Faith Nyasuguta
In a stunning medical first, a baby boy known as KJ has become the first person in the world to be successfully treated with a personalized CRISPR gene-editing therapy – a custom-built medical treatment that edits faulty genes in the body to fix the root cause of a disease. This groundbreaking procedure was carried out at Children’s Hospital of Philadelphia (CHOP) in collaboration with Penn Medicine, and early signs show that the treatment is working.
So what exactly is CRISPR, and why is this story so important?
Imagine our DNA- the code that runs every part of our body – as a long book filled with instructions. If there’s a typo in this book, our body may not work the way it should. CRISPR is a revolutionary tool that scientists can use like a pair of smart scissors to find that exact typo and fix it. In short, CRISPR gene editing is a technology that allows scientists to correct mistakes in a person’s DNA. This means it doesn’t just treat the symptoms of a disease – it goes in and tries to fix the cause.
Until now, CRISPR had mostly been used in clinical trials to treat more common inherited diseases like sickle cell anemia or beta thalassemia. Those diseases affect thousands of people, so companies are willing to invest in large-scale solutions. But when it comes to rare diseases, ones that may only affect one person in the world, treatments like these were considered too complicated and expensive to develop for just a single patient.
That’s why KJ’s case is such a breakthrough. He didn’t get a one-size-fits-all treatment. Doctors built a gene editing solution specifically for him, based on the exact mutation he was born with.
A Life-threatening Condition With Few Options
KJ was diagnosed shortly after birth with carbamoyl phosphate synthetase 1 (CPS1) deficiency, a rare and life-threatening condition that affects the body’s ability to get rid of ammonia- a waste product that forms when the body digests protein. For most people, the body turns ammonia into urea, which is then passed out in urine. But for KJ and others with CPS1 deficiency, the enzyme responsible for this process doesn’t work correctly. That means ammonia builds up in the body and becomes toxic, especially to the brain and liver.
When KJ was born, he had to stay in the hospital for months. He was placed on a very strict, low-protein diet, and his care team worked around the clock to prevent ammonia from reaching dangerous levels. Still, his condition was unstable, and every infection, fever, or small mistake in diet carried serious risks.
Most children with CPS1 deficiency eventually need a liver transplant– a major surgery that replaces their defective liver with a healthy one. But babies like KJ are often too small and medically fragile for that. The period before they’re strong enough for a transplant is the most dangerous, and it’s when irreversible damage can happen.
That’s where this new gene-editing idea came in.
Dr. Rebecca Ahrens-Nicklas from CHOP and Dr. Kiran Musunuru from the University of Pennsylvania had been researching gene therapy for rare diseases for years. In 2023, they began to explore whether CRISPR could be used in a more personalized way- not just for large patient groups, but for individuals like KJ. They were part of a research program called the Somatic Cell Genome Editing Consortium, which supports this kind of advanced work.
When KJ’s mutation was identified, Ahrens-Nicklas and Musunuru got to work. Using a method called base editing, they designed a therapy that would correct the specific genetic error causing KJ’s faulty enzyme. In February 2025 , KJ, who was between six to seven months old, received the first dose of the therapy. It was delivered to his liver using lipid nanoparticles- tiny, fat-like particles similar to the technology used in COVID-19 vaccines. These particles help carry the gene-editing tool directly into the body without surgery.
A First-Of-Its-Kind Treatment
In February 2025, when KJ was about six months old, he received his first infusion of the experimental treatment. Over the next two months, he received two more doses, one in March, and another in April. So far, he has not shown any serious side effects from the treatment.
Even better, there are signs that the therapy is starting to work.
KJ has been able to eat more protein- something that used to be dangerous for him -without having ammonia build up in his body. He also needs less medication to clear ammonia and has recovered from typical childhood illnesses like colds without going into crisis. His body appears to be slowly learning to manage ammonia more like a healthy child’s would.
Doctors are still closely monitoring him, as it’s too early to declare the treatment a complete success. However, the improvements are incredibly promising, and researchers believe this could be the start of a new era in medicine.
“This is just one patient, but we hope it’s the beginning of something big,” said Dr. Ahrens-Nicklas. “We want this approach to be used for many other children with rare diseases.”
Her colleague Dr. Musunuru added, “We’ve dreamed about this kind of gene therapy for decades. It’s finally becoming reality, and it’s going to change how we treat disease.”
Hope For Other Families
For KJ’s family, the past few months have been full of both fear and hope. His parents, Nicole and Kyle Muldoon, were at first overwhelmed when doctors suggested an experimental gene-editing treatment for their baby. But after learning about the science and seeing how carefully the team had planned everything, they decided to go ahead- not just for KJ, but in the hopes of helping others.
“We would do anything for our kids,” Nicole said. “When the doctors came to us with their idea, we trusted them. It felt like a way to help KJ – and maybe other families, too.”
Kyle said the journey has been exhausting but worth it. “Since KJ was born, our whole world has revolved around him and his care,” he said. “Now we’re finally home, and he’s with his siblings. It feels like we can finally breathe.”
This achievement wasn’t just the work of one hospital. The study was supported by several grants from the U.S. National Institutes of Health (NIH), as well as donations and contributions from biotech companies including Acuitas Therapeutics, Integrated DNA Technologies, and Aldevron. CHOP’s own Gene Therapy for Inherited Metabolic Disorders Frontier Program also played a key role.
So what’s next?
Doctors will continue to follow KJ’s progress for years to come. There are still risks, and no one knows yet if the improvements will last long term. But the team behind his treatment is already working to apply this model to other rare conditions.
There are more than 7,000 rare genetic diseases, many of which have no treatment options. While each one may affect only a small number of people, together they impact millions worldwide. If CRISPR can be tailored for individuals, even just a few at a time – it could be life-changing for families who currently have nowhere to turn.
KJ’s story is more than a success. It’s a signal that medicine is entering a new era, where even the rarest diseases are no longer too rare to treat.
RELATED:
