Rewritten Medical History: Scientists perform the first in-human gene editing therapy, saving a newborn.

Philadelphia team performs first in vivo gene editing to save a boy with Hunter’s disease, setting a global record for “in vivo gene editing” to save a life. This article is adapted and compiled from a Popular Mechanics piece.

The ability to successfully copy 3 billion completely different letters of the human genome is an absolute biological miracle, but sometimes, errors are still unavoidable. According to data from the Centers for Disease Control and Prevention (CDC), genetic diseases or other birth defects caused by genetic mutations during conception or embryonic development are quite common, affecting 1 in every 33 babies in the United States.

Throughout human history, individuals born with such conditions have typically had to live with them for life, and depending on the severity of the defect, these lives can be extremely short.

But in 2025, human history was forever changed.

In a groundbreaking study published in the New England Journal of Medicine (NEJM), scientists, doctors, and experts from top U.S. institutions including Philadelphia Children’s Hospital, University of California, Berkeley, and University of Pennsylvania School of Medicine, detailed how they successfully saved the life of a newborn named KJ.

To accomplish this incredible medical feat, the doctors used the world’s first customized “in vivo” (inside the living body, not in a petri dish) CRISPR gene therapy. This technology, supported by decades of federally funded medical research, aims to alleviate the suffering of millions affected by genetic diseases each year.

“Advances in gene editing over the years, along with close collaboration between researchers and clinicians, made this moment possible,” said Rebecca Ahrens-Nicklas, co-author of the study and from Philadelphia Children’s Hospital, in a press release. “Although KJ is just a single case, we hope he is the first of many beneficiaries. This approach can be expanded to meet the specific needs of individual patients.”

Like a medical drama, a real rescue

This astonishing medical intervention was like a scene from a TV medical drama, but the stakes were extremely real and severe. One week after birth, doctors noticed something was off with KJ. After ruling out several possibilities, they uncovered a cruel answer: a rare genetic disease called severe carbamoyl-phosphate synthetase 1 (CPS1) deficiency, affecting only 1 in 1.3 million infants.

This disease impairs the body’s ability to eliminate “ammonia” (a byproduct of protein metabolism). It can lead to fatal consequences, affecting brain development and causing severe liver damage. The usual treatment for such conditions is a liver transplant, but for a very young KJ who was not suitable for surgery, this was not an option.

Therefore, after diagnosis, Ahrens-Nicklas contacted Kiran Musunuru, a gene editing expert at the University of Pennsylvania. “My internal clock started ticking,” Musunuru later told The New York Times. Ahrens-Nicklas and Musunuru worked with a team of experts nationwide for six months to develop a precise targeted gene therapy specific to KJ’s CPS1 mutation.

Meanwhile, KJ was under medical supervision in the hospital and maintained his life through a completely protein-free diet. When the CRISPR treatment was ready, KJ’s weight percentile had dropped to the 7th percentile for his age.

From “fear” to “homecoming”

On February 25, 2025, the team began the treatment. Ahrens-Nicklas and Musunuru described the process as both exciting and terrifying.

“One of the most frightening moments for me was walking into the room and saying, ‘I don’t know if this will succeed, but I promise I will do everything I can to ensure its safety,’” recalled Ahrens-Nicklas.

The first infusion took two hours. Less than two weeks later, KJ started to intake proteins like a healthy baby. A second dose was administered 22 days later, and about two weeks prior, KJ received a third treatment.

While it is still unclear whether he will need a liver transplant in the future, doctors can now confidently say that thanks to the world’s first “tailored” in vivo gene therapy, a life has been saved. This is a huge proof of decades of research and experimentation. KJ is now back home with his family.

“We hope every patient has the potential to achieve the same results as this first patient,” Musunuru said in the press release. “The promises of gene therapy we’ve heard for decades are coming true, and this will fundamentally change how we approach medicine.”