Ambitious doctors thought they could fix this with a viral infusion that would import a healthy gene.

The limited gene pool makes it harder to recreate healthy genes.

For example, we don't know how the trial embryos managed to take copies of relevant healthy genes from the egg after the faulty one was snipped away.

The healthy gene's mission: to produce a protein critical to translating light waves into nerve impulses that can be interpreted as images by the brain.