Infant is World's First Patient Treated with Customized CRISPR Gene Editing Therapy

In a historic breakthrough, a child with a rare and life-threatening condition became the first person to receive a customized CRISPR gene editing therapy by a team at the Children's Hospital of Philadelphia (CHOP) and Penn Medicine. The treatment is the first of its kind designed to correct a disease-causing mutation found only in that individual.

The infant, KJ, was diagnosed with a rare urea cycle disorder. He was born with a rare metabolic disease known as severe carbamoyl phosphate synthetase 1 (CPS1) deficiency. He inherited mutations in each of his two copies of a gene for the liver enzyme CPS1. Without this enzyme, ammonia builds up in the blood when proteins, including ones found in food, are broken down, damaging the brain. According to Rebecca Ahrens-Nicklas, who led the development of the CRISPR therapy, more than half of children born with a CPS1 deficiency die. Patients with CPS1 deficiency, like KJ, are typically treated with a liver transplant.

Ahrens-Nicklas, together with cardiologist, geneticist, and gene editor Kiran Musunuru at Penn, created a base-editing therapy that corrects one of KJ's two copies of the CPS1 gene. After the treatment received approval from the U.S. Food and Drug Administration (FDA), KJ was given a low dose of the treatment in February 2025 when he was 6 months old, followed by larger doses in March and April. He has not shown any serious side effects and is now able to eat more protein than before.

Read more details about this breakthrough in the news releases from CHOP and the New Scientist. Get to know KJ's story here.