Gene therapy has restored vision partially in patients who were blind in experiments by Seattle biotech company Targeted Genetics.

Three young adults at Children's Hospital of Philadelphia, who suffered from a rare and as yet incurable form of congenital blindness, were given a genetically engineered virus as a vector to carry millions of copies of a normal version of the defective gene. Two weeks after the surgeries, performed between October 2007 and January 2008, all three showed significant improvements in the one eye that was injected with the gene-carrying virus.

"In the field of retinal dystrophies, this is, I believe, the most important therapeutic discovery" in four decades, said Dr. Morton Goldberg, an ophthalmologist at John Hopkins University's Wilmer Eye Institute, quoted by Seattle Times. "It's a landmark," he said.

The most successful operation was that of a 17-year-old man, Steven Howarth. The research was published online Sunday by the New England Journal of Medicine. The patients allegedly became three times as sensitive to light and could read lines on an eye chart, compared to near-blindness before the surgery.

Targeted Genetics's future was previously under question as federal regulators have recently investigated the firm following the death of a woman who died during its lead drug candidate's clinical trial. However, the FDA eventually determined that the company's product was not at fault for the tragic outcome.

The problem is that interaction of the immune system with the viruses carrying the replacement DNA can lead to further problems. Some gene therapy patients have developed leukemia in previous studies, and a teenager named Jesse Gelsinger died after gene therapy at the University of Pennsylvania in 1999. However, this is not such a problem with eye therapy, because within the eye the immune response is lower than in other areas of the body.