KJ Muldoon became the first patient to undergo personalized CRISPR treatment, a therapy that found the one uniquely mutated gene out of 20,000 in his little body, and fixed it.

Soon after KJ Muldoon was born in the summer of 2024, he was diagnosed with a rare genetic disorder that is fatal for about half the infants who are born with it.

Until now, the only effective long-term treatment for the rare metabolic disease known as severe Carbamoyl Phosphate Synthetase 1 deficiency, or CPS1, had been a liver transplant.

Instead, doctors at the Children’s Hospital of Philadelphia told KJ’s family they could try something never done before. They would use a technology known as CRISPR, a personalized gene-editing therapy, to find the one uniquely mutated gene out of 20,000 in his little body, and fix it.

He became the first known patient in the world to be treated using CRISPR personalized just for him, according to a news release from Penn Medicine. His case was published Thursday in the New England Journal of Medicine.