Philadelphia - Shortly after his birth in the summer of 2024, KJ Muldoon was diagnosed with a rare genetic disease that is fatal to about half of the babies.

Until now, the only effective long-term treatment for rare metabolic diseases is called severe phosphophosphate synthase 1 deficiency or CPS1 is liver transplantation.

Instead, doctors at Children's Hospital in Philadelphia told KJ's family that they could try things they had never done before. They will use a technique called personalized gene editing therapy CRISPR to find a unique mutant gene in his tiny body and repair it.

KJ then performed three experimental therapies on the liver treated by the hospital and Pennsylvania Medical Team earlier this year.

According to a press release from Penn Medicine, he became the first patient in the world to use CRISPR to personalize treatment. His case was published Thursday in the New England Journal of Medicine.

Pennsylvania Medical says that so far he has shown no serious side effects to treatment. He is still in the hospital, gaining weight and thriving. He will be 1 year old in August.

“Although it takes a careful monitoring of KJ for the rest of his life, our initial findings are very promising,” Dr. Rebecca Ahrens-Nicklas, director of gene therapy at the Genetic Metabolic Diseases Border Program at Children’s Hospital in Philadelphia, said in a statement.

"Now, when I was hugging him, he was laughing and jumping around, which was very heart-warming because I don't know if this would happen at some point," his mother Nicole Muldoon told CBS News on Zoom on Thursday.

According to Penn Medicine, CRISPR (a cluster of short plastid repeats representing a short interval) can “precisely correct disease-causing variants in the human genome.” Researchers warn that while personalized CRISPR treatment is in its early stages, the technology may one day help solve other rare genetic diseases.

“It’s exciting and we should all be very, very excited.” Bioengineer and immunologist, Dr. Brian Brown, serves as director of the Icahn School of Medicine at Mount Sinai, New York.

"You're the first day and you know how we're going to treat the future of different diseases," Brown said. "...we may be accompanied by treatments that may have terrible diseases for children."