fuzzy outlines during the day but little at night. His vision has also worsened with age.

The doctors — Prof. Robin Ali of the Institute of Ophthalmology at University College London and his colleagues at Moorfields Eye Hospital in London — said they injected non-defective copies of the RPE65 gene under the retina in one of Johnson's eyes.

A harmless virus was used to deliver the gene into the cells.

"It is very encouraging that we can deliver genes to an extremely fragile site in the eye without complications," ophthalmologist James Bainbridge, the leader of the surgical team, said in a release issued Tuesday.

The researchers said they hope the procedure will allow Johnson's retina to detect light properly.

Experiments in dogs have suggested the gene therapy treatment can improve and preserve vision.

However, it will be months before the team knows whether the approach worked for Johnson.

The researchers intend to perform the experimental treatment on 11 other young people as well, in a trial aimed at determining its safety and effectiveness in humans.

Younger patients may have better outcomes since the disease is in early stages of development, the researchers said.