PHILADELPHIA -

In bright daylight, 10-year-old Mark DeVoe has no trouble seeing his friends, but inside, or even in the shade, Mark's eyes sometimes don't work.

"I have trouble seeing like, trees, when the road ends, and when there's like a drop there," Mark said.

At age six, Mark's doctors diagnosed him with the genetic condition choroideremia, which causes people to progressively lose vision until they are completely blind.

"I don't know what it's like to live in darkness, but I've seen it," said Susan DeVoe, Mark's mother.

Susan is a carrier of the blindness gene. Mark's grandfather has the condition.

"Watching my father go blind was devastating. I was a little girl. You know, you count on daddy to do things, and daddy couldn't do them," she recalled.

Dr. Jean Bennett is one of two U.S. researchers preparing to test a gene therapy for choroideremia in humans.

"I think gene therapy holds a huge promise for developing treatments for blinding diseases," said Bennett, ophthalmologist and molecular geneticist at the University of Pennsylvania.

Researchers will use a virus, carrying a normal choroideremia gene and inject the virus just under the retina. The gene should begin to work in a few weeks.

More:

Health Beat: Gene therapy: From bench to bedside: Blindness