Because this type of blindness is based on a single, small gene in the retina, it's relatively easy for scientists to fix. They take an adeno-associated virus—a virus which is usually present in humans but not known to cause disease—remove most of its genes and patch a shiny, new, properly functioning version of the rpe65 gene into it.

Once injected into the eye, the virus goes to work doing what viruses do, i.e. invading cells and using their machinery to replicate its genetic information. But, in this case, that information is the rpe65 gene. Within a few weeks or months, the person has a supply of working rpe65 genes, churning out the enzyme they need to see.