Gene therapy can be particularly effective in treating inherited sight problems in children, fresh trials show.
US doctors treating 12 patients with a rare genetic eye disorder were able to significantly improve vision in the youngest.
The study, published in The Lancet, builds on work carried out by doctors at London's Moorfields Eye Hospital.
The eye is proving to be a particularly successful target for gene therapy.
The principle is simple: to replace a defective gene and restore function to a part of the body affected by a genetic disorder.
Treatment so far has focused on Leber's congenital amaurosis (LCA), a rare inherited disorder which causes gradual deterioration in vision and can lead to blindness by the time the patient is 20.
It occurs when faulty genes, called RPE65, stop the layer of cells at the back of the eye working.
LCA affects approximately one in 80,000 people, and is responsible for one in 10 severe sight disorders in children.
This latest study from the University of Pennsylvania involved patients aged between eight and 44, each of whom had the genetic material needed to correct LCA injected into the eye with the worst vision.
The study reported that all patients responded well to the treatment.
There were measurable improvements, including an at least 100-fold increase in pupillary light response - when the pupils constrict in brightness.
But the most marked improvements were seen in the young.
One eight-year-old developed the same degree of light sensitivity as an unaffected contemporary, while all the children involved gained vision sufficient to walk unaided.
The researchers noted the results showed early intervention produced the best results.
"The visual recovery noted in the children confirms the hypothesis that efficacy will be improved if treatment is applied before retinal degeneration has progressed," said lead author Professor Jean Bennett.
"Assessment of whether the treatment alters the natural progression of the retinal degeneration will be possible in follow-up studies."
Gene therapy has already been successfully deployed in the eyes by a team at the Institute of Ophthalmology and Moorfields Eye Hospital in London
The first such operation was carried out in 2007, and last year a further three patients received it - one of whom reported significant improvement.
The eye is seen as a particularly attractive target for these new treatments.
The genes are contained in a harmless virus which is unlikely to be attacked by the body as the immune system is not strong in the retina.
The eye is also readily accessible, and a relatively simple organ.
It is hoped that all this work will provide the basis for age-related as well as other genetic sight disorders, but also different conditions altogether.
Gene therapy has already been successfully deployed to treat children with X-SCID - a life-threatening immune deficiency also known as "boy in the bubble syndrome".
Professor Robin Ali of the UCL Institute of Ophthalmology, who has led the British trials, said the latest study was "very encouraging".
"The findings provide further evidence that gene therapy can be safe and can improve retinal sensitivity, particularly in dim light, even when a relatively small area of retina is treated.
"The next challenge is to determine the dose and the extent of retina that needs to be treated in order to slow retinal degeneration and preserve sight."
Barbara McLaughlan, of the charity RNIB, said: "We welcome the continuing research in this field and hope that at some point it will be possible to use gene therapy for conditions such as age related macular degeneration which are more complex because of a complicated interaction between multiple genetic and environmental factors."
But a lot more research was needed, she added, "to maximise the benefits of gene therapy techniques and understand how they can then be turned into effective treatments for a variety of more common degenerative eye conditions."