Long-term follow-up of 12 persons with Leber's congenital amaurosis treated with gene therapy showed that about half of them had improvements in retinal sensitivity (although the extent varied markedly among patients), followed by a decline. Leber’s congenital amaurosis is a group of inherited, early-onset, severe retinal dystrophies that cause substantial sight impairment in childhood. 1 One of the causes of this condition is mutations in the gene encoding RPE65 (retinal pigment epithelium–specific protein 65 kDa). The encoded retinoid isomerase converts all- trans retinyl esters to 11- cis retinal for the regeneration of visual pigment after exposure to light. RPE65 deficiency causes photoreceptor-cell dysfunction and impaired vision from birth. Severe dysfunction of rod photoreceptor cells, which are wholly reliant on retinal pigment epithelium–derived RPE65, causes severely impaired night vision. The function of cone photoreceptor cells, which mediate . . .