A FEW blind mice have had their sight restored. The process, which involved transplanting precursor retinal cells into their damaged eyes, promises a cure for age-related macular degeneration or blindness due to diabetes.

The mice were blind because they had been bred to have non-functional photoreceptor cells, the eye’s rod and cone cells that convert light into electrical signals to be sent to the brain. Elderly people and people with diabetes can also lose their vision when these cells fail.

In principle, restoring sight to animals that have simply lost photoreceptor cells should be relatively easy, because most of the …