The ability to culture pluripotent cell lines, introduce into them targeted gene mutations, and then use these mutant cell lines to generate chimeric animals (1-4), has had a major impact on many fields; not least on ophthalmology and vision research (5-8). By generating animals lacking in the product of a single gene, we can examine the biochemical, physiological, and structural effects of its loss in vivo. Often this can yield insights, sometimes surprising, into human diseases caused by mutations in the same gene.