This baby boy received the first custom gene-editing medication.

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The researchers used a new version of CRISPR technology known as base editing, which can replace a single letter of DNA at a specific location. Previous versions of CRISPR have typically been used to delete genes rather than rewrite them to restore their function. The researchers were on the lookout for a patient to treat when they learned about KJ. After his birth in August, a doctor noted that the infant was lethargic. Tests revealed he had a metabolic disorder that causes ammonia buildup, a condition often fatal without a liver transplant. In KJ’s case, gene sequencing indicated that the issue stemmed from a misspelled letter in the gene CPS1, which prevented it from producing a vital enzyme.

The researchers approached KJ’s parents, Nicole and Kyle Muldoon, with the proposal of using gene editing to attempt to correct their baby’s DNA. After their agreement, a race began to design the editing drug, test it in animals, and secure permission from the US Food and Drug Administration to treat KJ in a one-off experiment. The team reports that the boy, who hasn’t yet turned one, received three doses of the gene-editing treatment, with gradually increasing strength. They are not yet able to determine exactly how effective the gene editing has been, as they prefer not to perform a liver biopsy, which would be necessary to verify if KJ’s genes have truly been corrected. However, Ahrens-Nicklas suggests that because the child is “growing and thriving,” she believes the editing has been at least partly successful and that he may now have “a milder form of this horrific disease.” “He’s received three doses of the therapy without any complications and is showing some early signs of benefit,” she says. “It’s really important to emphasize that it’s still very early, so we will need to continue to monitor KJ closely to fully understand the complete effects of this therapy.”

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