Here’s what you’ll learn in this story.

• The world’s first bespoke gene therapy saved the life of a newborn with a rare genetic disorder that cause the build-up of life-threatening ammonia in the body.
• In a race against time, scientists and doctors across the U.S. developed the first in vivo gene therapy, thanks to decades of medical research.
• After three doses, the newborn patient showed drastic improvement, and this new era of in vivo gene therapies could save the lives of millions more in the future.

Life’s ability to successfully copy three billion distinct letters in the human genome is an absolute biological wonder—but sometimes, mistakes are made. Whether inherited or formed in utero, genetic disorders and other birth defects are common, and occur in one in every 33 babies in the U.S., according to the Centers of Disease Control and Prevention (CDC). For all of human history, a person born with such a disorder likely had to live with the condition, and depending on the defect, those lives could be brutally short.

But in 2025, human history changed forever.

In a groundbreaking announcement, detailed in a study published in the New England Journal of Medicine, scientists, doctors, and specialists from institutions around the U.S.—including the Children’s Hospital of Philadelphia, University of California-Berkeley, and Penn Medicine—successfully saved the life of a newborn patient named KJ, who had been born with a rare genetic disorder. To pull off this incredible medical feat, doctors employed the world’s first custom in vivo (i.e. inside a living organism, rather than in a petri dish) CRISPR gene therapy. This technique, developed over decades thanks to U.S.-funded medical research, could help alleviate painful lives for millions of people born every year with now-fixable genetic disorders.

“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,” Children’s Hospital of Philadelphia’s Rebecca Ahrens-Nicklas, a co-author of the study, said in a press statement.

The details of this incredible medical intervention play out like a made-for-tv medical drama, but the stakes were incredibly real and deadly serious. A week after his birth, doctors noticed something wasn’t quite right with KJ. After ruling out a few possibilities, they stumbled across the unfortunate answer—a rare genetic disorder called severe carbamoyl phosphate synthetase 1 (CPS1) deficiency that affects only one in every 1.3 million babies. This disorder inhibits the body’s ability to get rid of ammonia, a product of protein metabolism. This can have deadly consequences, impact brain development, and wreak havoc on the liver. Usually, the treatment for a disorder like this is a liver transplant, but that was not an option for the infant boy, who was still too young to be considered for the surgery.

So, once arriving at a diagnosis, Ahrens-Nicklas contacted a gene-editing specialist at the University of Pennsylvania named Kiran Musunuru and “the clock start[ed]in my mind,” he later told The New York Times. Working with a team of specialists across the country for six months, Ahrens-Nicklas and Musunuru developed a targeted gene therapy to fix KJ’s specific variant of CPS1. Meanwhile, KJ was kept under medical surveillance at the hospital and subsisted on a diet completely devoid of protein to avoid making his condition worse. By the time the CRISPR treatment was ready, KJ was in the 7^th percentile for his weight.

On February 25, the team began administering the treatment, with Ahrens-Nicklas and Musunuru describing the process as both exciting and terrifying.

“One of the most terrifying moments was when I walked into the room and said, ‘I don’t know if it will work but I promise I will do everything I can to make sure it is safe,’” Ahrens-Nicklas told The New York Times.

The first infusion took two hours, and within two weeks, KJ began eating protein like a healthy baby. A second dose arrived 22 days later, and about two weeks ago, KJ received a third. Although it’s unknown if he will eventually still need a liver transplant, doctors can now safely say that a human life has been saved thanks to the world’s first bespoke in vivo gene therapy—a huge testament to decades of a research and experimentation. KJ is now at home with his family.

“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,” Musunuru said in a press statement. “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.”