In a groundbreaking medical first, a ten-month-old baby boy named KJ Muldoon has become the first person in the world to receive a personalized CRISPR gene-editing therapy designed exclusively for him. The bespoke treatment, which corrected a rare and life-threatening mutation affecting his ability to metabolize proteins, marks a major milestone in the future of precision medicine.
KJ was born with a severe form of carbamoyl phosphate synthetase 1 (CPS-1) deficiency, a genetic disorder that prevents the body from properly breaking down nitrogen-containing waste products. Without treatment, the condition leads to dangerously high levels of ammonia in the blood — a compound toxic to the brain. Left untreated, the disease often results in brain damage or death within the first months of life. For many, a liver transplant is the only long-term option — but KJ was too young to qualify.
Faced with limited time and options, Dr. Rebecca Ahrens-Nicklas, a pediatrician and genetic disease specialist at the Children’s Hospital of Philadelphia (CHOP), proposed an unprecedented intervention: a custom CRISPR-based gene therapy designed specifically to fix KJ’s unique genetic mutation.
“We knew we had to act quickly,” Dr. Ahrens-Nicklas said. “Every day mattered. This was truly a race against time.”
A Race to Rewrite DNA
In just six months — an extraordinarily short timeframe in the world of biomedical research — a global team of scientists, clinicians, biotech firms, and U.S. regulatory agencies collaborated to develop and approve the therapy. The treatment uses base editing, a cutting-edge form of CRISPR gene editing that allows scientists to make precise, single-letter changes to DNA without cutting the DNA strands.
The therapy was designed to correct a mutation in KJ’s CPS1 gene, inherited from both his parents, which rendered his body unable to produce a key liver enzyme. After screening for the most effective editing strategy, researchers tested it in animal models including mice and monkeys to ensure safety.
The U.S. Food and Drug Administration (FDA) expedited its review process, allowing the treatment to be administered within half a year — a timeline experts have called “remarkable.”
A Hopeful Outcome
KJ received the first of three doses in late 2024. After the initial treatment, he was able to tolerate normal dietary protein levels for his age, though he still needed medication to control ammonia levels. With the second and third doses, doctors gradually reduced his reliance on those drugs.
“We are now cautiously tapering down his medications,” said Dr. Ahrens-Nicklas. “Every improvement is a milestone.”
KJ’s mother, Nicole Muldoon, described the emotional moment when she walked into her son’s hospital room and found him sitting up in his crib unaided. “We never thought this was going to happen,” she said through tears. “He’s smiling, he’s alert — he’s our miracle.”
The Future of “N-of-1” Therapies
While the success of KJ’s case opens exciting new doors in medicine, it also raises challenging questions. Personalized therapies like his — often called “N-of-1” treatments — are expensive and labor-intensive. The highly customized nature of such treatments means they are unlikely to be reused for other patients.
“There’s no clear path right now to make these treatments scalable or affordable for everyone who needs them,” said Dr. Waseem Qasim, a gene therapy researcher at University College London, who was not involved in the study. “But what this team achieved gives us a glimpse of what’s possible.”
Still, researchers say the case proves that with coordinated global effort, personalized gene therapies for rare diseases can be developed — and quickly.
“This is just the beginning,” said Dr. Ahrens-Nicklas. “We hope KJ’s story will inspire a new era of hope for families around the world facing rare genetic disorders.”
As KJ celebrates his tenth month of life, his story symbolizes not only a medical victory but a profound shift in how we think about treating the untreatable — one patient, one genome, one breakthrough at a time.
