Doctors Rewrite Baby's DNA in World-First Breakthrough to Cure Deadly Genetic Disease

Doctors at the Children's Hospital of Philadelphia (CHOP) have announced a breakthrough that sounds like something from science fiction: they have successfully rewritten the DNA of 15-month-old KJ Muldoon, making him the first infant in the world to undergo CRISPR gene editing as a treatment for a life-threatening genetic condition.

For the Muldoon family, this announcement, shared on CHOP's YouTube channel on Friday, 16 May 2025, couldn't have come at a more critical moment. Their lives were turned upside down when KJ was diagnosed with CPS1 deficiency, a rare disorder that disrupts the liver's ability to process ammonia. Without intervention, toxic ammonia levels can quickly cause permanent brain damage—or worse.

CRISPR Breakthrough: A Baby's DNA Rewritten to Save His Life

KJ's story began with a devastating diagnosis. According to The Sun, CPS1 deficiency compromised the infant's liver function, rendering it unable to eliminate excess ammonia from the bloodstream. If untreated, the condition can prove fatal.

The disorder is exceptionally rare, affecting just one in 1.3 million people. Those who suffer from it are at high risk of organ failure, in addition to neurological damage.

'You Google CPS1 deficiency and it's either fatality rate or liver transplant,' said Nicole Muldoon, KJ's mother, in a video released by CHOP.

The diagnosis was particularly distressing because infants are too small to receive immediate liver transplants. As explained by The Guardian, patients must grow large enough to safely undergo the procedure. But for babies with CPS1 deficiency, waiting carries life-threatening risks.

Thanks to pioneering work by CHOP and the University of Pennsylvania, however, doctors were able to rewrite KJ's DNA and give him a second chance at life.

Tailor-Made Treatment: Personalised Medicine in Action

In February 2025, KJ's DNA was altered using a gene-targeting infusion, specifically designed to repair the exact mutation causing his condition.

'The drug is really designed only for KJ, so the genetic variants that he has are specific to him. It's personalised medicine,' said Dr Ahrens-Nicklas, one of the lead physicians at CHOP.

She admitted that her greatest concern had been raising false hope. But a dedicated drug development team succeeded in engineering a solution tailored to KJ's unique genome.

The baby received his first dose via infusion, and just days later, doctors observed marked improvement in his condition. He went on to receive two more doses in March and April.

Rapid Recovery with No Side Effects So Far

The treatment worked. Thanks to the experimental gene therapy, KJ can now tolerate more protein in his diet without risking ammonia buildup. His doctors have also been able to reduce his reliance on ammonia-lowering medication.

KJ remains under close observation at CHOP to ensure his ongoing safety, but so far, no side effects have been reported.

'We've been operating in fight-or-flight mode for so long, that now it's finally starting to look like the light at the end of the tunnel,' said Nicole.

Dr Ahrens-Nicklas noted that KJ is just the beginning. The team hopes many more young patients could benefit from similar treatments in the near future.

A Milestone for Science—And Ethical Debate

While KJ's recovery represents a major medical breakthrough, it also raises profound ethical questions around gene editing in children. Are we entering an era where genetic conditions can be cured, rather than merely managed?

For the Muldoons, those questions are secondary to the joy of seeing their son beat the odds. And for now, KJ stands as living proof of the power of science, determination, and hope.