Scientists used gene therapy to improve vision in six people with a rare inherited disorder called Leber's congenital amaurosis, USA Today reported. The condition causes a gradual deterioration of eyesight beginning at birth, leading to blindness by a person's mid-20s.

While none of the six patients regained normal vision, four have much better eyesight than before the gene therapy, according to two studies published online in the New England Journal of Medicine. The studies were conducted by an international team led by doctors at the Children's Hospital of Philadelphia and the University of Pennsylvania.

Experts hailed the achievement as a welcome success in gene therapy, which has shown promise but has had significant setbacks.

"It's a breakthrough. It really helps to validate that gene therapy can work and is going to be important," Ronald Crystal, chairman of the department of genetic medicine at New York's Weill Medical College of Cornell University, told USA Today.