"The clinical benefits have persisted for nearly two years since the first subjects were treated with injections of therapeutic genes into their retinas," Bennett said.

Four children aged 8, 9, 10, and 11 can now walk unaided.

Corey's father, Ethan Haas, from Hadley, New York, said they embraced the experiment.

"You start to think of what could happen -- he could go completely blind. And then it's like, well, he may go blind in the future anyway because it's degenerative, so I decided to try it now and see if we could stop it and correct it," Haas said.

Corey's mother, Nancy Haas, said it was worth the risk.

"It's hard to see a child not be able to play like he should with his other friends, and then to have shortly after surgery, he's out there with his friends, playing, being able to see things coming from his peripheral vision, noticing other kids," she said, beginning to cry.

"It's all worth it."