. >> reporter: caroline and cole carper have been through more of your typical siblings. >> i can't imaginet being any different than the two of them being together and going through this together. you know, they've had to learn braille together and gone to school for the blind together. >> reporter: they were both diagnosed with lca, a rare retinal disease. >> the doctor really said there's nothing -- there's no medical treatments at this time. >> reporter: both caroline and cole were expected eventually to go blind. but their parents found an experimental treatment and two years ago, the kids enrolled together in a clinical trial. biotech company formed out of the research of children's hospital of philadelphia. >> the idea is we're taking the correct functional copy of the gene and adding it back into the cells that have dysfunctional or missing copy of that gene or blueprint. >> reporter: it's called gene therapy. and it's relatively unchartered territory medicine. the technology uses a modified virus to deliver a healthy copy of the gene to make up for one that causes disease. scientis