The field of gene therapy has had its share of troubles, including two studies in which patients died and others in which already-ill subjects developed additional conditions. But now a hint of success has arrived, in the form of two small clinical trials involving people with a rare blinding disease.

The subjects suffer from Leber’s congenital amaurosis, which causes poor vision at birth and sight that steadily worsens. Because the disease is caused by an error in a single gene, researchers at the University of Pennsylvania and at University College London independently decided to try to correct it with gene therapy.

They report online in The New England Journal of Medicine that three patients in each study experienced improvement in their vision after receiving an injection of a virus containing the healthy gene.

The improvements were modest, and a few other patients did not improve, but the results still give hope that a better version of the treatment will prove itself in larger, longer-term studies. Videos at the journal’s Web site show patients before the procedure bumping into walls and obstacles, and afterward navigating the courses with ease. —Lila Guterman