A year after receiving gene therapy for a condition that causes total blindness by age 30, three people continue to see better and one has improved enough to read the digital numbers on the clock in her parents' car, U.S. researchers said on Wednesday.

Improvements in the vision of the three volunteers -- all legally blind and in their 20s -- has not deteriorated over time, doctors involved in the experiment reported in New England Journal of Medicine.

One of the three has also developed a kind of second sight because her brain has learned to tap information from an area of the retina rejuvenated by gene therapy.

The initial improvements were very substantial and occurred in a matter of weeks, Dr. Artur Cideciyan of the University of Pennsylvania said in a telephone interview.

But in one patient, a woman, her vision has continued to improve because her brain has apparently learned to use information from the treated portion of the eye, allowing her to read the clock.

It's very encouraging to see that the adult brain seems to be adapting to this vision, Cideciyan said.

The three -- two men and the woman -- suffer from a form of Leber's congenital amaurosis, an otherwise-untreatable condition that robs infants and children of most of their vision. All had been legally blind since birth because a gene known as RPE65, which produces a special form of vitamin A for retinal cells, was defective.

The experiment was one of several attempts by various research teams to fix genetic problems by injecting properly functioning DNA into a virus and using the virus to deliver the healthy DNA to individual cells.

Three months after doctors injected patients in the eye with healthy versions of the gene, all three patients could detect dim lights they were unable to see before the treatment.

In the case of one woman, she was using her right eye - initially her worst - to read the dashboard clock for the first time in her life. The researchers found that she was seeing the light using a portion of the eye that had been injected with the healthy DNA, and not the usual spot used for reading and seeing detail.

She either uses her normal gaze straight ahead or, under other conditions, she switches to this treated area. So she has two loci for gaze and she switches between the two. That took a while to occur, Cideciyan said.

A year after their treatments, the therapy has not caused an immune response in the eye or in the body, the team said.

Research into gene therapy was dealt a blow by the 1999 death of a volunteer in a University of Pennsylvania study who received a corrective gene in a deactivated virus to treat a liver defect. He died four days later.

Cideciyan said in a statement the RPE65 gene therapy appears to be safe and leads to a stable visual improvement in the patients studied. We are cautiously optimistic about these results and look forward to additional reports that address the key issues of safety and effectiveness, he said.

The researchers will continue to monitor the patients over several years and are experimenting with other patients to see if injecting different amounts of DNA is more effective.

Leber congenital amaurosis type 2 affects about 2,000 people in the United States and is one of several incurable forms of blindness collectively known as retinitis pigmentosa, which affects about 200,000 Americans.