Nine-year-old Corey Haas can ride his bike alone now, thanks to an experimental gene therapy that has boosted his fading vision with a single treatment.

The gene therapy helped improve worsening eyesight caused by a rare inherited disease called Leber congenital amaurosis, or LCA, which makes most patients blind by age 40.

Twelve treated patients, including Corey, now have better vision, their doctors told a joint meeting of the American Academy of Ophthalmology and Pan-American Association of Ophthalmology in San Francisco on Saturday.