Philadelphia doctors shocked the medical world with a breakthrough that could save countless lives.
The treatment they developed is unlike anything seen before in medicine.
And the doctors just treated a baby with revolutionary gene editing therapy that could change medicine forever.
First-ever personalized CRISPR treatment successfully administered to infant
In a historic medical breakthrough, doctors at Children’s Hospital of Philadelphia (CHOP) and Penn Medicine successfully treated a baby with a custom-designed CRISPR gene editing therapy.
The infant, identified only as KJ, was born with a rare and potentially fatal genetic disorder called severe carbamoyl phosphate synthetase 1 (CPS1) deficiency.
After spending the first several months of his life in the hospital on a highly restrictive diet, KJ received the first dose of his personalized therapy in February 2025 when he was between six and seven months old.
The groundbreaking case was detailed in a study published last week in The New England Journal of Medicine and presented at the American Society of Gene & Cell Therapy Annual Meeting in New Orleans.
Dr. Rebecca Ahrens-Nicklas, who led the treatment development along with Dr. Kiran Musunuru, called it a watershed moment in medicine.
“Years and years of progress in gene editing and collaboration between researchers and clinicians made this moment possible, and while KJ is just one patient, we hope he is the first of many to benefit from a methodology that can be scaled to fit an individual patient’s needs,” Ahrens-Nicklas said.
A race against time to develop a lifesaving treatment
KJ’s condition prevents his body from properly processing nitrogen during protein breakdown, causing potentially lethal ammonia buildup that can severely damage the brain and liver.
The traditional treatment for his condition would be a liver transplant, but KJ was too young and medically fragile for such a major surgery.
Meanwhile, any episode of increased ammonia in his system risked causing permanent neurological damage or even death.
After identifying KJ’s specific genetic mutation shortly after birth, the research team sprang into action. Within just six months, they designed and manufactured a base editing therapy delivered via lipid nanoparticles specifically targeted to correct KJ’s faulty liver enzyme.
KJ received his first infusion of the experimental therapy in late February 2025, followed by two additional doses in March and April.
Treatment shows promising early results
The results so far have been nothing short of remarkable.
As of April 2025, KJ had received three doses of the therapy with no serious side effects. In the short time since treatment began, he has been able to tolerate increased dietary protein and needed less medication to manage his condition.
Perhaps most importantly, KJ has been able to recover from typical childhood illnesses like rhinovirus without dangerous ammonia buildup in his body – a crucial milestone that suggests the treatment is working.
“While KJ will need to be monitored carefully for the rest of his life, our initial findings are quite promising,” Dr. Ahrens-Nicklas noted.
A new frontier in personalized medicine
What makes this case so revolutionary is that the treatment was custom-designed for KJ’s specific genetic mutation.
Until now, gene therapies have primarily targeted more common diseases that affect thousands of patients, such as sickle cell disease and beta thalassemia, which already have FDA-approved CRISPR treatments.
However, millions of patients worldwide suffer from ultra-rare genetic diseases that don’t benefit from a “one-size-fits-all” approach. This breakthrough suggests a new path forward for treating these conditions.
Dr. Musunuru expressed hope that the methodology could be replicated for many rare diseases.
“We want each and every patient to have the potential to experience the same results we saw in this first patient, and we hope that other academic investigators will replicate this method for many rare diseases and give many patients a fair shot at living a healthy life,” he said.
“The promise of gene therapy that we’ve heard about for decades is coming to fruition, and it’s going to utterly transform the way we approach medicine,” Musunuru added.
A family’s leap of faith
For KJ’s parents, Kyle and Nicole Muldoon, the decision to try the experimental therapy was an act of faith and love.
“We would do anything for our kids, so with KJ, we wanted to figure out how we were going to support him and how we were going to get him to the point where he can do all the things a normal kid should be able to do,” Nicole said.
“We thought it was our responsibility to help our child, so when the doctors came to us with their idea, we put our trust in them in the hopes that it could help not just KJ but other families in our position.”
After months in the hospital, the family is looking forward to bringing KJ home to be with his three siblings.
“We’ve been in the thick of this since KJ was born, and our whole world’s been revolving around this little guy and his stay in the hospital,” Kyle said. “We’re so excited to be able to finally be together at home so that KJ can be with his siblings, and we can finally take a deep breath.”
The study was supported by grants from the National Institutes of Health Somatic Cell Genome Editing Program and other NIH grants, with additional contributions from Acuitas Therapeutics, Integrated DNA Technologies, Aldevron, and Danaher Corporation.
