Gene therapy has always held enormous promise to correct genetic diseases, but turning that potential into treatments has been challenging.

In a study published May 15 in the New England Journal of Medicine and presented at the American Society of Gene and Cell Therapy, researchers led by teams at Children’s Hospital of Philadelphia and University of Pennsylvania report on the first use of the gene-editing technology CRISPR in a customized therapy designed to treat one patient with a rare disease. CRISPR is already approved by the U.S. Food and Drug Administration (FDA) to treat sickle-cell anemia and beta thalassemia, in which patients receive the same gene therapy to treat an abnormality in their red blood cells.

In the latest case, the scientists developed a CRISPR treatment for a boy named KJ, who was born with genetic mutations in his liver cells that prevent him from breaking down proteins properly. As a result, ammonia builds up in his body, which can be toxic to the brain, potentially leading to developmental delays. Led by professor of medicine Dr. Kiran Musunuru at University of Pennsylvania, and Dr. Rebecca Ahrens-Nicklas, director of the Gene Therapy for the Inherited Metabolic Disorders Frontier Program at Children’s Hospital of Philadelphia, the scientists designed a CRISPR gene therapy to specifically address one of KJ’s mutations. “This drug was designed and made for KJ, so in reality this drug will probably never be used again,” says Ahrens-Nicklas of the bespoke nature of the therapy.

While the therapy was created for him, the team is hopeful that the process can be made more universal and applied to other genetic mutations, for which they can plug in the appropriate genetic change to correct a disease.